I have two Model 254 colorimeters and when I put the same solution in each, even after blanking, why do I get different results?
This is because each instrument and particularly each filter have slight differences in the way they pass the light through . You must therefore, for quantitative work , not only blank the colorimeter, but a standard of known concentration must be measured and the absorbance of the unknown sample be measured and compared with that of the standard.
(When blanking (zeroing), the cuvette ,which will be used for measuring the standard and samples, should be filled with the solvent (usually water) that is to be used for the sample and standards (including all other reagents except the sample and standard substance). The cuvette is placed in the cuvette holder and the absorbance is set to 0.00 (0.000 for the Model 257) by adjusting the coarse and fine controls.
How do I choose the correct wavelength filter for my measurement.
If you are using a commercial kit of chemicals to perform an assay the kit manufacturer will give the wavelength in the kit “insert” or instructions.
If you are developing a new method and want to measure the concentration of a substance only at one wavelength then perform a spectral scan of the substance under investigation on a spectrophotometer to determine the wavelength of maximum absorbance. Determine how broad the wavelength peak is at half peak height. If it is less than 10nm then choose an interference filter with the wavelength the same as the peak. If it is > 20nm then you could choose a gelatin filter with a wavelength within 10nm of the peak wavelength. If you have no spectrophotometer available then choose a filter of complementary wavelength.
How long does a bulb last
About 1000 hrs.
Theory of Colorimetry
Before we can discuss the technique of colorimetry it is important to understand and to differentiate this technique and others which involve measuring electromagnetic radiation from various parts of the spectrum.The following table should clarify the types of radiation that constitute the electromagnetic spectrum. For those who are unfamiliar with the definition of wavelength and its units here is a brief guide:
Radiation may be considered as a wave. The wavelength is the distance between two successive peaks of that wave.
The wavelengths in the table are expressed in nanometers (nm) these are related to metres thus:
1 nanometer = 10-9 metre
Colorimetry is just one of the types of photometric analysis techniques i.e. it is a light measuring analytical procedure. Colorimetric measurements are made using a white light source which is passed through a colour filter or alternative wavelength selection device. This incident light then passes through a cuvette containing a chemical compound in solution. The intensity of the light leaving the sample will be less than the light entering the cuvette. The loss of light or absorption is proportional to the concentration of the compound.Colorimetry however only applies to measurements made in the visible region of the electromagnetic spectrum e.g. (380 – 780 nm). The extent to which light is absorbed by a sample is dependant upon many factors. The main general contributors are the wavelength of the incident light and the colour of the solution.
Each compound in solution has a typical (and usually unique) absorption spectrum, an example is shown in fig. 4.
The spectrum is a pattern of the amount of light absorbed by the substance in the solution plotted against the wavelength of the light. In most cases the spectrum will have a peak i.e. a wavelength at which absorption is at a maximum. This is often referred to as the max for the compound in question. If the absorption is being quantified it is essential that it is measured as close as possible to the max. Sensitivity is reduced at any other wavelength. From the example our sample has a max at about 460 nm in the blue part of the spectrum. So what colour will it appear to be? Well the answer is yellow!
Confused? Well here is the explanation:
Inert materials whether solid or liquid appear coloured due to the way they modify light illuminating the object. Thus different objects absorb some wavelengths and reflect others. If white light passes through a yellow solution, it absorbs all colours except yellow. Similarly, a book cover appears red since it absorbs all colours except red.
If a solution is clear and colourless it has not absorbed any visible radiation and therefore all the white light is transmitted ie. it is transparent.
See the example of the spectral distribution curve solution in figure 5. The solution absorbs blue light strong a max at 460nm and therefore appears yellow.
If the concentration of the yellow solution is reduced by half the two solutions will give curves shown. Therefore for greatest sensitivity and linearity it is essential to limit the measuring wavelength to the area of highest absorption. Figure 5 shows that the correct wavelength at which to measure a solution is the one which gives greatest absorption.
The wavelength or colour filter that will produce the maximum absorbance can be selected in two ways:
There are several options open to the manufacturer of a colorimeter when deciding how to select the wavelength i.e. produce monchromatic radiation (one wavelength band) from polychromatic radiation (white light). These basic options are-
Gelatin Filters (now replaced by Polyester filters) These are low cost selection devices which produce or transmit a wide band of radiation usually ą 20 nm. Fortunately most colorimetric analyses have a wide absorption band which allows excellent results to be obtained from a simple colorimeter. The most common type of gelatin filter is constructed by sandwiching a thin layer of dyed gelatin of the desired colour between two thin glass plates (polyester filters are sheets of polyester with carefully selected coloured saying material incorporates within the sheet).
There are two drawbacks which can be encountered using gelatin/polyester filters:
However these filters are eminently suitable for most general applications.(Glass Filters Coloured glass filters are now more or less historical selection devices in colorimeters and have very wide bandposses often up to 150nm. Specific wavelengths can however be achieved by using a combination of glass filters.)
To ensure all wavelengths in the visible spectrum are catered for, approximately 8 Gelatine or Polyester filters are required.
These are used to select wavelengths more accurately by providing a narrow bandpass typically of around 10nm. The interference filter also only absorbs approximately 10% of the incident radiation over the whole spectrum thereby allowing light of higher intensity to reach the detector.
The theory of operation of an interference filter is fairly complicated but has been simplified below.
An interference filter comprises of several highly reflecting but partially transmitting films of silver separated by thin layers of transparent dielectric material (often magnesium fluoride (MgF2) This is also referred to as an MD or metallic dielectric filter). When white (polychromatic) light passes through the dielectric layers multiple reflections appear between the semi-transparent mirrors. However some energy from the light beams passes straight through the filter. It is this wavelength which is desired for analysis. If the dielectric layer thickness is altered slightly the resultant wavelength is changed.
Before the analyst attempts to perform quantitative colorimetric analysis it is important to understand the theoretical aspects of the technique. The relationship between concentration and the light absorbed is the basis of the following theoretical consideration; The seemingly obvious way of taking readings on a colorimeter is to measure % transmission and adjust the ‘blank” to 100%.
For example, consider a situation where a blank is measured followed by three standard solutions having concentrations of 1, 2 and 3 units respectively. Ideally, a colorimeter should be giving concentration readings directly, but consider the above solutions when analysed.
The solution with a concentration of 1 unit reduces the light to 50% therefore, the solution with a concentration of 2 units will reduce the light to 25% and the solution with a concentration of 3 units will reduce the light to 12.5%.Therefore if the colorimeter is calibrated using a transmission scale, the following graph is produced.
The calibration in %T has the drawbacks of being non- linear and readings decreasing with increasing concentration. Bonguer first investigated this type of relationship for changes in thickness of solid materials. His work was followed by Lambert and Beer in 1852, who extended the studies to solutions. All three investigators contributed what is universally known as The Beer Lambert Law.
This states that:
The light transmitted through a solution changes in an inverse logarithmic relationship to the sample concentration.
In order to take measurements both directly and linearly in terms of concentration, %T readings must be converted into an inverse logarithmic form which are called optical density units (OD) or absorbance (A).
The formula is: = OD = log10100/%T
Therefore, for the given example, the relationship of OD to concentration is shown in the table below.
A calibration curve of OD against concentration linear and directly proportional.
Optical density (absorbance) is used for colorimetric analysis so that readings relate directly to concentration.
Similarly, optical density changes directly with sample path length. Thus we arrive at.
Abs = E x c x l
Abs = Absorbance
E = Extinction coefficient or molar absorptivity
c = Concentration
l = Path length
l is fixed by the pathlength of the cuvette (usually 10mm) and E is a constant for each chemical species hence Abs
What does the "mg%" button do?
Firstly the relationship between the chloride and salt content
The Atomic weight of Na = 22.98 Cl = 35.45
so the molecular weight of Na Cl is 58.43
so to convert from Cl to NaCl we multiply by 58.5/35.45 = 1.6482.
This is the factor which is used when you press the “mg%” button
e.g. A reading of 200 on the display for chloride is converted by pressing the mg% button to 200 x 1.6482 = 329 mg/I NaCl = 033 mg % (mg per 100 gms water) we then multiply by the 100: 1 dilution factor = 3300 mg % or 3.3 %. (gms per 100 gms water) which is the concentration in % of the original sample before diluting 1:100
Therefore the mg% button on the model 926 only works if the original sample is diluted 1:100.
What does the "Select” button on the M926 MkIII do?
It converts the mg/l Chloride result to mg% Salt – but see below
Chloride and Salt are linked as follows:
The atomic weight of Na is 22.98 and Cl is 35.45: so the molecular weight of NaCl is 58.43
To convert a Cl result to NaCl we multiply by 58.43/35.45 = 1.6482 which is the factor used when the “Select” button is pressed.
e.g. For a displayed Chloride reading of 200, pressing the “Select” button results in 200 x 1.6482 = 329 mg/I NaCl; displayed as 033 mg % (mg per 100g water). We then multiply by the 100:1 dilution factor = 3300 mg % or 3.3 %. (g per 100g water) which is the concentration in % of the original sample before diluting 1:100
Therefore the Select (mg%) button on the Model 926 only works correctly if the original sample was prepared/diluted 1:100 prior to the Chloride content being determined
How can the M926S MkIII measure the concentration of Chloride with a 20µl aliquot and a 100µl aliquot from the same sample and get the same answer? Surely the amount of Chloride added is 5 times less in the 20ul aliquot?
Yes there is 5 times less Chloride in 20µl of a sample as compared with 100µl but when the “Select” button (20µl button on the M926S MKII) is pressed, to indicate a selected sample volume of 20µl, the M926S adjusts the constant current to 20% of the value used for a 100µl sample.
With the rate of formation of silver reduced to 20% the instrument takes the same time to react with the smaller sample of Chloride and thus displays the same concentration. The 20µl and default 100µl settings are both, independently, factory calibrated.
How long does an Anode last (925 11 003 contains 3 Anodes).
The M926 works by generating Silver ions at the Anode. These dissolve in the Chloride Analyser Buffer and react with any Chloride ions present from the sample/standard. The rate of generation of the Silver is governed by the electrochemical equivalency. This states that 1 Faraday (96500 coulombs of current) will release 1 mole Silver (107.87 grams).This is also exactly equivalent to 1 mole Chloride (35.45 grams).
Each Anode weighs 1.4g and a third is lost when the Anode is finished. Mass Ag = 1.4/3 = 0.467g. Mass Cl = 0.467/107.87 x 35.45 = 0.1534g Chloride.
So, if 50 samples a day each containing 200 mg/l Chloride and 0.5 mls aliquots are used
1 day = 50 x 0.5 x 200/1000000 grams Cl = 0.005g
Therefore 1 Anode will last 0.1534/0.005 days = 30.68 or just about 1 month
Will Active Salt Software work with the M926S Chloride Analyser
The M926S Chloride Analyser measures Chloride concentration in mmol/l and therefore the data generated is not suitable for use with Active Salt which performs a calculation based on the premise the Chloride data entered is in units of mg/l
Can I collect data from more than one M926 Chloride Analyser with Active Salt Software?
Only one copy of Active Salt Software may be hosted on a PC but there is provision to run two instances of Active Salt; each collecting data from one M926 and one RS232 enabled balance
So with one PC it is possible to collect data from two Chloride Analysers and two balances using one copy of Active Salt Software (from revision 1.07 onwards)
Can I run Active Salt Software on a 64Bit Windows system?
Yes, the software will run OK on 64Bit Systems. You will get an error message when the software is installed. Just choose “Continue” and the software will load. When finished the Active Salt icon will appear on the PC or Laptop’s Desktop. Right click the application, select compatibility mode and check the option to always run as an Administrator. You only need do this once and the settings will be remembered so the program will then open up without error next time you use it.
I conditioned the Buffer and then checked the calibration using the provided Chloride Standard Solution but the displayed result is 000. What should I do?
The condition process should take longer than 10 seconds but unfortunately sometimes, after the Electrodes have been cleaned and/or reinserted in to the analyser, the sense (sleeved) Electrodes don’t always make the correct contact with the Electrode Board Assembly and a false endpoint is determined – usually resulting in the conditioning cycle lasting ten seconds or less. If that short condition is not noticed then subsequent titrations will all return a value of zero and any Chloride in the beaker will not have been titrated
To correct this situation you can try rotating the sense Electrodes in the Electrode Board Assembly or remove them, wipe, then re-insert and then try conditioning again. If conditioning now takes longer than ten seconds the likelihood is the problem is cleared and titrations can be successfully carried out.
If the condition cycle still takes ten seconds or less, then the Electrodes should be removed, cleaned and re-inserted again until the conditioning sequence works properly.
I conditioned the Buffer and then checked the calibration using the provided Chloride Standard Solution but the displayed result is higher than expected, e.g. 210 not 200. How do I re-calibrate the analyser?
The M926 MkII Chloride Analyser had visible/labelled calibration adjustment ports on the side panel; the M926 MkIII Chloride Analyser does not. Whilst that may appear to be an inconvenience and an oversight in the new design; in our opinion it is not. The M926 and M926S are factory calibrated devices that should not lose their calibration. Re-calibration should only be necessary if some critical component has failed and been replaced.
Most M926 MkII Analysers that were user re-calibrated had in fact been erroneously calibrated to accommodate either a Standard Solution that was no longer the correct concentration or a Pipette that was not delivering the correct sample volume or in some cases, both.
The most likely reason for a result of 210 instead of 200 when checking the calibration with the Standard Solution is someone having left the lid off the standard bottle for some time or routinely every time it has been used since first opened.
If you contact Sherwood Scientific asking how to re-calibrate your M926 MkIII Chloride Analyser we will first ask what results are being achieved, then if the calibration standard is freshly opened and in date, if the Buffer is in date and the lot numbers of both solutions. We will then ask when the pipette being used was last calibrated and if any attempt was made to verify if it is delivering the correct volume when the Chloride Analyser was giving the “wrong” results.
If all that information then points to the instrument not being calibrated correctly we will consider providing guidance to enable you or your distributor to re-calibrate the analyser.
The M926 is calibrated to work with a 0.5ml (500µl) sample. Can I use a different sample volume?
You can use a different sample size/volume – then adjust the result for the sample volume used versus the calibration volume. For example; for a titrated volume of 250µl, a displayed result of 150mg/l Chloride is corrected as 150 x (500/250) = 300mg/l or if the sample titrated was 1ml (1000µl) a result of 150mg/l Chloride would be 150 x (500/1000) = 75mg/l.
The Active Salt Software package we offer allows the user to enter the sample volume and so the correct product Salt content calculation is completed automatically for the operator.
Note: The ability to use a larger sample volume than the instrument is calibrated for, means the effective detection limit of the instrument may also be “improved”. The lower measuring concentration is stated to be 10mg/l Chloride. If, however, a 1000µl sample volume is used, the lower measuring limit effectively becomes 5mg/l and for a 2500µl sample it would be 2mg/l. (Note: if larger sample volumes are used, then the titration beaker buffer will need to be changed sooner than indicated by the on-screen messages; which work on the basis of the beaker volume allowing for a total of seven 500ul sample volumes).
Can I use a different size beaker to the one provided?
The instrument is factory calibrated using the standard beaker supplied. One critical factor, impacting the basic principle of operation of the M926/M926S, is time and related to that; stirrer speed and mixing efficiency. If the beaker dimensions are changed – especially diameter – the instrument may not work to the stated specification.
I removed the Electrodes and Stirrer to clean them. I have tried to put the Stirrer back in but it drops back down. What should I do?
There are two possible explanations for this problem:
1) The Motor Shaft coupling (925 09 006) that holds the Stirrer has come off, or more likely
2) The O-ring (001 31 058) that helps the coupling grip the Stirrer Shaft has perished and will need to be replaced
How long should it take to analyse a sample?
A stable reading should be displayed within 36 seconds of pressing the ‘titrate’ button, at a Chloride concentration level of 200mg/l
Can I convert my mg/l Chloride readings, obtained using my M926, to mmol/l Chloride readings?
The relative atomic mass of Chloride is 35.45, so by definition one Mole of Chloride has a mass of 35.45g and 1mmol a mass of 35.45mg. Therefore 1mg/l Chloride is equivalent to 1/35.45 = 0.02821mmol/l and so, for example, a reading of 200mg/l ≡ 5.64mmol/l
We have a M926 MkII. We switched on the analyser this morning and the display is showing EEE. What’s wrong?
The Error code EEE indicates the M926 MkII is “looking” for an external device which is not responding; in other words, something is plugged into the RS232 socket on the rear panel but the external device (printer) is not connected or not switched on. However if your instrument is not connected to an external device/printer, EEE indicates the small round switch on the rear panel, just above the on/off switch, has been accidentally moved to a position wherein the M926 is “looking” for an external device. This can happen when you reach for the on/off switch over the top of the instrument to switch the M926 on. Please make sure the switch (item 4 below) is positioned with the slot horizontal and with a 0 in the notch at the top. You may have to switch the instrument off and then on again with the switch in the correct position to clear the error message. If that doesn’t help, contact: email@example.com or call +44 (0) 1223 243444
What is BlueNotes?
“BlueNotes” is our dedicated software package that can be used to control our Flame Photometer range from a Windows PC. There are currently three versions, i) BlueNotes 410 for our M410* Flame Photometers, ii) BlueNotes 420 for our M420 series** of Flame Photometers and, iii) Regulated BlueNotes 420 for 420 series instruments being operated in an environment that requires adherence to 21 CFR Part 11 and similar regulation. (* – see FAQ 3 below) (** M420 series of Flame Photometers: M420, M425 and M420Cs)
Software requirements and compatibility
BlueNotes 420 and Regulated BlueNotes 420 require Windows 7 or later. BlueNotes 410 operates on Windows XP or later. The PC requires a serial connection to link to the flame photometer and an additional one for the optional autosampler accessory for which a USB to RS-232 adapter can also be used (supplied separately).
I already own a Model 410 Flame Photometer; does BlueNotes 410 software require any additional accessories?”
To work with 410 BlueNotes, the Model 410 requires a digital interface to enable output via a serial connection to a PC or printer. The Models 410 Clinical and Industrial are supplied with the interface already installed. The Model 410 Classic requires purchase and installation of the Digital Interface module to enable work with BlueNotes 410 software. A number of “Upgrade & Automate” packages are available: please consult your local supplier or contact Sherwood Scientific directly for more information.
“Does Regulated BlueNotes 420 comply with 21 CRF part 11?
Software alone cannot provide compliance to regulations pertaining to GCP (Good Clinical Practice), GLP (Good Laboratory Practice), GMP (Good Manufacturing Practice) or FDA 21 CFR part 11. No instrument or analytical software package can provide full compliance. The software must be operated in conjunction with the standard operating procedures (SOPs) defined by the end user and an organisation’s maintained quality management system. The software does contain features (in other words; provides the tools) that make it possible for the user to operate the connected Flame Photometer in a compliant manner.”
How is data integrity ensured in BlueNotes?
“All settings, methods, results files, etc. are stored locally on the host PC in a folder that is hidden, by default, in Windows. These files are stored in a digitally signed XML format and so are tamper evident. To guard against data loss, your IT administrator will be able to automatically back up this folder to a networked location using existing Windows features. In Regulated BlueNotes 420 there is no way to for a user to delete any results or method files through the software user interface once those files have been generated. In standard BlueNotes 420 it is only possible to delete these files via the password protected supervisor mode. Therefore, as long as regular backups of the BlueNotes main data folder are taken and supervisor
Can I export BlueNotes data to an external database?
“There is no built-in way to export to specific, external databases. BlueNotes software allows results to be exported as either a PDF report or a CSV* file which can be manipulated by spreadsheet software. While there is no way to automate this process from within the software, third party software such as Microsoft Excel could be used to automatically extract data from the CSV results files once they have been manually exported.”(*CSV Files: only possible for 410 and 420 BlueNotes Software. Regulated BlueNotes 420 does not allow export of CSV files)
Why is the display varying?
1 – If the variation consistently in one direction this is Drift. This is caused by the instrument characteristics changing with temperature of the flame system.
Solution to this problem :
2 – If the variation is random (Up and Down ) this is instrument noise.
Solution to this problem:
Specification for short term drift is ± 1%
Can the Model 410 measure Calcium on biological fluids?
Yes but; The samples must be pretreated to remove phosphate. The method is listed in the applications section Below
Why? – The Model 410 is a low temperature flame photometer and the flame has not enough energy to break the calcium phosphate bond.
How long will a cylinder of gas last?
The instrument uses 440 mls gas per minute (propane)
1 mole of propane is equivalent to approx 22.4 litres gas.
Thus 1 mole would last 22.4/0.44 minutes = 51 minutes
I mole propane weighs 44 gms (C4H10)
So 7 kg cylinder lasts 7000/44 x 51 minutes = 135 hours
Can the Model 410 measure Magnesium?
Magnesium atoms emit light at UV wavelengths after excitation at higher energies than available from the M410 Flame.
I have replaced the Lithium filter (top) with Calcium filter but can get no response even from 20 ppm Calcium standard solution
The top position is occupied by the Sodium Filter, not Lithium, even though on the Model 410 Li is the top element printed on the front panel. (This is because the optical path used in the flame photometer is opposite the bottom element position. So, for Na to be in the optical path, the filter holder lever must be pushed to the bottom position.)
So to replace Lithium with Ca, the bottom filter should be changed and Ca is measured when the filter holder lever is in the top position.( newer Model 410 Flame Photometers from serial # 15659 are fitted with Calcium as standard.)
Why are my results non linear when I measure Clinical samples for sodium even though I dilute the samples 200:1?
Even after dilution Na is outside the its linear range 140 mmo/l is normal serum value for Na = 140 x 22.9 =3206 ppm After 200:1 dilution it is still 16 ppm. Non-linearity starts at 10 ppm Solution is to fit a Lineariser Sherwood Part 47829800 See Non Linearity of sodium
Why does my MnCl2.4H2O solution filled tube that accompanied my MK1 MSB vary its reading?
Like many paramagnetic species, the reading MnCl2 solution gives is dependent on temperature. Always note the temperature when you measure the MnCl2 containing tube. Refer to the Mk1 manual for how to correct for sample temperature.
If the reading changes over time despite the temperature remaining constant, then it is possible there is a tiny hole in the seal which would allow water vapour to escape thereby altering the concentration and reading.
Why can't you tell me the concentration of MnCl2.4H2O in the solution in the tube?
The concentration of MnCl2.4H2O is known to be approximately 1M at the time of manufacture but, because it is adjusted to match a reference sample the final concentration in the sealed tube cannot be precisely known due to the method of manufacture.
Each tube is sealed by hand and it is not possible to guarantee that there is no tiny gas leak which over time will cause the concentration to change due to solvent evaporation. If the length of liquid in the tube has reduced since it first arrived – it should be at least 2.0cm – then the readings will be suspect. An air lock can sometimes trap some liquid at the top of the tube. Always make sure all the liquid is together and at the bottom of the tube before checking the length or taking any readings.
How can I use the MnCl2.4H2O tube to monitor the performance of the MK1 balance?
Regular measurement of the tube and recording the temperature at the time of measurement will allow confidence to be gained that the tube is gas tight. A plot of temperature versus tube reading should give a straight line. Ideally, keep the tube with the balance so they are at the same temperature.
How do I know if the reading I'm getting is right?
To work out if the balance is reading the ‘right’ result for a sample you have to check that it gives the right reading for a pure substance with a known magnetic susceptibility or a solution of such a substance with a pure solvent of known magnetic susceptibility. For the latter, see How do I work out the concentration of the solute in the liquid sample? below. A good start would be to use the standard tube delivered with your instrument; subject to the three answers above. Consider the choice of known substance carefully. Using solid substances in powder form requires allowances for the actual density of sample in the sample tube at time of measurement. Other things to bear in mind are; that the susceptibility of many paramagnetic species is temperature dependent, the ideal substance should measure a little bit higher than the highest expected test sample, liquids are easier to measure reliably.
What is the relationship between Molar Magnetic Susceptibility and Mass Magnetic Susceptibility?
For a pure compound the Mass Susceptibility is equal to the Molar Susceptibility divided by the molecular mass of the substance.
What are the different types of susceptibility and how are these related?
There are 3 commonly used types of susceptibility each with its own symbol. These types are shown below together with how they are related to each other in the cgs system
|Xv||Volume susceptibility||Xv = Xg ✴ ρ||and||Xv = XM ✴ ρ / M|
|Xg||Mass susceptibility||Xg = Xv / ρ||and||Xg = XM / M|
|XM||Molar susceptibility||XM = Xv ✴ M / ρ||and||XM = Xg ✴ M|
|Where:||ρ is the substance density in g·cm−3 e.g. water 0.9982 gmcm-3 (20°C)|
|M is the relative molecular mass e.g. water 18g/mol|
Literature values of susceptibility are often quoted as XM – Molar susceptibility, sometimes described as susceptibility per gram formula weight.
How do I work out if two tubes are matched?
At the simplest level, two tubes are ‘matched’ if they give the same reading in the instrument when clean and empty. For very precise work you can go further and check that they give the same reading when filled with the same volume of a sample – for example, pipette 300µl of 1M MnCl2 solution into each clean tube and put them in the balance. Rotate the tube about a clock face and record the results. Calculate the mean for each tube. If the means agree then the tubes can be said to be matched.
At a more detailed level matching tubes is usually intended to save having to empty and clean a tube to measure another sample under identical conditions. Tubes would be said to be matched if the readings obtained with them were the same for all samples. This requires the tubes to pass (at least) 2 tests:
If both apply then the tubes are matched. If the empty tubes read the same but filled tubes do not this indicates that the sample space defined by the insides of the tubes are different although the amount of glass being measured is the same.
If the filled tubes read the same but the empty tubes do not this indicates that the sample space defined by the insides of the tube is consistent between the tubes but the amount of glass being measured is different.
The explanation of how these situations can occur is that the key quality control dimension of this process is the distance from the inside of the closed end to the lower end of the rubber sleeve around the upper part of the tube which doesn’t necessarily account for the shape of or amount of glass in the closed ends of the sample tubes (see 2 examples in picture below) which are are formed by hand.
How do I measure strong samples which are over range?
The instrument reading is related to the volume of sample in the measuring space. This means that one answer for samples that are over range is to put less material between the magnets whilst maintaining a depth of at least 1.5cm. For a soluble substance it is simply a matter of dissolving it and diluting until the reading is in range. For large dilution ratios the presence of the solvent can easily be accounted for by measuring the sample using a tube containing just the solvent and set the display to read 000 with the zero knob before measuring the diluted sample. Prepare the same dilution ratio of all the samples and the results will be relative and directly comparable.
For smaller dilution ratios refer to the principles described in How do I work out the concentration of the solute in the liquid sample? If the sample is not soluble (or if you prefer) then a narrower tube could be used to present less of the sample to the instrument. Sherwood can provide tubes of 1 or 2mm internal diameter which would reduce the readings by 1:10.5 and 1:2.62 respectively, relative to the same substance in a normal tube of 3.24mm i.d.
For insoluble samples an alternative that is sometimes possible is to mix the sample with an inert substance with low mass susceptibility. It is important that the mixing is very thorough and this may not be easily achieved. One inert solid ‘diluent’ used has been icing sugar. Again it is necessary to account for the presence of the diluent. For large dilution ratios this can be most simply done by setting zero with the tube containing just the diluent. For smaller dilutions refer to the principles described in How do I work out the concentration of the solute in the liquid sample?
How do I work out the concentration of the solute in the liquid sample?
The simple answer is to plot a 2-point linear calibration from the MSB readings (R-R0) of:
The equation of this line will allow us to calculate the concentration of a solution of unknown concentration from its reading.
Here’s an example based on aqueous solutions of MnCl2.4H20. Ideally the same tube should be used for all measurements and, if doing so, it should be rinsed carefully a number of times with the fresh solution and tipped to waste so that the solution in the tube isn’t contaminated by the previous solution measured. At the start take a reading for the empty sample tube; this will be R0 that has to be subtracted from the readings of the other samples to give the (R-R0) values below.
For sample 1 we choose a 1M aqueous solution of MnCl2.4H20 made with 197.9gm (molecular mass) made up to 1 litre in water and get a reading that gives (R-R0) of 1149.
Sample 2 is water and its reading gives an (R-R0) of -59.
We plot these 2 points on our graph and draw a straight line connecting them as below.
Which we can rearrange to calculate concentration from (R-R0), thus (R-R0)
Our unknown solution gives a (R-R0), reading of 1000 so its concentration is
(1000+59) / 1208=0.877 M
This process can be followed on the graph by starting with the red value of 1000 on the (R-R0) axis traced horizontally across to meet the calibration line then, in blue, followed vertically down to the concentration axis at a value of 0.88M
A more detailed explanation from first principles and using literature values instead of having to make a solution of known concentration is also available on this page under the question How do I work out the expected MSB reading of a solution of a known solid in a known solvent?
What about the 'air correction term'?
Some chemists use an air correction term to account for the displacement of air in the balance when a sample is introduced. The air correction term makes a very small contribution to results, is referred to in section 1.1 of the Mk1 manual and the equation for it is given in Appendix A to that manual.
Why do I need to add 1.5cm depth of sample?
The section of the tube which is ‘exposed’ to the measuring magnetic field inside the instrument extends to just less than 1.5cm up from the bottom of the tube. More information is also available under the question Is it ever possible to work with less than the 1.5cm depth?
Is it ever possible to work with less than the 1.5cm depth?
It can be possible to get reliable relative readings with a depth of sample less than 1.5cm in the tube, however samples can only be compared to one-another when the same volume (not mass) is put in the bottom of the tube. Due to this constraint it is probably only suitable to fixed volumes of liquid that can be precisely pipetted into the bottom of the tube. When the depth of sample is > 1.5cm the reading is proportional to the sample Xv (volume susceptibility). Below 1.5cm the reading becomes a function of Xv and the sample volume. Any difference in volume will cause an error in the relative reading that is not necessarily proportional to the sample volume.
Are the MSBs able to measure high or low temperature samples?
Neither the MK1 or Auto Magnetic Susceptibility Balances have temperature control but it is possible to put hot or cold samples into either instrument and get readings.
Measurements made on such samples will be subject to errors for two reasons; the sample temperature will not be constant so any temperature dependent magnetic property will be changing and there are magnets inside the balance, very close to the sample, whose strength varies as these are heated or cooled by the sample. The user would have to make allowance for these effects. If any form of sensor is in the sample to measure its temperature to help with interpreting the readings then the effect of the sensor itself on the magnetic measurement must be understood.
How do I work out the expected MSB reading of a known, solid substance?
To do this using first principles and literature values, and without doing any measurements we will need some fundamental relationships involving magnetic susceptibility so let’s start with these.
Literature values of susceptibility are often quoted as Molar susceptibility, XM which is related to Mass susceptibility, Xg by the formula:
Mk1 readings for samples, in a standard sample tube with an internal diameter of 0.324cm, are related to Mass susceptibility by the formula printed on the instrument top panel
|Where:||C||is the balance constant that is factory set at 1.0|
|l, m||are the length of sample in the tube, in cm and sample mass, in g.|
|R||The balance reading for the sample and tube|
|R0||The balance reading for the empty tube|
Now we can go from the susceptibility of a known substance to its reading on the Mk1 balance.
Take for example the compound MnCl2.4H2O of relative molecular mass M = 197.9 and literature Molar susceptibility XM of +14,600 x10-6 cgs at 20°C. We start by converting this to Mass susceptibility using, from above, equation i
In our thought experiment we do not have a length or mass for equation ii, but we can fnd the literature value for the density of our substance, 2.01gmcm-3. Knowing that a standard 0.324cm internal diameter sample tube has a crosssectional area 0.0824cm2 and that the volume of a cylinder is equal to the cross sectional area multiplied by its length, we can rearrange the formula for density, ρ
To get our final answer we substitute these numerical values into equation ii
This would lead us to expect an MSB reading of +1222 on the x10 scale. This reading would be for a solid cylinder that fills the standard sample tube. In practice we are likely to be using a powder sample with a density smaller than the literature value and will therefore get a proportionally smaller MSB reading. To correct for this, without having to measure the actual sample density, equation ii conveniently allows us to measure just l and m. Let’s say we get l = 2.2cm and m = 0.193g. Now we can calculate, for the sample that we are measuring, a value for the effective density
or about half of the literature value which would lead to a realistic MSB reading of
Were we doing a practical experiment we would get the same result by substituting these measured values of l and m directly into equation ii as follows
How do I work out the expected MSB reading of a solution of a known solid in a known solvent?
To do this we will need some fundamental relationships involving magnetic susceptibility so let’s start with these.
Literature values of susceptibility are often quoted as Molar susceptibility, XM which is related to Mass susceptibility, Xg by the formula
The Mass susceptibility, Xs of a solution of a single solute in a single solvent is made up of the relative contributions from the solute and solvent according to the formula
|Where:||m1, m0||are the masses of solute and solvent respectively in the solution|
|Xg, X0||are the Mass susceptibilities of the solute and solvent respectively|
Mk1 readings for samples in a standard sample tube with an internal diameter of 0.324cm are related to Mass susceptibility by the formula printed on the instrument top panel
|Where:||C||is the balance constant that is factory set at 1.0|
|l, m||are the length of sample in the tube, in cm and sample mass, in g.|
|R||The balance reading for the sample and tube|
|R0||The balance reading for the empty tube|
Now we can go from the susceptibility of a known concentration to its reading on the Mk1 balance.
Take for example MnCl2.4H2O of relative molecular mass M = 197.9 dissolved in water, M = 18. The literature Molar susceptibilities, XM are respectively +14,600 and -12.96 x10-6 cgs at 20°C which convert to Mass susceptibility using, from above, equation i
To use equation ii we also need to know m1 and m0. These will come from the solution concentration. Let’s choose a 1M aqueous solution. This would contain 197.9gm of solute made up to 1 litre with water. We would fnd that 10cm3 of the solution would weigh 11.03gm so the 1 litre contained (1103-197.9) = 905.1 gm of water and had a density, ρ of 1.103 gmcm-3
With m1 = 197.9 and m0 = 905.1 we can calculate the Mass susceptibility of our 1M solution of MnCl2.4H2O by substituting these numerical values into equation ii
In this thought experiment we do not have a length or mass to use in equation iii. We do however have the density of our solution, 1.103 gmcm-3. Knowing that a standard 0.324cm internal diameter sample tube has a cross-sectional area 0.0824cm2 and that the volume of a cylinder is equal to the cross sectional area times its length, we can rearrange the formula for density, ρ
The final step is to substitute these numerical values into equation iii
So the answer to our question is that we would expect a 1M aqueous solution of MnCl2.4H2O to give an MSB reading of
MSB Mk I Calibration and use of formula
Each balance is supplied with a calibration standard (a sealed tube containing a solution of MnCl2.4H2O) and a test sheet showing the value of the instrument’s balance constant, CBal that is factory set at 1.000 ± 0.020
Mk1 readings for samples in a standard sample tube with an internal diameter of 0.324cm are related to Mass susceptibility Xg, by the formula printed on the instrument top panel.
|Where:||CBal||is the balance constant that is factory set at 1.0|
|l, m||are the length of sample in the tube, in cm. and sample mass, in gm.|
|R, R0||are the balance readings of the sample and empty tube|
Using a substance of known Xg and measurements of the other variables in the formula the value of CBal can be checked.Note this new value of CBal and use it in all subsequent calculations of susceptibility until the next calibration check.If no such substance is available and there is doubt about the value of CBal the standard tube can be used as follows.
Put a clean, empty standard sample tube (internal diameter of 0.324cm) in the balance and use the zero knob to set the display to zero on the x1 range.Remove the tube and insert the standard. Gently and slowly rotate the tube while it is in the balance and note the extremes of the readings. Note the average of these extremes; this will be R to be used later.
R should be the same as Cstd If it is larger than Cstd then the balance constant has decreased by the factor of Cstd/R.
A worked example should make this clear.
|CBal||= 1.005||from test sheet|
|Cstd||= 1019||from the label on the tube|
|R||= 1025||from measurement|
|New CBal||= 1.005 x 1019/1025 = 0.999|
Note this new value of CBal and use it in all subsequent calculations of susceptibility until the next calibration check.