Colorimetric analysis is performed on solutions and as such the sample can only be analysed within a container. The container must therefore be (or made to be) transparent at the wavelength at which the sample is being measured.
Sample containers are offered in a variety of forms depending on whether the important factor is sample size, speed of measurement or accuracy.
For colorimetric work all containers should be made of glass or clear plastic. The types of container are listed below.
Matched Test Tubes
These are made of soda glass. They are individually matched at the Sherwood factory and supplied in matched sets, i.e. each tube in any set has exactly the same absorbance characteristics, this ensures that once a blank has been measured every sample absorbance readout will be due to the sample only no matter which of the tubes in the set is used.
Various diameter tubes are offered so that the sensitivity of measurement can be varied i.e., if a sample is concentrated (high absorbance values) then measurement in a small test tube is suitable as the light has to travel a smaller distance through the sample leading to lower absorbance values. However if dilute samples are to be analysed it is necessary to increase sensitivity by offering more sample to the light hence larger test tubes are used.
Each test tube size has a corresponding holder. The holders can also accommodate a square cuvette of the appropriate size as indicated in the table
| Test Tube Part No Set of 12 |
Diameter |
Holder Part |
Cuvette Path Length |
Cuvette Part number |
| 00299011 |
15 mm |
25212001 |
not used |
N/A |
| 00299012 |
10 mm |
25211001 |
5 mm |
25205001 |
| 00299013 |
5 mm |
25210001 |
2.5 mm |
25205002 |
| N/A |
|
25211002 |
10 mm |
22299013 |
Cuvettes
Sherwood Scientific supplies these in several versions:
General purpose Plastic ( Polystyrene) 10 x 10 x 45 mm. (47188300)
Supplied with every CHROMA colorimeter. Beware of reusing disposable cuvettes which can become scratched and unusable. They should not be used for any measurement involving light scattering such as Sperm counting as the slightest surface imperfection will ruin the precision of the measurement
Semi micro version Plastic 10 x 4 x 10 mm (47188400)
Also made of polystyrene these are used where the volume of sample is restricted for example with biological samples. The pathlength is still 10 mm.
Glass Cuvettes
These are basically fused glass cells with an exact path length (usually 10 mm). Glass Cuvettes generally offer reusability provided they are handled carefully and cleaned well.
The most common colorimetric measurements are made in 10 mm x 10 mm cuvettes. These cuvettes are usually 45 mm high and hold 4 ml of sample solution.
Cuvettes of lower pathlength (e.g. 5 mm) can be supplied for samples with higher absorbances. (Allowance for this shorter path length must be made in quantitative measurements.. see Theoretical aspects of colorimetery)
Pour In - Suck Out Cell (Sherwood Part 25213001)
For routine work it is extremely convenient to have a built-in sample tube with the outlet connected to a vacuum.
Solutions are poured into the container and after measurement are sucked out through the tube to waste.
This technique has several advantages:-
1. A high throughput of samples is achieved.
2. All measurements are made in the same tube eliminating the need for matched tubes.
3. Blank measurement is made only once and by nature of the method is now 100% accurate for all tubes.
4. Vacuum can be achieved by a water pump extremely cheaply
Flow cells Sherwood Part (25316001) When on line work is required, flow cells are ideal. Most flow cells have a small total volume but maintain the 10 mm path length. This ensures fast response and good sensitivity. Several factors however should be taken into account
1. The Sherwood Model 254 and Model 257 both have recorder output, which is required to record the data.
2. Air bubbles must be eliminated at all costs. Our flow cell incorporates a de-bubbler.
3. Sample tubes must remain free from kinks etc. to ensure consistent flow.
4. All connections must be secured by clips or wire fittings.
Optical Filters
Gelatin Filters
These are low cost selection devices which produce or transmit a wide band of radiation usually ± 20 nm. Fortunately most colorimetric analyses have a wide absorption band which allows excellent results to be obtained in conjunction with the CHROMA colorimeters.
We make this type of filter by sandwiching a thin layer of dyed gelatin of the desired colour between two thin glass plates.
Coverage of all wavelengths in the visible spectrum is catered for using a set of 8 Gelatin filters Sherwood part Number is (25226001). However most colorimeters are used for only one type of analysis. This will require the use of only one filter and the expense will be greatly reduced as compared with manufacturers who supply colorimeters with a full set already fitted and charged for!
Interference Filters
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.
Sherwood supplies a range of interference filters and, in principle, any wavelength can be accommodated.
| Filter type |
Peak wavelength nm |
Bandpass |
Part number |
| Set Gelatin filters |
410-710 |
± 20 nm |
25226001 |
| Gelatin |
410 |
± 20 nm |
25231001 |
| Gelatin |
430 |
± 20 nm |
25215001 |
| Gelatin |
470 |
± 20 nm |
25216001 |
| Gelatin |
490 |
± 20 nm |
25217001 |
| Gelatin |
520 |
± 20 nm |
25218001 |
| Gelatin |
540 |
± 20 nm |
25219001 |
| Gelatin |
580 |
± 20 nm |
25220001 |
| Gelatin |
600 |
± 20 nm |
25221001 |
| Gelatin |
710 |
± 20 nm |
25222001 |
| Interference |
340 |
± 5nm |
25227021 |
| Interference |
405 |
± 5nm |
47188900 |
| Interference |
430 |
± 5nm |
47179700 |
| Interference |
430 Brewing* |
± 1nm |
47179800 |
| Interference |
510 |
± 5nm |
47189000 |
| Interference |
546 |
± 5nm |
47189100 |
| Interference |
600 |
± 5nm |
47189150 |
| Interference |
660 |
± 5nm |
47189200 |
| Interference |
670 |
± 5nm |
47189250 |
| Interference |
680 |
± 5nm |
47189300 |
| Interference |
725 |
± 5nm |
47189400 |
*Sherwood Scientific has worked with a leading Brewery Group in the UK to develop a method using the CHROMA Model 257 for the colour of Beer. The European Brewing Conference approved this method, which requires very precise wavelength control and uses the 430 Brewing filter with its 2 nm bandwith. The Model 257 has the ability to input a "Factor" which relates the absorbance to a particular concentration value. In the case of the EBC method, the factor is 25. So the "colour " of beer is the absorbance at 430 x 25.
