Instrumentation
The Model 926 (and Model 926S) directly measure the chloride ion concentration of an aqueous sample by a coulometric technique, which involves reacting with silver ions generated in-situ by an electrical current passing between two silver electrodes immersed in an acid buffer. The reaction produces an insoluble silver chloride precipitate, which is held in suspension by gelatin and other components of the Acid Buffer.
The Model 926 is intended for general laboratory use and is calibrated in ppm (mg/l) chloride. It has a simple factor switch enabling the immediate translation of chloride content into % Salt (sodium chloride) of the original sample (See Below)
The Model 926S is intended for chloride measurements in biological samples and is calibrated in mmol/l. The Model 926S can accept original samples of as little as 20µl. It is of particular use in the final confirmation of Cystic Fibrosis by the analysis of Sweat samples.
Reagents
Acid Buffer (00156608) 6 x 500 ml
This is a non-hazardous mixture of Glacial Acetic and Nitric acids together with gelatin and other components designed to give highly reproducible results when used with the Models 926 and 926S. It provides the strongly acidic medium required for the dissolution and reaction between the silver and chloride ions.
Chloride standards (00156203 200mg/l) and (00156202 100mmol/l) both 6 x 100 ml
Traceable to NIST standards these stable solutions are used to verify the accuracy of the instrumentation and pipetting operation.
Silver polish (00156205 25 gm )
Used to keep the pure silver electrodes in good condition.
Electrodes
The anodes are pure silver electrodes, which are "sacrificed" by dissolution into silver ions. Once the end is dissolved the anode can be repositioned for extended use.
The cathode completes the electrical circuit with the anode.
Two detector electrodes monitor the conductivity of the reacting solution.
Pipette (not supplied)
The accuracy of the method is directly affected by the volume of sample dispensed by the pipette. A fixed volume (500 µl for the Model 926; 100 or 20 µl for Model 926S) is pipetted directly into the Acid Buffer solution. A further 6 samples ( 21 samples for the Model 926S) may be added into the same buffer before the instrument prompts a new aliquot be used.
Sample Preparation.(see table below)
Solid Food samples
Sodium chloride is easily soluble in water, so the sample preparation is usually to mix/shake the solid sample with water for 15 minutes- 1 hour depending on the sample, warming if necessary. Allow to cool, filter and wash filter paper, and make up to a 1% solution (1 gm sample in 100 mls distilled water).
Measurement
A 500 µl sample is carefully added to the acid buffer and the Titrate button pressed
The result is automatically displayed (and sent via the RS232 output to a any printer connected) in mg/l chloride.
Calculation of Salt Concentration
The "mg% Salt" button can convert mg/l chloride reading to mg% salt (mg/100 gms) in the original sample ONLY if the solution tested is a 1% solution of the original sample.
Firstly the relationship between the chloride and Salt content
The Atomic weight of Na = 22.98 CI = 35.45
The molecular weight of NaCl is 58.43
To convert from CI to NaCl we multiply by 58.5/35.45 = 1.6482.
This is the factor that is used when the "mg%" button is pressed
For example:
After pressing the mg% Salt button, a reading of 200 ppm for chloride is converted to 200 x 1.6482 = 329 mg/l NaCl = 033 mg % (mg per 100 gms of the diluted sample aliquot)
This is displayed on the Model 926 as 033.
This reading is then multiplied by the 100: 1 dilution factor = 3300 mg % or 3.3 %. (gms per 100 gms original sample) which is
the concentration of Salt (sodium chloride) in % of the original sample.
Salt in certain food products measured using Model 926
| Product | Extraction step | Expected value 1 % Salt |
| Bacon, raw | Macerate, Filter/centrifuge | =3.5 % |
| Breadcrumbs | Heat to 60° C , filter | 0.4-2.0% |
| Butter | Melt in warm water, gently mix. Cool, puncture fat layer, pipette from aqueous layer | 1.8 %, Unsalted >0.03% |
| Cheese | Macerate, Filter/centrifuge | 0.8-3.8 % |
| Curry Powder | Heat to 60° C Filter through Whatman No4 | 0.8% |
| Fish (smoked) | Macerate, Filter/centrifuge | 1.8-4.6% |
| Milk | Dilute with nitric acid | .2% |
| Mustard | Mix vigorously water | 5.9% |
| Sauce mayonnaise | Mix vigorously water | 1.2% |
| Tomato ketchup | Mix vigorously water | 3% |
Ref 1: The composition of foods 5th Edition 1991 MAFF
