Techniques for the optimum use of the Sherwood Model 926 and M926S Chloride Meters
The Operator's Manual details all of the procedures necessary for the correct operation and maintenance of the chloride meters.
Here we stress those aspects which may not be obvious:
1 - The instruments works by passing a constant current between the anode and cathode. The number of silver ions liberated is directly related to this current by the laws of Electrochemical equivalence, which states that 1 coloumb of electricity will release exactly 0.001118 gms of silver, which in the Model 926 is reacted to a stoichimetric amount of chloride ions (the Faraday or 96500 coloumbs releases exactly 1 mole of a substance).
The instrument is calibrated at the factory by adjusting the frequency of the display clock to an absolute value, which, for an aliquot of 500 microlitres of standard containing 200 mg per litre, will read 200.
Provided that the same volume of sample is pipetted the display value or " time" is directly proportional to the concentration of chloride ions in that sample.
Because the provision of a constant current and the precision and stability of a frequency signal can be made to very high tolerances (variability <2ppm), the Model 926 will retain this absolute value unless some electrical or mechanical change to the circuit is made.
2 - The Instrument needs 15 minutes for the electronics to stabilise.
3 - From item 1 it can be seen that the variables to perfect performance in practical use will be found in the sample /standard handling and most importantly the sample pipetting.
4 - The pipette used should be calibrated by weighing 10 aliquots of 500 microlitres distilled water. These should weigh 5.00 gms at 20 degrees Celcius.
This will measure the mean aliquot but not the variation, which is done by recording the individual aliquots and determining the relative standard deviation from the mean.
Once consistency is achieved in pipetting then the technique in adding the standard or sample to the buffer solution needs to be acquired.
5 - One method is to take up the sample/standard, gently wipe the side (not the end) of the tip, place the tip on the surface of the buffer then expel the aliquot and removing the pipette while still holding the "button" depressed.
You should expect to get results within 2 units (i.e. 002 on the display) for the same sample every time.