FAQ's Model 926 Chloride Analyser

Answer 1 - 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.

Question 2 How can the Model 926S measure the concentration of chloride with a 20 µl aliquot and a 100 µl aliquot from the same sample and get the same answer? Surely the amount of chloride added is 5 times greater in the latter aliquot?

Answer 2 - Yes there is 5 times less chloride added in 20 µl sample than in 100 µl but when the "20µl" button is pressed the Model 926S adjusts the constant current to 20% of the value it uses for a 100 µl sample.

The rate of formation of silver is only 20% and so the instrument still takes the same time to react with the smaller sample of chloride and thus it displays the same concentration. The exact 20% rate is calibrated at the factory at the same time as the default 100 µl rate.

Question 3 How long does an anode last ( 92511003 contains 3 anodes).

Answer 3 - The Model 926 works by generating silver ions at the anode. These dissolve in the acid 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 1.4 grams and a third of this is lost when the anode is finished

Mass Ag = 1.4/3 = 0.467 gms

Mass Cl = 0.467/107.87 x 35.45 = 0.1534 gms 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 = 0.005 grams