The problem of the determination of chlorides in chromate solutions came to me from galvanometry experts. The point is that it was necessary to control the content of chlorides in pickle baths. I still do not know why they need to do it, but I have solved the problem of chloride control.
The solution in the bath was absolutely deadly: 400 g/L CrO3. In that solution, chloride in concentration of above 10-3M was to be determined.
Counting on the chloride-selective electrode EM-Cl-01 (AgCl+Ag2S) I started the experimental work. At first, I diluted the solution by a factor of 10 in order to reduce the medium corrosiveness. As a result, the pH of the medium became close to 1 at the content of CrO3 = 40 g/L. Then the problem of neutralizing the acidic medium of the sample arose. Further investigation showed that the neutralizing deteriorated the conditions for chloride determination, making the further analysis impossible. Why?
The point is that CrO3 mainly exists in acidic medium in the form of Cr2O72-. With the increase of pH, the form CrO42- becomes favorable. On the other hand, the selectivity of the chloride electrode to this or that anion is determined by the solubility of the compound of that anion with silver cation. (Chloride-selective electrode is an extruded tablet of a mixture of chloride and silver sulphide.) Since the solubility of Ag2CrO4 is apparently lower than that of Ag2Cr2O7, it means that a chloride-selective electrode has a higher sensitivity to CrO42- ions. Thus, a conclusion arises that there is no need to try to neutralize the solution.
Electrode calibration in CrO3 solutions gave the following results.
First, before analysis in the beginning of working day the electrode needs to be prepared for operation by putting it for an hour in the CrO3 solution. Without that, the electrode potential experiences a considerable drift.
Second, the calibration by chloride is linear at least in the concentration range pX=4-2.5, although the slope is somewhat different from the theoretical one (see the figure).
Third, it turned out that the calibration curve technique could not be used in analysis because slight changes in the sample composition caused considerable change of the potential (see the figure). It is only the addition method that can be used, where the calibration slopes do not change that much. Calculations show that reducing the concentration of CrO3 from 40 g/L to 20 g/L causes an error of 20% in the analysis by the addition method.
In conclusion, it should be mentioned that measurements in such an aggressive medium as the one formed by chromates considerably shorten electrode lifetime. In this connection it is recommended that the electrode should be stored dry instead of being left it in the analyzed solution.