Why do methods 8206 and 8207 differ in the ideal sample pH compared to the Standard Methods pH?

Document ID

Document ID TE1567

Published Date

Published Date 27/05/2017
Question
Why do methods 8206 and 8207 differ in the ideal sample pH compared to the Standard Methods pH?
Summary
Chloride pH Methods 8206 and 8207
Answer
The conditions for the methods are different. For the mercuric nitrate method the Standard Method calls for a pH of 2.3-2.8. The buffer in Hach indicator powder sets the pH at 2.7 +/- 0.1. Adjust the pH with sulfuric acid if possible. Nitric can also be used but is usually avoided due to its tendency to fume. The silver nitrate Standard Method calls for "neutral or slightly alkaline conditions". The chloride indicator powder contains a mixture of sodium bicarbonate and potassium chromate, which makes the solution slightly alkaline. Having a slightly alkaline solution to start with should be fine.