Question

This is a limit test for chlorides.
1.0g of NaOH sample was dissolved in 5ml of deionized water, acidified with 4ml of concentrated nitric acid (65%) and diluted to 15ml with deionized water. (Test tube A)
10ml of 50ppm Standard NaCl solution was prepared. 5ml of water was added, followed by 1ml of dilute nitric acid. (Test tube B)
1ml of AgNO3 was added to each of the test tubes and compared for opalescence.

Can you explain why we need to add 4ml of concentrated nitric acid and how was the 4ml derived?

Answer 1

Ag ion reacts with strongly basic solution to produce a hydrated Ag2O sometimes simply written as Ag(OH)2. It's a dark brown/black gelatinous ppt when Ag ion is added to a basic solution. So the 4 mL HNO3 is to neutralize the NaOH and make sure the solution is acidic. HNO3 is used because neither HCl nor H2SO4 can be used; HCl for the chloride content and H2SO4 because of solubility concerns. 65% HNO3 is very close to 15 M and 4 mL of that is enough to neutralize all of the NaOH (about 1/40 = 0.025 moles) (4 mL of 15 M HNO3 is about 0.06 mols).

Answer 2

Thank you very much. Another question, why do we need to add 1ml of dilute nitric acid to the 50ppm standard NaCl solution before adding AgNO3?

Answer 3

Same answer. You want the Ag^+ to be an acid solution. AgCl needs to be pptd from an acid solution. HNO3 is the preferred acid to use. Too much can go wrong with Ag^+ from neutral or basic solution.