Common Operations and Wet Chemical Methods in Environmental Laboratories - Fundamentals of Environmental Sampling and Analysis - page 152

(Mohr method) is suitable for relatively clean water when 0.15 to 10 mg Cl is
present in the portion titrated. The end point of the mercuric nitrate method is easier
to detect.

Argentometric Method (Mohr Method)
In a neutral or slightly alkaline solution, potassium chromate (CrO 2 4 ) can indicate
the end point of the silver nitrate titration for a solution containing Cl . Silver
chloride (AgCl, K sp ¼ 310 10 ) is precipitated quantitatively before orange silver
chromate (Ag 2 CrO 4 , K sp ¼ 510 12 ) is formed.

AgNO 3 þCl AgCl þNO 3

ð6 : 31Þ

@

2Ag þ þCrO 2
4

Ag 2 CrO 4 ðorangeÞ ð6 : 32Þ

@

Mercuric Nitrate Method
Cl is titrated with mercuric nitrate, Hg(NO 3 ) 2 , because of the formation of soluble,
slightly dissociated mercuric chloride. In the pH range from 2.3 to 2.8,
diphenylcarbazone (DPC) indicates the titration end point by the formation of a
purple complex with excess mercuric ions.

Hg 2þ þ2Cl @ HgCl 2 ð6 : 33Þ

Diphenylcarbazone

N N

H

Purple Complex

NNC

ð6 : 34Þ

O

HN N Hg O

H H

O C H

C N N

N N NN

6.3.7 Ammonia in Wastewater

Ammonia is produced mostly by decomposition of organic nitrogen-containing
compounds and by the hydrolysis of urea in wastewater. Ammonium concentration
in surface and groundwater is usually low. The titrimetic method can be used directly
in the case of drinking water or highly purified wastewaters. A preliminary
distillation step is needed for other sample matrices, which transfers NH 3 into a
receiving flask containing pH 9 borate acid (H 3 BO 3 ) (Eq. 6.35). The ammonia in the