Common Operations and Wet Chemical Methods in Environmental Laboratories - Fundamentals of Environmental Sampling and Analysis - page 151

iodine. A handy source of iodine is from potassium biiodate, KH(IO 3 ) 2 , which can
be prepared as a primary standard according to the following reaction in the presence
of an acid:

KHðIO 3 Þ 2 þ10 KI þ11H þ ! 6I 2 þ6H 2 Oþ11K þ

ð6 : 29Þ

It is possible to use the back titration method to measure separately free chlorine
residuals and combined chlorine residuals by conducting a two-stage titration.
Phenylarsine oxide (PAO) is the reducing agent normally used for this purpose. At
pH above 7, PAO reacts with free chlorine residuals in a quantitative manner,
whereas at pH below 4, only chloramines react with PAO.

DPD-FAS Titration The N,N-diethyl-p-phenylenediamine (DPD) is added to a
sample containing free chlorine residuals and an instantaneous reaction occurs,
which produces a red-color (Eq. 6.30). At pH 6.2 to 6.5 the reaction is rapid and
reversible. Then the solution is titrated with ferrous ammonium sulfate (FAS)
[Fe(NH 4 ) 2 (SO 4 ) 2 ] until the disappearance of the red color as the end point. If a
small amount of iodide is further added to the sample, monochloramine reacts to
produce iodine, which in turn oxidizes more DPD to form additional red color.
Titrate with FAS. If a large quantity of iodide is then added, dichloramines will
react to form still more red color. Titrate with FAS again. By three successive
titrations, free chlorine, monochloramine, and dichloramine residuals can be
determined.

DPD

Red-pink colored

+ N(CH 2 CH 3 ) 2

N(CH 2 CH 3 ) 2

+ 2Cl - + H + Cl 2 +

ð6 : 30Þ

NH 2 NH

Fe 3+ Fe 2+

2. Chloride
The measurement of chloride (Cl ) is necessary to determine the suitability of
water for domestic, industrial or agricultural uses. High chloride concentration may
be a result of seawater intrusion due to excessive pumping of groundwater,
evaporation of irrigated water leaving salts behind in the soil, or discharge of human
excreta into municipal wastewater. In addition, chloride is a common tracer in
chemical fate and transport studies, when a simple and reliable analytical method is
required. Two wet chemical methods are available for the measurement of chloride.
The selection is largely a matter of personal preference. The argentometric method