Common Operations and Wet Chemical Methods in Environmental Laboratories - Fundamentals of Environmental Sampling and Analysis - page 149

lubricating oils. ''Grease'' represents higher molecular weight hydrocarbons and all
glycerides of animal and vegetable origin. The term ''oil and grease'' is defined by
the operational procedure rather than the representation of distinct chemicals or
groups of compounds.
The partition-gravimetric method described herein includes acidification
and extraction followed by a gravimetric measurement. Acidification is required
before extraction if fatty acids are present in the sample. The fatty acids occur
primarily in a precipitated form as Ca and Mg salts with soaps. As such, they
are insoluble in the solvents. Acidification with HCl to PH 1.0 will release the
free fatty acids for analysis. The reaction involved may be represented by the
equation:

CaðC 17 H 35 COOÞ 2 þ2H þ ! 2C 17 H 35 COOHþCa 2þ ð6 : 19Þ

Hexane is then used to extract oil and grease in a separatory funnel. The extracted
phase is drained to a tarred distilling flask through anhydrous Na 2 SO 4 to remove
residual water, and then hexane is separated by evaporation in a distillation device
(boiling point is 69 C for hexane; maintain water bath temperature at 85 C). The
tarred flask is weighed again and the gain in the weight of the tarred flask is due to oil
and grease.
Modification of the above procedure is needed for solid samples such as sludge.
The method for solid samples consists of weighing a definite amount of sample,
acidifying it to release fatty acids, and then adding sufficient MgSO 4 H 2 Oto
combine with all free water by forming higher hydrated forms, MgSO 4 7H 2 Oto
ease extraction with hexane. The extraction is then performed in a Soxhlet extractor
in place of a separatory funnel for liquid samples.

6.3.6 Residual Chlorine and Chloride
in Drinking Water

1. Residual Chlorine

Disinfection using gaseous chlorine (Cl 2 ) is the most widely used in domestic water
supply and sewage treatment plants. When chlorine is added to water, it rapidly
hydrolyzes to form hypochlorous acid (HOCl) and hydrochloric acid (HCl). HOCl is
a weak acid that can be further dissociated into hypochlorite (OCl ). The reactions
are:

HOCl þH þ þCl Cl 2 þH 2 O ð6 : 20Þ

@

H þ þOCl HOCl ð6 : 21Þ

@

At equilibrium, aqueous phase Cl 2 normally is negligible above pH 3. Hence the two
species formed by Cl 2 in water are primarily HOCl and OCl . These two species are
termed as free chlorine residuals that have the disinfection capacity in killing
bacteria and pathogens. Unlike HOCl and OCl , chloride (Cl ) is not effective in
disinfection at all.