The Determination of
Total Organic Carbon in Water
T. E. LARSON, Head
F. W. SOLLO, Principal Chemist
B. J. GRUNER, Research Assistant
Chemistry Section
Illinois State Water Survey
Urbana, Illinois
The principal methods now being used for measurement of the degree of pollution of water or wastes are the biochemical oxygen demand (BOD) and the
chemical oxygen demand (COD) (1). In terms of oxygen demand, the BOD test
measures that portion of the contaminants which can be oxidized biologically in
five days at 20 C. Many of the synthetic organic chemicals in common use today
are not oxidized biologically, or only to a limited extent, and are not measurea
by this procedure.
In its original form, the COD test (2) failed on several simple compounds,
such as ethanol and acetic acid, and required a correction for chloride ion which
was quantitatively oxidized. A modification using silver sulfate as a catalyst (3)
gave complete oxidation of most compounds but complicated the chloride correction. More recently, a further modification using mercuric sulfate to complex
the chloride (4) was introduced. Using both of these modifications, precise results may be obtained on almost all organic compounds.
Both of these tests are useful in many situations, but both give results in
terms of the oxygen required for stabilization of a waste. A more fundamental
measure of pollution would be the concentration of organic carbon, since this
element is characteristic of organic matter. The importance of this determination has been recognized for many years, and a variety of techniques for its
measurement has been devised.
Almost invariably, the methods used for the determination of carbon have
been based on complete oxidation of the organic matter and measurement of the
carbon dioxide formed. Differences are noted in the methods of oxidation and
the measurement of the resulting carbon dioxide.
The methods of oxidation have included ultra-violet irradiation (5), oxygen
at elevated temperatures (6,7), and a variety of oxidants based on chromic and
sulfuric acids (8,9,10,11,12,13). Systems for measurement of the carbon dioxide include absorption in Ba(OH)2 and back titration (8), manometric procedures (9,11,13), mass spectrometry (5), infra red absorption (6,7,14), and gas
chromatography (15,16,17).
ANALYTICAL PROCEDURE
The present method is essentially that suggested by Buswell (18), but adapted
for measurement of the very low concentrations of organic matter found in polluted streams or sewage after "complete" treatment. Oxidation is accomplished
with the Van Slyke liquid reagent, followed by catalytic oxidation of any vola-
tiles evolved. The carbon dioxide is trapped at -195 C, purified by distillation
under high vacuum, and measured manometrically.
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