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Chemical Analyses: Water: Sulfate
Sulfate (SO42-) is a naturally occurring ion and may be present
in waters over wide concentration range. Acidic drainage may contribute
large amounts of sulfate from oxidation of pyrite and other sulfide
minerals. Increased levels of sulfate are usually the first indication
of acid generation. Sulfate concentrations can be measured by ion
chromatography, colorimetry, gravimetrically, or turbidimetrically
using either manual and/or automated procedures. The main limitation
of sulfate occurs in the interpretation of data. The source of the
sulfate ion can be incorrectly attributed to acidic drainage, when
in fact it may be from solubilization of salts such as gypsum which,
release sulfate but do not produce acid mine drainage.
The standard method for sulfate analysis is the turbidimetric procedure
whereby, sulfate ion in acidic medium is precipitated with barium
chloride to form barium sulfate. The resulting suspension is measured
by a photometer at 420 nanometers or with a turbidity meter. The
instrument of use is calibrated with sulfate standards of known
concentrations. Initial turbidity in the samples must be removed
by filtration prior to analysis. The minimum detectable sulfate
concentration by this method is 1 mg/L. The procedure is outlined
in Standard Methods, Section 4500 E (Standard
Methods, 1998).
Sulfate concentrations can also be determined by ion chromatography.
In this procedure, the sample anions of interest are separated on
the basis of their relative affinities for a low capacity, strongly
basic anion exchanger. The advantage of this method is that other
anions can be determined at the same time (i.e. phosphate, nitrate)
(Standard Methods, Section 4500 B) (Standard
Methods, 1998). The gravimetric procedure is a classical method
base on the low solubility of barium chloride (Standard Methods,
Section 4500 C) (Standard
Methods, 1998). The procedure is time consuming and is only
used when sulfate levels are very high. Sulfate can be determined
colorimetrically by the methylyhymol blue method (Standard Methods
Section 4500 F) (Standard
Methods, 1998). This colorimetric procedure can only be conducted
on samples that are free of color and turbidity.
For more information on measuring sulfate concentrations, see MEND,
2001.
Chemical
Analysis | Physical
Properties |
