EPA Method 375.2

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EPA Method 375.2:
Sulfate by Colorimetry. Official Name: Sulfate (Colorimetric, Automated, Methylthymol Blue, AA II)

Summary:
A sample is first passed through a sodium form cation-exchange column to remove multivalent metal ions. The sample containing sulfate is then reacted with an alcohol solution of barium chloride and methylthymol blue (MTB) at an acidic pH to form barium sulfate. The combined solution is raised to a basic pH so that excess barium reacts with MTB. The uncomplexed MTB color is gray; if it is all chelated with barium, the color is blue. Initially, the barium and MTB are equimolar and equivalent to 300-mg SO4/L; thus the amount of uncomplexed MTB is equal to the sulfate present. The amount of sulfate in the sample is measured by measuring the absorbance of uncomplexed MTB using a spectrophotometer.

Scope:
This method determines sulfate in drinking, ground and surface water; domestic and industrial wastes.

Citation:
Methods for the Determination of Inorganic Substances in Environmental Samples (EPA/600/R-93/100)

Interferences:
(A) Ion exchange column breakdown: Run sulfate/Ca+2standard to ensure proper column operation.(B) Turbidity: Filter or centrifuge turbid samples.(C) Contamination: Reagents or glassware can be contaminated.

QC Requirements:
The minimum recommended quality control requirements include an initial demonstration of laboratory capability (determining the linear calibration range and method detection limit, and analyzing a quality control sample; LCR and MDL, and QCS), and the periodic analysis of a laboratory reagent blank.

Maximum Holding Time:
28 days (MCAWW, Table 1).

Media:
WATER

Subcategory:
Inorganic

Concentration:
3 - 300 mg/L and 0.5 - 30 mg/L (with minor modification)

Precision:
-

Detection:
DL is taken as lowest point of the effective range.

Revision Number:
Revision 2.0, August 1993