1. Total mercury in chlor-alkali effluents shall be determined using the test method prescribed in the chapter entitled Mercury in Water, Storet No. Total 71900 in the publication “Methods for Chemical Analysis of Water and Wastes”, 1974, published by the U.S. Environmental Protection Agency, with the following modifications:

• (a) section 3.1 under the heading ”Sample Handling and Preservation” shall be replaced by the following:

  • “3.1 Composite samples shall be collected in nitric acid cleaned glass carboys and representative portions transferred to nitric acid cleaned glass bottles with Teflon lined caps or in high density polyethylene bottles. Prior to sampling, the carboys shall be prepared by one of the following methods using the quantity of reagents required as determined from past experience with that particular type of sample.

  Chlor-Alkali Plants Using Fresh Water:

  Method A or Method B

  	A —	add sufficient nitric acid to lower pH of the total sample to 1 or less and an excess of saturated potassium permanganate solution to maintain a purple colour,
  	B —	add sufficient nitric acid to lower pH of the total sample to 0.5 and add sufficient potassium dichromate to make the sample contain 0.05 per cent potassium dichromate by wt.

  Chlor-Alkali Plants Using Natural Sea Water:

  Add sufficient nitric acid to lower the pH to 0.5 or less.

  In either case a representative portion of the composite sample of a suitable volume should be carefully transferred.”

• (b) section 4.3 under the heading “Interference” shall be replaced by the following:

  • “4.3 Chlorides: Since free chlorine also absorbs radiation at 253 nm, assurance must be made that free chlorine is absent before the mercury is reduced. The type of sample being tested will determine the procedure to be followed:

  Chlor-Alkali Plants Using Fresh Water:

  Potassium permanganate oxidation converts Cl to Cl₂. This free chlorine is removed by re-conversion with excess hydroxylamine sulfate and subsequent purging of the dead air space in the sample bottle prior to the addition of stannous sulfate. Some mercury losses may be attributed to the possible formation of volatile mercury-chlorine compounds and mercury-hydroxylamine reduction compounds.

  Chlor-Alkali Plants Using Natural Sea Water:

  Sulfuric acid, nitric acid and potassium persulfate as oxidation reactants are sufficient since mercury in natural sea water is in the inorganic form in equilibrium between HgCl₂, HgCl₃^-, HgCl₄^-2. Consequently, the problems associated with Cl₂ generated are eliminated.”