Fluoride Limit Test

Perform a blank determination (see General Provisions), and make any necessary correction. Each milliliter of 0.1 N perchloric acid is equivalent to 20.12 mg of C4H4KNO4S. Fluoride Determine as directed in Method III under Fluoride Limit Test, Appendix IIIB, using a 4-g sample. 

Fluoride Determine as directed in Method V under Fluoride Limit Test, Appendix IIIB. 

Assay Dissolve about 1 g of sample, accurately weighed, in 50 mL of water, add 50.0 mL of 0.05 M disodium EDTA and 20 mL of pH 4.5 buffer solution (77.1 g of ammonium acetate and 57 mL of glacial acetic acid in 1000 mL of aqueous solution), and boil gently for 5 min. Cool, and add 50 mL of alcohol and 2 mL of dithizone TS. Back titrate with 0.05 M zinc sulfate to a bright rose-pink color. Perform a blank determination (see General Provisions), and make any necessary correction. The milliliters of 0.05 M disodium EDTA consumed is equivalent to 50 minus the milliliters of 0.05 M zinc sulfate used. Each milliliter of 0.05 M disodium EDTA is equivalent to 23.72 mg of A1K(S04)2T2H20. Ammonium Salts Add 1 g of sample to 10 mL of 1 N sodium hydroxide in a small beaker, and heat on a steam bath for 1 min. The odor of ammonia is not perceptible. Fluoride Determine as directed in Method V under Fluoride Limit Test, Appendix IIIB. 

Arsenic Determine as directed under Arsenic Limit Test, Appendix IIIB, using a solution of 1 g of sample in 35 mL of water. Fluoride Determine as directed in Method /Funder the Fluoride Limit Test, Appendix IIIB, using a 2-g sample, Buffer Solution B, and 0.1 mL of Fluoride Standard Solution. 

Fluoride Determine as directed under Fluoride Limit Test, Appendix NIB, using 1.0 g of sample, accurately weighed. Lead Determine as directed under Lead Limit Test, Appendix IIIB, using 10 mL of the following solution and 4 pig of lead (Pb) ion in the control Transfer 4.0 g of sample, accurately weighed, into a 250-mL beaker, cautiously add 50 mL of nitric acid, and evaporate just to dryness on a steam bath. Add 20 mL of nitric acid, repeat the evaporation, cool, and dissolve the residue in sufficient water containing 4 drops of nitric acid to make 40.0 mL. 

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