Standard Methods for the Examination of Water and Wastewater. 16th Edition: 1985. Method 507, pg 525-532.
Standard Methods for the Examination of Water and Wastewater. 13th Edition: 1971. Method 218B, pg 477-481.
The laboratory procedure with which I am familiar also makes reference to the United States Environmental Protection Agency Methods for Chemical Analysis of Water and Wastes. March 1979: 1974 editorial revision. Method 405.1.
Additional information not available in either of these methods was performed or modified at the discretion of the laboratory based on necessary requirements for approval.