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A Modified Procedure for the Determination of Phosphorus in Detergents

Section 1: Introduction

Regulations made under the Canada Water Act limit the amount of phosphorus that can be present in laundry detergents made in or imported into Canada. From 1st August, 1970 to December 31, 1972, the regulation specified the maximum permissible concentration as 20% expressed as phosphorus pentoxide (P2O5). Since January lst, 1973 the maximum permissible concentration has been 5% P2O5. The regulations further decree that the method of analysis shall be that prescribed from time to time by the American Society for Testing and Materials.

In earlier work[1] using ASTM Method D820-58 (Reapproved 1965) it was found necessary to add a specific step for removal of borate interference, and with this modification the method was found to be satisfactory for analysis of detergents against the "20% P2O5 limit". The latest ASTM Method published (ASTM D820-72)[2] is more specific in its description of silicate-dehydration and contains a step for borate removal. However at low phosphate levels (5% P2O5) some problems have been experienced, both in the Environmental Protection Service and Water Quality laboratories, in obtaining reproducible results with the method. This seems to be recognized in the ASTM write-up, which states "This method does not apply when the level of phosphate present is equivalent to or less than 2 percent P2O5". From a study of the method, it was concluded that two problems exist in the ASTM procedure:

  1. In the presence of the electrolyte from the detergent and the digestion-neutralization procedure, titration between two fixed pH's does not give a valid measure of the normality of a weak-acid solution; and,
  2. There is potential interference from carbonate carried in with the sodium hydroxide.

This study concluded that the ASTM procedure as written is not a satisfactory method for the analysis of phosphorus in detergents at the 5% P2O5 level.

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