Section 2: Background

The determination of phosphates in a detergent as described in ASTM D820-72, involves the conversion of the phosphates to orthophosphate by boiling with hydrochloric acid. The orthophosphate is then determined by a "weak-acid" titration between the pH's of 4.3 and 8.8. However, it was found that the presence of an added electrolyte (e.g. NaCl) changes the shape of the pH-titration curve obtained when a solution of orthophosphate is titrated with sodium hydroxide solution. This is shown in Figure 1, where the pH-titration curves of KH₂PO₄ solution titrated with NaOH solution alone and in the presence of NaCl at three concentrations have been plotted. Similar results were seen with Na₂SO₄. It is apparent that, except in the absence of added electrolyte, the points of inflexion of the titration curves do not correspond to the two pH's given in the ASTM method and marked in Figure 1. Shown in Figures 2 - 5 are the titration curves for 4 detergents. Figure 6 shows a phosphate standard solution, analysed by the modified procedure described below. From Figures 1 - 6 it is evident that the only satisfactory way to determine the end-points in the titrations is to plot a titration curve for each determination. The points of inflexion may best be determined by plotting A pH against volume of titrant added (Figures 7 & 8).

The determination of orthophosphate is affected by other weak acids such as silicates, borates, carbonates etc., and these materials, which enter with the detergent, are removed in the procedure. However, the hydrochloric acid used to hydrolyze the phosphates must be neutralized with concentrated sodium hydroxide. Absolutely carbonate-free sodium hydroxide is difficult to prepare (the procedure being to decant the clear 50% sodium hydroxide from settled carbonate (D820-72)) and requires special precaution to prevent subsequent absorption of CO₂ from the atmosphere. The carbonate which may enter with sodium hydroxide is determined as phosphate in the ASTM procedure and, at low levels of phosphate, can significantly distort the results obtained. The procedure below overcomes this possible interference and essentially calibrates the phosphate determination and all the reagents.