The effect of oxalic acid on phosphate interference in the colorimetric analysis of Si.

Without oxalic acid, phosphorus (P) interference caused the reading to over-range (4 ppm silica equivalents). The addition of 0.6 M oxalic acid reduced the interference to 36%, and at 1.1 M oxalic acid interference was 4% (0.06 / 1.49). This test included a high P background. Lower P background levels would have less interference.