A new indicator for the direct negative colloid titration

The colloid titration is used as a determination method for the positive or negative charged colloid. For the negative one, the direct titration with Cat-Floc(poly-N, N-diallyldiinethyl-ammonium chloride) is impossible, because of the indistinct color change of TB(toluidine blue indicator) at the end-point. A new indicator, NBS[3-(2-hydroxy-3-carboxyanilid-1-azonaphthalene)-4-hydroxybenzene sulfonic acid], was synthesized by the coupling reaction of Naphthol AS with 3-diazo-4-hydroxybenzenesulfonic acid for the direct titration of negative colloid solution with Cat-Floc. NBS can be used at pH 6.5 to 11, and the color change at the end point is from violet to blue at pH about 7 and from bluish-violet to blue in alkaline range. NBS was successfully applied for the direct titration of negative colloids such as PVSK, sodium alginate and sodium polyacrylate. The results for PVSK are shown in Fig. 1. The value 5.71 meq/g(indicating 92.7% purity) by the direct method was fairly coincident with 5.68 meq/g(92.2% purity) by the indirect one. For sodium alginate, almost the same results were obtained between pH 7 and 10 by the both methods; 4.56 and 4.51 meq/g, correspoding to the purity of 90.3% and 89.3%, respectively. For sodium polyacrylate, the value by the direct method was 9.18 meq/g, indicating 86.4% purity, while a constant value was not obtained by the indirect one as shown in Fig. 2.