The purpose of this paper is to investigate about the discoloration of chrome vermilion in sulfuric acid solution, and also about its prevention method by surface treatment with reagents such as sodium lignin sulfonate, soluble starch, saponin, gum arabic and zein.
The measurement of spectral reflectance, the X-ray analysis, the solubility of chrome vermilion composition in sulfuric acid solution and also the adsorption of zein on chrome vermilion in DMF (N, N-dimethylformamide) solution were carried out.
The results obtained were as follow;
Chrome vermilion is decomposed with sulfuric acid, and the color of chrome vermilion changed from reddish orange to yellow with decreasing pH.
Chrome vermilion treated with certain surface treating agents showed no color change with increasing acidity. And the minimum concentrations of the surface treating agents to prevent discoloration were found as follow :
0.08% sodium lignin sulfonate, 0.40% soluble starch, 0.16% saponin, 0.40% gum arabic in aqueous solutions and 0.56% zein in DMF solution.
The solubility of the sufficiently surface treated chrome vermilion in sulfuric acid solution became remarkably small.
The adsorption of zein on chrome vermilion (at 25°C) followed Langmuir's isotherm.
From the results of adsorption isotherm of zein on chrome vermilion and the minimum concentration of zein to prevent discoloration, it is concluded that the prevention of discoloration occurs nearly at the amount of saturated monolayer adsorption
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