Mamoru Saito

Studies on the Preparation of Chrome Yellow. III.

Lead chromates were prepared by simultaneously adding 125 ml of 0.25 molar Na2Cr207 solution and 125 ml of 0.5 molar Pb (NO3)2 solution to 500 ml of water kept at 25-80°C. The Na20/Cr03 and N205/Pb0 ratios in the solutions were controlled by the addition of NaOH and HNO3, respectively. The pH of the resulting solution was 3. 2 when the alkali and acid mole ratios were 1.0 . With the increase in pH the red hue of the precipitates dried at 110°C decreased in the range pH 1-3 and increased in the range pH becoming very stronger at pH 8. The green hue and brightness increased between pH 1 and 2, becoming unchanged between pH 3 and 6 and then decreasing above pH 7. The effects of reaction temperature on colour depended on the pH at which the precipitates were prepared. Solubilities of the precipitates in O. 125 molar NaOH, 0.204 molar H2SO4 and 1. 0 molar AcOH were determined. The amount of Cr03 dissolved in the NaOH solution from the precipitates produced at pH 1-5 was not affected by the pH, the amount however decreased with the increase in pH in the range pH 5-7. The amount of Cr03 dissolved in the H2SO4 solution increased constantly with the increase in pH. The amount of Pb0 dissolved in the AcOH solution from the precipitates produced at pH 1-5 was negligible, while a gradual increase in the amount was observed in the range pH 5-7. The precipitates produced at pH 8 were very soluble in the H2SO4 and AcOH solutions, and very insoluble in the NaOH solution. The PbO/Cr03 mole ratios of the precipitates produced at pH 1-5, pH 5. 7-7. 4 and pH 8. 3 were 0. 98-1. 0, 1. 01-1. 03 and 1. 28, respectively. The X-rays diffraction patterns indicated the absence of basic lead chromate in the precipitates produced at pH 1-4, a trace of it was first detected at pH 5. 7, and in the precipitates prepared at higher pH values lead chromate became predominant. From the change in the X-rays diffraction patterns by the H2SO4 and NaOH treatments of the precipitates, it is suggested that the behaviours against acid and alkali and colours of the precipitates produced at various pH values are caused mainly by the presence of basic or acidic lead chromates in the precipitates produced at pH below 3. 2, respectively.