Determination of Soluble and Insoluble Iron in Kaolin by Phenanthroline Spectrophotometry

The whiteness and whitening effect of kaolin are influenced by the status and content of iron in kaolin. It is meaningful in practical application to determine the contents of different species of iron in kaolin rapidly and accurately. However, it is difficult to analyze the contents of free and structural iron in kaolin. In this article, iron in kaolin is divided into soluble and insoluble iron. The soluble iron in kaolin can be obtained by leaching with hydrochloric acid, while the total iron is achieved by calcining with sodium hydroxide. The content of insoluble iron is equal to the content of total iron minus soluble iron. The phenanthroline spectrophotometry method is used to determine the content of iron. The relative standard deviation of this method is less than 5%, and sample recoveries range from 95% to 105%. The detection limit of iron is 1.06 μg/g. Other elements in kaolin did not interfere with iron determination by this method. Soluble iron was almost free iron with little structural iron in kaolin. The maximum content of removal iron for the whitening technique can be estimated by quantitative analysis of soluble iron in kaolin.