Effect of the Grain Refinement of Iron(III) Hydroxide Precipitation for Removal of Chromium(III) by Iron(III) Hydroxide Coprecipitation Method for Chromium(VI) Determination

This study demonstrates that grain refinement of iron(III) hydroxide under alkaline conditions affects the chromium(III) removal efficiency of the coprecipitation method for chromium(VI) determination. After the addition of an FeNH4(SO4)(2) center dot 12H2O solution to a simulated chromium solution, the pH was adjusted using a sodium hydroxide solution and chromium(III) was removed by filtration after heating and cooling the simulated solution. When the pH was in the range of 10.78-12.67 after heating and cooling, the chromium(III) removal efficiency decreased, causing a positive error for chromium(VI) determination. This may be because the iron (III) hydroxide precipitate undergoes grain refinement and passes through the filter because the iron concentration increased, and thereby the filtrates were colored. The addition of calcium chloride dihydrate facilitated the flocculation of the iron(III) hydroxide precipitate and increased the chromium(III) removal efficiency. The addition of calcium chloride did not affect the chromium(VI) recovery rate. When the pH was in the range of 8.95-13.07 after heating and cooling, the chromium(VI) recovery rate was over 90-% and the chromium(III) removal efficiency was over 98-%. Therefore, this method was confirmed to be suitable for chromium(VI) determination.