Abstract:

The determination of trivalent chromium content was based on the titration method of “HG/T 4311—2012 Industrial Chromium Chloride”.By using a multi-step oxidation method，Cr³⁺ in the iron chromium mixture was completely oxidized to Cr⁶⁺，achieving accurate and stable testing of Cr³⁺ content within a large concentration range，and solving the interference of iron ions.At the same time，the main conditions that affected the accuracy of the measurement results，including the amount of distilled water，the amount of sodium hydroxide solution and the amount of hydrochloric acid，were experimentally verified and compared，and the experimental conditions were optimized.The main measurement steps of this method were as follows：firstly，in an acidic environment，Fe²⁺ was oxidized to Fe³⁺ with an excess sodium chlorate solution，then a sodium hydroxide solution was added，and in alkaline environment，Cr³⁺ was preliminarily oxidized to Cr⁶⁺ with hydrogen peroxide.At this point，as some of the Cr³⁺ encapsulated by the iron complex was not oxidized，a hydrochloric acid solution was added to adjust to the appropriate pH value to completely release the Cr³⁺ encapsulated by the complex.Then，hydrogen peroxide was used to completely oxidize it to Cr⁶⁺.Then，sulfuric acid solution and phosphoric acid were added to mask the iron ions，and titrated with ammonium ferrous sulfate standard solution.The results showed that the relative standard deviation of Cr³⁺ determined by this method was 0.12%~0.51%，and the recovery rate of the added standard was 95.0%~99.2%.The determination results were basically consistent with those determined by spectrophotometry，meeting the requirements for the determination of trivalent chromium content in the electrolyte of iron chromium flow batteries.

Key words: Fe-Cr mixture, trivalent chromium, RSD