Regarding Cr (VI) measurement by an iron coprecipitation operation, we found an effluent sample with a low recovery rate. Note 11 of JIS K 0102: 2016 65.2 Cr (VI) has no detailed description of the operation, so we explored an optimal operating conditions using ultrapure water. In addition, since the dominant Cr valence depends on the pH and Eh of the solution, a recovery test was performed using ultrapure water whose pH and Eh were adjusted with sodium sulfite. As the amount of sodium sulfate was added, the pH was increased while Eh was decreased, and the recovery rate of Cr (VI) was decreased, it was confirmed that pH, Eh, and reducing substance are related to the recovery rate of Cr (VI). In the Eh-adjusted sample, it was confirmed that the recovery rate was improved by adding ammonia water before the addition of recovery standard solution. Based on the results of these studies, an effluent sample with a low recovery rate was subjected to aeration to adjust the Eh, and ammonia water was added first so that the pH did not become acidic during the iron coprecipitation operation. The recovery test confirmed that Eh and the recovery rate were increased with the longer aeration duration. Therefore, if the actual sample is reduced condition, an aeration treatment in addition to particle coagulation period, filtration temperature, and reagent addition sequence improves the recovery rate of Cr (VI) and makes it possible to evaluate appropriately.