Influence of acidity in the precipitation process on Mohr’s method

Abstract

The main goal of this study was to demonstrate the importance of acidity on precipitation process ofAg₂CrO₄ and Ag₂O, which can be formed in Mohr’s method. This can be explained through the interpretationof a Predominance-Existence Diagram, which shows reliable results of a didactic experiment.The first part of this study consists of identifying two kinds of solids, Ag₂CrO₄, which is reddish,and Ag₂O brownish. The second part shows that acidity affects in speciation of Cr (VI) bychanges in the color of the aqueous solution. Finally, it has been demonstrated that pH affects onabsence or formation of certain solids. These changes can be explained by a predominance-existencediagram, which is useful to identify a logarithmic curve related with conditional solubility of Ag₂CrO₄or Ag₂O, the conditional solubility constant, and also concentration of silver at saturation. The findingof minimum solubility — without Ag₂O can be formed- justifies that optimal interval pH to titrateon Mohr’s method is 7.0 <pH< 10.