Manipulations, pM'-pH diagrams and accuracy in determination of metals

Manipulations done with metal ion solutions can influence the accuracy of a determination. During a manipulation, when a non-homogeneous solution in a non-equilibrium state is dealt with, polynuclear hydroxide compounds and/or a hydroxide precipitate may be formed, which generally do not immediately disappear after homogenization of the solution. Deviations from the equilibrium state that is expected after homogenization, may not be negligible during the subsequent analysis. A part of the metal content escapes determination and systematic errors arise. The magnitude of the error depends upon the nature of the particular metal to be determined and the manipulations performed. The influence of the manipulations can be understood and foreseen by studying the behaviour of the metal ions in solution by means of a pM'-pH diagram. In this diagram regions can be indicated where the experimental conditions correspond to the formation of a hydroxide precipitate and polynuclear hydroxide complexes. The momentary state of the solution during a manipulation can be represented by a point. This point changes its position as the manipulation proceeds, describing a locus through the diagram. The locus depends upon the initial conditions and the particular manipulation performed. If the locus penetrates one of the "error" regions, the corresponding compounds will be formed and the error of the determination will increase. Conversely, if the locus is adapted so as to avoid penetration of the regions for polycomplexation and precipitation, the error is found to decrease remarkably.