Principles of End Point Detection in Chelometric Titrations Using Metallochromic Indicators. Characterization of End Point Sharpness

Abstract

b The optimal use of metallochromic indicators in chelometric titrations requires a thorough understanding of the factors involved. Unlike acid-base titrations, the interplay of a large number of equilibria must b e considered. From pM-pH or pY-pH diagrams the sharpness of an end point may be quantitatively characterized by only two end point indices and the end point sharpness under different conditions may be readily compared. Such diagrams are useful in predicting the possibility of selective titrations and the feasibility of activating an indicator response for the titration of one metal ion by the addition of a second. The principles outlined are helpful in the search for new chelometric methods and indicators. HELoJiETRIc titrations occupy an C increasing role in the analysis of metal-ion mixtures. To n iden the scope of these applications, new indicators and titrants are being studied and proposed.In the evaluation of metallochromic indicators, two important but different characteristics of the end point must be distinguished and considered. First, a large degree of contrast must exist between the colors of the indicator before and after the end point. Second, this indicator color change must be sharp-i.e., produced by a minimum increment of titrant. In the literature, these two characteristics have often been confused.No quantitative scale has been set up for describing thme two characteristics of end point quality. This article proposes a numerical characterization of the second characteristic, end point sharpness. The relationship betneen such sharpness and the significant parameters involved in a titration are derived to create a method for selecting titration conditions from theoretical considerations. A quantitative characterization of the color contrast of an end point and the theoretical aspects of screening dyes will be the subject of a separate communication PH Figure 1. Titration of magnesium using Eriochrome Black T indicator leff. pM-pH diagram Righf. Corresponding pM-titration curvesThe sharpness of the end point in any titration with an indicator may be illustrated by a plot of the color change of the indicator us. the equivalents of titrant. In acid-base titrations, the shape of such a curve depends on only a few equilibrium constants and characterization of the end point sharpness is relatively simple. In chelometric titrations, the number of pertinent equilibria is considerably larger. The end point sharpness depends on the stability constants of the metal chelonates and the metal-indicator complexes, on the concentrations of metal ion and indicator, and on pH, buffer, masking agents, and other cations present in solution. Color change curves have been calculated for a number of highly specified systems (3,4, 16). Although the methods of calculation are of general applicability, separate calculations are required for each set of conditions. This procedure is tedious and the interplay of the various factors and equilibria is difficult to perceive.In this paper, phl-pH and pY-pH diagrams pr...

Patch electrode glass composition affects ion channel currents

“…Although the K1/2 for extracellular Mg2+ and Ca2+ block in the cGMP-activated channel is unknown, a reasonable estimate is 10-100 uM (Zimmerman and Baylor, 1987). Under this assumption, the end point of free metal titration determined by noting the rapid changes in membrane current with small increases of chelator would overestimate free metal concentration, at most, by 1.5-3 x only if the apparent stability constant of the metal-chelator were of the same order of magnitude as the metal-channel affinity constant (Reilley and Schmid, 1959). Of all the possible blocking species in Table I, only Mg2+ and Ba 2+ have sufficiently small apparent stability constants for this to occur (Martell and Smith, 1974).…”

Chelon approach to analysis: I. Survey of theory and application

“…In the former as many as 13 competitive equilibria can be considered, whereas in most acid-base titrations only 3 or 4 competitive equilibria are usually involved. A full description of these competitive interactions has been given (18).…”

“…pM-pH and pY-pH diagrams allow presentation in an easily digestible form of the competitive equilibria involved in chelometric titrations and are consequently quite useful in predicting suitable titration conditions. From such diagrams the sharpness of the end point can be quantitatively characterized and the effect of pH, buffer, type of chelon, and indicator can be readily seen (18). The pM-pH diagram is the ideal choice of presentation when comparing the applicability of various chelons and indicators for the titration of a single metal ion.…”

Alizarin Acid Black SN as a Metallochromic Indicator for Calcium. Nature and Stability of Its Calcium Chelates.

“…It is given by pCa = -2.33 -log ocz + pH + y2 log [ x 10" -S.gx S-] (10> Above the line (pCa effect) and to the left (pH effect) free AABSN is present, while below and to the right the indicator is converted to Ca2Z2. The sharp decrease in pCa below pH 11 and the asymptotic limit at pH 10.5 are caused by the decreased concentration of HZ, as was the case in AA. Below pH 10.8, both AA and BB are dashed, as Ca2Z and H2Z are the major species present.…”

“…A case in point is the present study. The species Ca2Z2 is stable only above pH 11 in the presence of calcium concentrations lower than 10 ~4M. A decrease in pH or increase in the calcium concentrations leads to the formation of Ca2Z which, in turn, dissociates stepwise to give the free indicator, the intermediate species being spectrally similar to Ca2Z.…”