Keihei Ueno Kyushu University Fukuoka, Japan
Guide for Selecting Conditions of EDTA Titrations
Titrations with EDTA have been widely used to determine the concentration of metal ions in solution. However, those inexperienced in the use of EDTA often find it difficult to select the proper pH and metal indicator. The accompanying chart is a convenient guide for selecting proper conditions for EDTA titrations of metal ions. The diagram indicates the pH range where EDTA reacts quantitatively with the metal ion, the pH range where a specific metal indicator may be used, and the pH range where an auxiliary complexing agent is necessary. The apparent stability constant of a metal-EDTA chelate determines the p H range where EDTA reacts quantitatively with the metal ion. The lower pH limit is governed by the extent of the competitive reactions of EDTA with protons and of the metal ion with any auxiliiry complexing agent. Consider the reaction:
with the use of the particular metal indicator is shown by the shadow on the principal line. The metal indicators are a kind of chelating agent; therefore the end point color change is governed by the solution equilibria involving the metal ion, EDTA, protons and the auxiliary complexing agent. The clear end point of the metal indicator for a given met,al ion can he ohnerved only in a
MfASMA
where the stability constant, KMA= [MA]/[M] [A]. The "apparent stability constant," K*MA, may be defined:
The parameter orH = Z[HiA]/[A], is a function only of the pH and the stability constants of the proton complexes. Similarly [A'] represents the concentration of the free ligand plus the concentration of all species of the complexing agent not bound to the metal. Then PA = [A']/[AI1. Therefore:
It has been proved that K*,A should not be less than 8 if the stoichiometric reaction of EDTA with the metal ions is to come within an experimental error.% Therefore, the lower pH limit can be estimated from equation (1) \-3.
The upper pH is limited by the acidic tendency of the metal ion and the hydrolytic tendency of the metalEDTA chelate as well as the ligand PA value. This relation must he determined experimentally. The pH range where EDTA reacts with the metal ion quantitatively is indicated on the chart by the principal line for each metal ion. The pH range where the end point can be determined Contribution Number 42 from the Department of Organic Synthesis, Faculty of Engineering, Kyushu University. G., "Die Komplexometrische Titration," SCHWARZENBACH, Ferdinand Enke, Stnttgart, 1955, p. 14. Z M ~ ~A. eE., ~AND~ CHABEREK, , S., A1zal. Chem. 26, 1692 (1954).
432
/ Journal of Chemical Education
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pH where EDTA reacts with met01 ion pH region where metal indicatort con b e used to determine the end point of EDTA titrotion uuuuul. p H region where ouxiliory complexing agent. should b e used to prevent hydroxide precipitation
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Bindrchedleis green, leuco bare B~omopyrogoilolred BT Eriochrome block T GCR Glycinecred red GTB Gly
