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Fresenius' Zeitschrift für analytische Chemie

, Volume 238, Issue 5, pp 321–414 | Cite as

Die voltametrische Indikation komplexometrischer Titrationen

  • Günther Kraft
Article

Summary

A survey of the principles of complexometric titration is undertaken, paying particular regard to the methods of indication employed. The fundamentals of voltametric indication are also discussed, with emphasis being placed on the interpretation given to the measured indication curves by various workers. It is then shown how voltametric indication may be successfully applied to complexometric titrations.

Complexometric titrations in acid solutions, can be exactly indicated with the aid of a Pt-double-electrode of 10 to 50 sq. mm. The electrode is polarized by means of a constant current of 1 to 10 μA, and its potential difference is measured throughout the course of the titration. If the electrode is electrolytically pretreated in a HNO3-Bi-solution for 10–20 min with a current of a few μA, Bi deposits on both the anode and the cathode, and the resulting activation of the electrode causes a markedly improved indication at the end point.

The untreated Pt-double-electrode is only in certain cases suitable for the indication of titrations in alkaline solutions. If, however, the electrode is electrolytically pretreated in an ammoniacal solution for approx. 15 min with a current of a few μA, Tl-oxide deposits on the anode, and the performance of the resulting indicator is extremely good.

Indication curves resulting from the titration of numerous elements in acid and ammoniacal solutions prove the high suitability of the above described electrodes for the purpose under review. The end point, which is indicated by a change in potential of several 100 mV, is found to be identical with the equivalence point of the reaction. This method of indication also lends itself unreservedly to indirect or substitution titrations, as well as to sequence titrations, when, for instance, one component is complexometrically titrated in an acid solution and the other in an ammoniacal solution. The electrodes described are not only suited for titrations with EDTA, but also with more recent complexing agents, of which DTPA, EGTA and DCTA have been examined. As the electrodes may be used for titrations with solutions having a concentration of as low as 0.0001 M, they enable the titrimetric determination of amounts of as little as about 1 μg in a volume of about 50 ml. This has been clearly shown by the titration of magnesium.

The reproducibility of the indications, and thus the accuracy of the titrations, has been found to be ± 0.01 ml when working with 0.1 M and 0.01 M titration solutions. In the case of 0.001 M solutions, the results are only slightly less accurate. A few μg of material may be titrated with an accuracy of at least ± 10%.

As a general rule, the voltametric indication curves of complexometric titrations are dependent only on the depolarising effect of the free complexing agent upon the anode. If, however, ions such as Cu, Tl or Pb, which may themselves give cathodic and/or anodic electrode processes, are present in the solution under investigation, their action interferes with the basic run of the indication curve. A number of different curve shapes thus occur, all of which can be evaluated very accurately. The metal complexes themselves, do not have any significant influence on the electrochemical process, in so far as this method is concerned.

This method of indication is not applicable, if the solution under investigation contains substances which depolarize the anode more than the free complexing agent, e.g. reducing agents.

The investigations are extended to cover polarized Au, Ag and Cu electrodes, with discussion of their differences in properties as compared with platinum. The study is rounded off by considering the slope of the indication curves as a function of various parameters, among others the amount of substance to be titrated, and also the question of the evaluation of the curves.

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Copyright information

© Springer-Verlag 1968

Authors and Affiliations

  • Günther Kraft
    • 1
  1. 1.Analytisches Laboratorium der Metallgesellschaft A.G.Frankfurt a. M.

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