Elsevier

Talanta

Volume 118, 15 January 2014, Pages 162-171
Talanta

Review
Free acidity measurement – A review

https://doi.org/10.1016/j.talanta.2013.10.017Get rights and content

Highlights

  • A literature review of free acidity measurements in the presence of hydrolysable ions has been done.

  • The review has been classified with respect to the method used, complexant employed.

  • Importance has been attributed to measurements involving actinide solutions.

Abstract

Free acidity is an important parameter especially in the presence of hydrolysable ions. Several methods have been developed for the determination of free acidity, attributing due importance to the accuracy and the precision of the measurement with the aim of the easiness of the methodology as well as post-measurement recovery in mind. This review covers important methods for the determination of free acidity with emphasis on actinide containing solutions, reported in the literature over the past several decades classifying them into different categories.

Introduction

Free acidity of a solution containing hydrolysable metal ions is defined as the acidity in excess of the stoichiometrically balanced salts or the acidity without taking into account that contributed by the hydrolysis of such metal ions. It can also be simply defined as the acidity of the solution when the hydrolysable metal ions are removed or complexed so as not to interfere during the measurement. The term “free acidity” is used always to quantify the acidity of solutions containing hydrolysable ions such as UO22+, U4+, Pu4+, Th4+, Zr4+, trivalent lanthanides, actinides, transition metal ions such as Al3+, Fe3+, Cr3+, Co3+. It should be noted that anions such as phosphate, acetate, and oxalate and cations such as ammonium ion present in the solution also can interfere in free acidity measurement through buffering action during alkalimetric titrations.

Typical chemical engineering operations such as spent fuel reprocessing involving the separation and recovery of highly hydrolysable actinide ions Pu(IV) and U(VI) employing the PUREX process [1], [2] demand free acidity measurements during dissolver solution conditioning, process control analyses, partitioning of Pu and U, stripping steps and also during the final reconversion steps. Waste management operations [3], which follow the spent fuel reprocessing operations, involve neutralization, precipitation, solvent extraction, ion-exchange, etc., wherein free acidity measurements are essential. Studies on polymerization, hydrolysis and complexation involving hydrolysable ions also necessitate free acidity measurement. The measurement is carried out using alkalimetric titration after the removal of the hydrolysable ion by a suitable method such as ion exchange, precipitation or solvent extraction or suppression by complexing the metal ions. The free acidity measurement becomes increasingly inaccurate when the ratio of the concentrations of the hydrolysable metal ion to the acid increases. The difficulty in the preparation of standard solutions with known free acidity makes the assessment of a particular method with respect to the accuracy or the choice of a suitable complexant difficult.

The literature on measurement of free acidity dates back to 1890 [4] or earlier, wherein measurement of free acid in the presence of aluminum salts have been discussed. The present review gives an overview of the literature reported methods for the determination of free acidity, describing the techniques used for the suppression of hydrolysis as well as for the measurement of the acidity. This review also describes methods developed to simultaneously measure the concentrations of hydrolysable metal ions and other additives such as hydrazine, in addition to the free acidity. Importance has been given to free acidity measurement in the presence of actinides.

The suppression methodologies adopted for overcoming the influence of the hydrolysable metal ions and subsequent free acidity measurement techniques developed are listed below. There are further choices in the use of titrants, which can be NaOH, Na2CO3 or alkali generated electrochemically. Also several non-aqueous titrations involving non-aqueous titrants such as cyclohexylamine and pyridine have been used. All the methods have attempted in addition to getting good accuracy and precision, simplification of the procedure, reduction of the corrosion problems of the waste generated, making the recovery of the hydrolysable ions such as actinides which are both valuable and radiotoxic, easier and ensuring long life for the detection equipment such as pH measuring electrodes.

Usually the metal salts are purified by recrystallization from saturated solutions in high purity water under near neutral pH conditions and do not contain any associated acid. For highly hydrolysable metal ions such as Pu(IV), U(IV), U(VI) and Th(IV) getting near neutral salts without accompanying acid is very difficult which in turn makes getting “free acidity standards” through this route nearly impossible. This monograph also describes some of the methods used for the preparation of “free acidity standards” though their reportings are scarce in the literature.

  • Suppression techniques

    • Precipitation

      • iodate, ferrocyanide, peroxide

    • Complexation

      • Sulfate, oxalate, fluoride, oxalate–fluoride, thiocyanate, citrate, tartrate, EDTA, DTPA, TTHA

    • Addition of a neutral salt and titration in non-aqueous media

    • Removal by cation-exchange

    • Removal by solvent extraction

  • Measurement techniques

    • Measurement of pH

    • Alkalimetric titration with pH measurement

    • Alkalimetric titration with visual indicators

    • Alkalimetric titration using constant current coulometry

    • Alkalimetric titration using conductometry

    • Alkalimetric high frequency titrations

    • Alkalimetric titration using thermometry

    • Gran titration with alkali

    • Potentiometry with standard addition of acid

    • Non-aqueous titrations

Section snippets

Hydrolysis suppression by precipitation

Hexacyanoferrate (Fe(CN)64−) has been employed for the precipitation of uranium [5], [6] for the free acidity determination. The method for uranium however required addition of methanol for preventing the uranyl hexacyanoferrate complex from dissociation and the dark color precipitate prevented subsequent visual indicator based titrations. The recovery of uranium from the waste was also found to be difficult. Ferrocyanide precipitation method is not applicable for Al(III) containing solutions.

Measurement techniques

The following sections describe the measurement techniques employed for the determination of free acidity with or without using suppression techniques.

Conclusions

It can be seen that from the above review that the methods which employ complexants and alkalimetric titration either using manual titration with visual indication of end point or pH meters appear to be most versatile and attractive with respect to simplicity in execution, acceptable bias and precision and also for the recovery of the metal ions after the free acidity measurement. It can be noticed that mostly researchers have used inflection point as the end point which may increase the

Acknowledgment

T.G. Srinivasan would like to acknowledge with thanks, the Department of Atomic Energy, India for the award of Raja Ramanna Fellowship.

References (78)

  • P. Pakalns

    Anal. Chim. Acta

    (1981)
  • J.L. Pflug et al.

    Anal. Chim. Acta

    (1960)
  • Z.I. Dizdar et al.

    Anal. Chim. Acta

    (1959)
  • S. Ganesh et al.

    Talanta

    (2011)
  • P. Pillay et al.

    Anal. Chim. Acta

    (2001)
  • H Schmieder et al.

    Talanta

    (1969)
  • V.K. Bhargava et al.

    Anal. Chim. Acta

    (1971)
  • D.D Sood et al.

    J. Radioanal. Nucl. Chem. Ar

    (1996)
  • A.P Paiva et al.

    J. Radioanal. Nucl. Chem.

    (2004)
  • Radioactive Waste Management – Status and Trends, Issue No.4, IAEA/WMDB/ST/4, February...
  • F Beilstein et al.

    Z. Anal. Chem.

    (1890)
  • C.J. Rodden

    Analytical Chemistry of the Manhattan Project

    (1950)
  • Uma Sundar et al.

    Analyst

    (1998)
  • J.W. Dahlby, G.R. Waterbury, C.F. Metz, USAEC Report LA-3876,...
  • M.E. Smith, Report LA-1864,...
  • W. Baehr, D. Thiele, Report KFK-499,...
  • W. Baehr, D. Thiele, Report KFK-1133,...
  • H.Vogg, Report KfK-622,...
  • S. Arhland

    Acta Chem. Scand.

    (1960)
  • J. Chwastowska et al.

    Nukleonika

    (1970)
  • K. Motojima et al.

    Anal. Chem.

    (1964)
  • M. Fatu

    Revta Chim.

    (1974)
  • M. Tomasevic

    Hem. Ind.

    (1973)
  • T. Nakashima et al.

    Radiochim. Acta

    (1986)
  • R.A. Schneider, M.J. Rasmussen, Report HW-53368,...
  • G.L. Booman et al.

    Anal. Chem.

    (1958)
  • P.C. Mayankutty et al.

    J. Radioanal. Chem.

    (1982)
  • D.V. Bhatnagar

    J. Sci. Ind. Res.

    (1957)
  • K. Benadict Rakesh, A. Suresh, P.R. Vasudeva Rao, in: Proceedings of Nuclear and Radiochemistry Symposium (NUCAR 2013),...
  • K. Benadict Rakesh, A. Suresh, P.R. Vasudeva Rao, unpublished...
  • M Solomons

    Natl. Inst. Metall (South Africa)

    (1980)
  • O. Menis, D.L. Manning, G. Goldstein, Report ORNL-2178,...
  • D. Scargill, M.J. Waterman, A.S. Kurucz, T.S. Hilton, Report AERE-M 3323,...
  • D. Crossley, Report AERE-R 9848,...
  • E.R Schmid et al.

    Fresenius' Z. Anal. Chem.

    (1971)
  • J.A Bishop et al.

    Chemist-Analyst

    (1954)
  • S.C. Soundar Rajan

    Talanta

    (1987)
  • E.W. Baumann, Report DP-1632,...
  • E.W. Baumann et al.

    Anal. Chem.

    (1984)
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