The estimation of acid dissociation constants in seawater media from potentionmetric titrations with strong base. I. The ionic product of water — Kw

The various assumptions implicit in the calculation of acid dissociation constants (based on ionic medium standard states) from potentiometric titrations using a cell with liquid junction (i.e. a pH measuring cell) have been examined. It was concluded that results can be obtained having an accuracy commensurate with the experimental precision. It has been shown that although the precise composition of the medium is a function of the hydrogen ion concentration (because of the protolytic nature of some of the ions in the media, e.g., sulphate and fluoride), the effect of such variations in the medium composition can be compensated for when defining the activity of hydrogen ion on an ionic medium standard state by defining the concentration of hydrogen ion as: [H]SWS=h(1 + βHSO4ST + βHFET) where βHSO4 and βHF are the relevant association constants and ST and FT are the total concentrations of sulphate and fluoride, respectively. This approach was used to obtain values for the ionic product of water (KW) in artificial seawater media at various temperatures and ionic strengths. These were fitted to give the equation (molal concentration units): pKw= 3441.0T+2.256-0.709112 (rms deviation 0.01) where I is the formal ionic strength of the artificial seawater medium and T is the absolute temperature. The values obtained are in reasonable agreement with those found by previous workers.

[1]  F. Ingman,et al.  Acid-base titrations by stepwise additions of equal volumes of titrant with special reference to automatic titrations-I Theory, discussion of the gran functions, the hofstee method and two proposed methods for calculating equivalence volumes. , 1976, Talanta.

[2]  C. Culberson,et al.  Ionization of water in seawater , 1973 .

[3]  L. G. Sillén.  Master Variables and Activity Scales , 1967 .

[4]  D. Jagner,et al.  Evaluation of the accuracy of gran plots by means of computer calculations , 1973 .

[5]  G. Schwarzenbach.  The Determination of Stability Constants , 1962 .

[6]  T. Vries,et al.  The thermodynamics of aqueous hydrofluoric acid solutions. , 1947, Journal of the American Chemical Society.

[7]  K. H. Khoo,et al.  Determination of hydrogen ion concentrations in seawater from 5 to 40.degree.C: standard potentials at salinities from 20 to 45% , 1977 .

[8]  D. Dyrssen,et al.  Dissociation of hydrogen sulphide in seawater and comparison of pH scales , 1976 .

[9]  Maxwell Len McGlashan,et al.  Manual of symbols and terminology for physicochemical quantities and units , 1970 .

[10]  F. Ingman,et al.  Acid-base titrations by stepwise addition of equal volumes of titrant with special reference to automatic titrations-II Theory of titration of mixtures of acids, polyprotic acids, acids in mixture with weak bases, and ampholytes. , 1976, Talanta: The International Journal of Pure and Applied Analytical Chemistry.

[11]  K. Leander,et al.  The Determination of Dissociation Constants of Carbonic Acid in Synthetic Sea Water in the Salinity Range of 20--40 ‰ and Temperature Range of 5--30 degrees C. , 1973 .

[12]  A. M. Feltham,et al.  Platinized platinum electrodes , 1971 .

[13]  D. Dyrssen,et al.  Determination of pH on the moles per kg seawater scale (Mw) , 1975 .

[14]  H. Rossotti.  Design and publication of work on stability constants. , 1974, Talanta.

[15]  K. Leander,et al.  Determination of the Acidity Constant of Boric Acid in Synthetic Sea Water Media. , 1973 .