A new method for determining chlorinity of water was developed in order to improve the old method by alleviating the environmental problems associated with the toxic chromate. The method utilizes a mediator, a weak acid that can form an insoluble salt with the titrant. The mediator triggers a sudden change in pH at an equivalence point in a titration. Thus, the equivalence point can be determined either potentiometrically (using a pH meter) or simply with an acid-base indicator. Three nontoxic mediators (phosphate, EDTA, and sulfite) were tested, and optimal conditions for the sharpest pH changes were sought. A combination of phosphate (a mediator) and phenolphthalein (an indicator) was found to be the most successful. The choices of the initial pH and the concentration of the mediator are critical in this approach. The optimum concentration of the mediator is ca. 1~2？mM, and the optimum value of the initial pH is ca. 9 for phosphate/phenolphthalein system. The method was applied to a sample of sea water, and the results are compared with those from the conventional Mohr-Knudsen method. The new method yielded chlorinity of a sample of sea water of (17.58 ± 0.22)？g/kg, which is about 2.5% higher than the value (17.12 ± 0.22) g/kg from the old method. 1. Introduction Chlorinity is one of the most fundamental quantities associated with water quality [1, 2], and it is directly related to the salinity of sea water [3] and often used to determine the salinity [4]. In recent years, physical methods of determining salinity [5, 6], such as coulometry, measurements of conductivity, refractive index, or density, have become more popular because of their speed. Nevertheless, chemical measurements of chlorinity still remains an important and independent method of characterizing water quality. Various chemical methods have been reported for chlorinity determination: the gravimetric method with AgCl precipitates [7–9], various volumetric precipitation titrations with silver (I) or mercury (II) ions [7–9], and a recent spectroscopic method utilizing the Raman scattering band of OH stretching of water [10]. Among these chemical methods, the volumetric titrations with precipitation with Ag+ have been most popular because it is less time consuming than the gravimetry. Several different ways of determining the equivalence point have been reported for the volumetric titrations with silver ion, such as (a) use of various visual indicators [11, 12], (b) thermometric titration measuring enthalpy changes [13], and (c) potentiometric titration [14]. The analytical methods
References
[1]
D. F. Martin, “Marine chemistry,” in Analytical Methods, vol. 1, p. 65, Marcell Dekker, New York, NY, USA, 1968.
[2]
A. H. Brownlow, Geochemistry, Prentice-Hall, Englewood Cliff, NJ, USA, 1979.
[3]
T. R. S. Wilson, “Salinity and major elements of sea water,” in Chemical Oceanography, J. P. Riley and G. Skirrow, Eds., chapter 6, Academic Press, New York, NY, USA, 2nd edition, 1975.
[4]
R. A. Cox, “The physical properties of sea water,” in Chemical Oceanography, J. P. Riley and G. Skirrow, Eds., vol. 1, chapter 3, Academic Press, London, UK, 1965.
[5]
K. Grasshoff, “Determination of salinity,” in Methods of Sea Water Analysis, K. Grasshoff, M. Ehrhardt, and K. Kremling, Eds., p. 31, Wiley-VCH, Weinheim, Germany, 1983.
[6]
F. Millero and M. L. Sohn, Chemical Oceanography, chapter 2, CRC Press, Boca Raton, Fla, USA, 1996.
[7]
I. M. Kolthoff and E. B. Sandell, Textbook of Quantitative Inorganic Analysis, MacMillan, New York, NY, USA, 1969.
[8]
D. C. Harris, Quantitative Chemical Analysis, W. H. Freeman & Co., New York, NY, USA, 1982.
[9]
R. A. Day and A. L. Underwood, Quantitative Analysis, Prentice Hall, New York, NY, USA, 5th edition, 1986.
[10]
J. Dubessy, T. Lhomme, M. C. Boiron, and F. Rull, “Determination of chlorinity in aqueous fluids using Raman spectroscopy of the stretching band of water at room temperature: application to fluid inclusions,” Applied Spectroscopy, vol. 56, no. 1, pp. 99–106, 2002.
[11]
H. A. Laitenen, Chemical Analysis, chapter 12-13, McGraw Hill, New York, NY, USA, 1960.
[12]
E. Pungor and E. Schulek, “Adsorption indicators,” in Indicators, E. Bishop, Ed., chapter 7, Pergamon Press, Oxford, UK, 1972.
[13]
F. J. Millero, S. R. Schrager, and L. D. Hansen, “Thermometric titration analysis of seawater for chlorinity, sulfate and alkalinity,” Limnology and Oceanography, vol. 19, pp. 711–715, 1974.
[14]
D. Jagner, “Potentiometric titrations,” in Marine Electrochemistry, M. Whitfield and D. Jagner, Eds., chapter 7, John Wiley & Sons, Chishester, UK, 1981.
[15]
L. S. Clesceri, A. E. Greenberg, and A. D. Eaton, Standard Methods for the Examination of Water and Wastewater, American Public Health Association, America Water Works Association, Water Environment Federation, Washington, DC, USA, 29th edition, 1998.
[16]
“Official and standardized method of analysis,” in Analytical Methods Committee, C. A. Watson, Ed., p. 556, Royal Society of Chemistry, Cambridge, UK, 1994.
[17]
Committee on Biological Effects of Atmospheric Pollutants, Chromium: Medical and Biological Effects of Environmental Pollutants, National Academy of Science, Washington, DC, USA, 1974.
[18]
R. A. Goyer, “Toxic effects of metals,” in Casarett and Doul’s Toxicology, C. D. Klaassen, M. O. Amdur, and J. Doull, Eds., pp. 582–635, Macmillian, New York, NY, USA, 3rd edition, 1986.
[19]
R. A. Anderson, “Essentiality of chromium in humans,” Science of the Total Environment, vol. 86, no. 1-2, pp. 75–81, 1989.
[20]
J. M. Eckert, J. J. Stewart, T. D. Waite, R. Szymczak, and K. L. Williams, “Reduction of chromium(VI) at sub-μg l-1 levels by fulvic acid,” Analytica Chimica Acta, vol. 236, no. 2, pp. 357–362, 1990.
[21]
F. C. Richard and A. C. M. Bourg, “Aqueous geochemistry of chromium: a review,” Water Research, vol. 25, no. 7, pp. 807–816, 1991.
[22]
B. Deng and A. T. Stone, “Surface-catalyzed chromium(VI) reduction: reactivity comparisons of different organic reductants and different oxide surfaces,” Environmental Science and Technology, vol. 30, no. 8, pp. 2484–2494, 1996.
[23]
C.-J. Lin, “The chemical transformations of chromium in natural waters—a model study,” Water, Air, and Soil Pollution, vol. 139, no. 1–4, pp. 137–158, 2002.
[24]
M. Pettine, L. D'Ottone, L. Campanella, F. J. Millero, and R. Passino, “The reduction of chromium (VI) by iron (II) in aqueous solutions,” Geochimica et Cosmochimica Acta, vol. 62, no. 9, pp. 1509–1519, 1998.
[25]
M. Pettine, D. Tonnina, and F. J. Millero, “Chromium(VI) reduction by sulphur(IV) in aqueous solutions,” Marine Chemistry, vol. 99, no. 1–4, pp. 31–41, 2006.
[26]
D. Michaels, C. Monforton, and P. Lurie, “Selected science: an industry campaign to undermine an OSHA hexavalent chromium standard,” Environmental Health, vol. 5, article no. 5, p. 8, 2006.
[27]
M.-Z. Czae, “The adsorption of hydrogen and hydroxyl ions during precipitation tritration,” Journal of the Korean Chemical Society, vol. 14, no. 1, p. 137, 1970.
[28]
L. Legradi, “Az adszorpcios indikacio mechanizmusa,” Magyar Kemiai Folyoirat, vol. 76, no. 1, pp. 30–40, 1970 (Hungarian).
[29]
N. Hota, “Use of a pH meter for indicating the end point of the precipitation titration of sulfate with a mixed solution of barium nitrate and lead nitrate,” Bunseki Kagaku, vol. 19, p. 780, 1970.
[30]
N. Hota, “Use of a pH meter for indicating the end point of the precipitometry. II. Titration of halides, iodate, chromate, and secondary phosphate,” Bunseki Kagaku, vol. 20, p. 522, 1971.
[31]
N. Hota, “pH meter for indicating the end point of various titrations in mixed solvent. Precipitometry using a titrant of silver nitrate, lead nitrate, sodium sulfide and neutralization titration of ammonium compounds,” Bunseki Kagaku, vol. 21, p. 1463, 1972.
[32]
D. Dob？nik and D. Brodnjak-Von？ina, “Hydrolytic potentiometric titration of sulphate with application in the analysis of waters,” Analytica Chimica Acta, vol. 177, pp. 209–212, 1985.
[33]
D. Dob？nik and J. Gibicar, Vestnik Slovenskega Kemijskega Drustva, vol. 34, p. 305, 1987.
[34]
D. Brodnjak Voncina, D. Dobcnik, and G. Gomiscek, “Hydrolytic precipitation titrations followed by a glass electrode,” Analytica Chimica Acta, vol. 263, no. 1-2, pp. 147–153, 1992.
[35]
T.-K. Hong, B.-H. Koo, M.-H. Kim, and M.-Z. Czae, “Use of pH electrode for precipitation titration analysis: theory and practice,” Journal of Analytical Chemistry, vol. 64, p. 1158, 2009.
[36]
J. F. Coetzee, “Precipitation equilibria and titrations in aqueous and non-aqueous media,” in Treatise on Analytical Chemistry, I. M. Kolthoff and P. J. Elving, Eds., vol. 3, pp. 370–378, John Wiley & Sons, New York, NY, USA, 1983.
[37]
J. P. Riley, in Chemical Oceanography, J. P. Riley and G. Skirrow, Eds., chapter 21, Academic Press, New York, NY, USA, 1965.
[38]
R. A. Berner, “Activity coefficients of bicarbonate, carbonate and calcium ions in sea water,” Geochimica et Cosmochimica Acta, vol. 29, no. 8, pp. 947–965, 1965.
[39]
G. S. Sodhi, Fundamental Concepts of Environmental Chemistry, Alpha Science International, 2000.
