Spectroscopic measurements of the pH in NaCl brines

Abstract

Spectrophotometric measurements of the pH in natural waters such as seawater have been shown to yield precise results. In this paper, the sulfonephthalein indicator m-cresol purple ( mCP, H 2I) has been used to determine the pH of NaCl brines. The indicator has been calibrated in NaCl solutions from 5 to 45 °C and ionic strengths from 0.03 to 5.5 m. The calibrations were made using TRIS buffers (0.03 m, TRIS/TRIS–HCl) with known dissociation constants pK TRIS in NaCl solutions [Foti C., Rigano C. and Sammartano S. (1999) Analysis of thermodynamic data for complex formation: protonation of THAM and fluoride ion at different temperatures and ionic strength. Ann. Chim. 89, 1–12]. The values of pH were determined from pH = pK m CP + log { ( R - e 1 ) / ( e 2 - Re 3 ) } where R = 578 A/ 434 A, the ratios of the indicator absorbance maximum at 578 and 434 nm, e 1 = 0.00691, e 2 = 2.222 and e 3 = 0.1331 [Clayton T. and Byrne R. H. (1993) Spectrophotometric seawater pH measurements: total hydrogen ion concentration scale calibration of m-cresol purple and at-sea results. Deep-Sea Res. 40, 2115–2129]. Measurements were also made in NaCl solutions with different levels of TRIS (0.01–0.11 m). At low levels of TRIS buffer (<0.03 m), the values of pK m CP increased significantly. This effect can lead to erroneous values of pK m CP at low ionic strengths in estuaries and lakes. The measured values of pK m CP in NaCl as a function of ionic strength ( I/m) and temperature ( T/K) were fitted to the equation ( σ = 0.0072) pK m CP = - 29.095 + 2639.2 / T + 5.0417 ln T - 0.3307 I 0.5 - 186.80 I 0.5 / T - 0.28346 I + 296.44 I / T + 0.12841 I 1.5 - 68.23 I 1.5 / T These results should be useful in determining the pH of NaCl brines in natural waters from 0 to 50 °C.