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. 2011 Jun 1;45(11):4862-8.
doi: 10.1021/es200665d. Epub 2011 May 12.

Purification and characterization of meta-cresol purple for spectrophotometric seawater pH measurements

Xuewu Liu  1 , Mark C Patsavas, Robert H Byrne
Affiliations
Free PMC article

Purification and characterization of meta-cresol purple for spectrophotometric seawater pH measurements

Xuewu Liu et al. Environ Sci Technol. .
Free PMC article

Abstract

Spectrophotometric procedures allow rapid and precise measurements of the pH of natural waters. However, impurities in the acid-base indicators used in these analyses can significantly affect measurement accuracy. This work describes HPLC procedures for purifying one such indicator, meta-cresol purple (mCP), and reports mCP physical-chemical characteristics (thermodynamic equilibrium constants and visible-light absorbances) over a range of temperature (T) and salinity (S). Using pure mCP, seawater pH on the total hydrogen ion concentration scale (pHT) can be expressed in terms of measured mCP absorbance ratios (R = λ2A/(λ1)A) as follows: [formula in text] where -log(K(2)Te2) = a + (b/T) + c ln T – dT; a = -246.64209 + 0.315971S + 2.8855 × 10(-4)S2; b = 7229.23864 – 7.098137S – 0.057034S2; c = 44.493382 – 0.052711S; d = 0.0781344; and mCP molar absorbance ratios (ei) are expressed as e1 = -0.007762 + 4.5174 × 10(-5)T and e3/e2 = -0.020813 + 2.60262 × 10(-4)T + 1.0436 × 10(-4) (S – 35). The mCP absorbances, λ1A and λ2A, used to calculate R are measured at wavelengths (λ) of 434 and 578 nm. This characterization is appropriate for 278.15 ≤ T ≤ 308.15 and 20 ≤ S ≤ 40.

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Figures

Figure 1
Figure 1
HPLC chromatographs of mCP. (a) mCP direct from vendor (Aldrich). Absorbances versus elution time were obtained using a broadband light source. The small inset panels show the UV–vis spectra (200 to 600 nm) of indicator components eluted at different times. (b) Purified mCP.
Figure 2
Figure 2
Comparison of pH values obtained using mCP from seven different vendors in strongly buffered solutions: (a) unpurified mCP, and (b) purified mCP. All pH differences are expressed relative to the pH measured with the reference mCP (purified Acros).
Figure 3
Figure 3
Residuals for pH (expected pH (eq 6) minus measured pH (eq 18)) determined in synthetic tris seawater buffer.
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References

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