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6402138 
Journal Article 
Spectrophotometric measurement of freshwater pH with purified meta-cresol purple and phenol red 
Lai, CZe; Degrandpre, MD; Wasser, BD; Brandon, TA; Clucas, DS; Jaqueth, EJ; Benson, ZD; Beatty, CM; Spaulding, RS 
2016 
1 
Limnology and Oceanography: Methods
ISSN: 1541-5856 
14 
12 
864-873 
WOS:000393047700008 
Impurities in indicator salts can significantly bias spectrophotometric pH determinations. In this work, two purified sulfonephthalein indicators, meta-cresol purple (mCP) and phenol red (PR), were tested for analysis of freshwater pH on the free hydrogen ion concentration scale. These two purified indicators were characterized for the first time under low ionic strength conditions, providing their molar absorption coefficients and dissociation constants along with their temperature dependence from 8 degree C to 30 degree C. At 25 degree C, the infinite dilution constants were determined to be 8.6606 and 8.0642 for mCP and PR, respectively. The accuracy and precision of the method, evaluated with a variety of buffers with known pH, were found to be +0.0014 pH units and plus or minus 0.0022 pH units, respectively (n=30). The pH values of different freshwater samples were also determined using both indicators. The mCP and PR results were all within plus or minus 0.01 pH units of each other with three out of seven pH differences within plus or minus 0.001 pH units, indicating the high consistency between these two indicator methods. The work presented here is the first parallel comparison with two purified indicators used to determine pH of the same freshwater samples. 
; Temperature effects; Dissociation; Absorption coefficient; Limnology; Oceanography; Inland water environment; Phenols; pH; Methodology; Ion concentration; Absorption coefficients; Hydrogen; Salts; Strength; Temperature; Absorption; Indicators; Wastewater Disposal; Hydrogen Ion Concentration; Marine/