Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

Purified meta-Cresol Purple dye perturbation: how it influences spectrophotometric pH measurements



Xinyu Li, Maribel Garc?a-Ib??ez, Brendan Carter, Baoshen Chen, Qian Li, Regina Easley, Wei-Jun Cai


Ocean acidification, a phenomenon of seawater pH decreasing due to increasing atmospheric CO2, has a global effect on seawater chemistry, marine biology, and ecosystems. Ocean acidification is a gradual and global long-term process that demands high-quality pH data. The spectrophotometric technique generates accurate and precise pH measurements but requires adding an indicator dye that perturbs the sample pH. While the perturbation is modest in well-buffered seawater, applications of the method in less well-buffered environments such as riverine, estuarine, sea-ice meltwater and lacustrine environments are increasingly common, and uncertainties related to larger potential dye perturbations need further evaluation. In this paper, we assess the effect of purified meta-Cresol Purple (mCP) dye addition on the sample pH and how to correct for this dye perturbation. We conducted numerical simulations by adding mCP to the MATLAB CO2SYS program to examine the changes in water sample pH caused by the dye addition and to reveal the dye perturbation mechanisms. Then, laboratory experiments were carried out to verify the simulation results. The simulations suggest that the dye perturbation on sample pH is a result of total alkalinity (TA) contributions from the indicator dye and chemical equilibrium shifts that are related to both the water sample properties (pH, TA, and salinity) and the indicator dye solution properties (pH, solvent matrix, etc.). The laboratory experiments supported the simulation results; the same dye solution can lead to different dye perturbations in water samples with different pH, TA, and salinity values. The uncertainties caused by the empirical dye perturbation correction can be as large as 0.005 pH units. Adjustments are minimized when the pH and salinity of the dye are matched to the seawater, but we suggest that the dye solution should be prepared in CO2 free deionized water to minimize the dye perturbation effect on pH when it is used in samples w
Marine Chemistry


Spectrophotometric pH measurement, meta-Cresol Purple (mCP), Dye perturbation


Li, X. , Garc?a-Ib??ez, M. , Carter, B. , Chen, B. , Li, Q. , Easley, R. and Cai, W. (2020), Purified meta-Cresol Purple dye perturbation: how it influences spectrophotometric pH measurements, Marine Chemistry, [online], (Accessed April 13, 2024)
Created September 9, 2020, Updated October 12, 2021