I am trying to use the SOP 6b from Dickson et al. 2007 (Determination of the pH of sea water using the indicator dye m-cresol purple). Unfortunately, I have been getting consistently low values (~0.1-0.4 units too low) and I'm wondering if my technique of preparing indicator stock solution is an issue.
The SOP calls for:
"A concentrated (at least 2 mmol dm–3) dye solution of known pH adjusted to be in the range 7.9 ± 0.1 pH units—chosen to match pH measurements from an oceanic profile—is required"
Does it matter how I adjust the pH of the indicator solution? Would it make a difference if I use bicarbonate vs. NaOH to make the solution more basic?
This problem is discussed in
Marine Chemistry, Volume 65, Issues 3–4, June 1999, Pages 281–290
Influence of m-cresol purple indicator additions on the pH of seawater samples: correction factors evaluated from a chemical speciation model
Melissa Chierici, Agneta Fransson, L.G. Anderson
and
Environ Sci Technol. 2012 May 1;46(9):5018-24. doi: 10.1021/es300491s. Epub 2012 Apr 11.
Spectrophotometric calibration of pH electrodes in seawater using purified m-cresol purple.
Easley RA, Byrne RH.
As you will notice the salinity and temperature bring important effects.
Hi Matthew,
I'm probably much too late to answer your question....But just thought to share my experience. I used 0.1M NaOH and adjusted the pH of the Na-salt of m-cresol purple to 7.9+/- 0.1 . I did this both using the spectrophotometric technique and the pH meter. It worked ok for me but I get absorbances not greater than 0.6. The pH values seem to be ok. I have done some open ocean and coastal measurements in the Arabian Sea. Let me know your status as of now. We can discuss in more details.
Bicarbonate or phosphate might influence due to introducing supplementary different anions.
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