pH measurement by using HPLC 8543 and UV 1601

[Olivine]

Hi everyone,

In my experiment, I have to monitor spectrophotometric pH of seawater sample over days by using both single beam (HPLC 8453) and double beam (UV 1601) spectrophotometers. However I can not obtain constant value of Spectrophotometric pH which is calculated from the ratio of two absorbances at 578 nm and 434 nm after correction for that of 730 nm. In the case of using HPLC 8453, I doubted that it is due to the inconsistency of wavelength 730 nm and as it was suggested by Mr Tom in this forum that the single beam spectrophotometer can not correct for the fluctuation of lamp output.

For the case of using double beam, I can take blank of my sample every measurement then i do not observe large fluctuation of wavelength 730 nm but the results of pH is still not constant over a period of several days (in the condition of no reaction affecting pH occur within the sample).

Could someone help me to explain about this phenomena that I have been encountering ?

Thank you so much

Olivine

[tom jupille]

It's really not a chromatography problem, but I'll take a stab at it anyway.

The procedure that I found (http://cdiac.ornl.gov/ftp/oceans/Handbo ... sop06b.pdf) calls for regulating the temperature at 25 degrees +/- 0.1 degree.

Temperature control is the first place I would look.

[Olivine]

Thank you a lot for your answer

The cell (400ml in volume)used for my spectrophotometric pH measurement is connected with the Thermol bath to keep the sample at a constant temperature of 25 degree C. I also wrap the water tube connection with insulator to prevent the effect of ambient temperature, especially the difference of daytime and nightime temperature.

If the factor of temperature is not the cause for my result. Is there any possibilities that the energy of light source getting weeken or unstable and there is aslo noise in the detector due to constantly operating spectrophotometer in a period of several days?

I highly appreciate your spending time to respond me.

[HW Mueller]

How do you know that your sample is stable? You can check the stability of your machines with all kinds of stable absorption standards.

[Markus Laeubli, Metrohm]

Maybe a pH meter could help.

But still Tom's and HW Mueller's remarks apply.

[Olivine]

My sample is composed of seawater and small amount of m- cresol purple. In my data of 5 days, I got fluctuation of spectrophotometric pH values which are measured automically every 10 minutes. However when I sort out of those data, I can get consistent values from the first day to last day. This convinces me that the degration of dye does not occur in my sample, then the sample is stable. I would like post some graphs of my result to illustrate my points but I do not know how to do?

Could you please name some kinds of stable absorption standards?

[tom jupille]

Instructions for linking to graphics are here:

http://www.sepsci.com/chromforum/viewtopic.php?t=2617

[Bruce Hamilton]

I would suggest using a suitable potentiometric pH meter system to confirm that the calculated spectrophotometric variation of pH is not correct.

I'd also be a little wary of choosing just 3 wavelengths when you have the capability of collecting the full spectrum with the HP 8453, as that should provide less noise.

My other concern would be if the instruments had two lamps ( D2 and Vis ) operating simultaneously, as any internal filters may be affecting the beam energy.

However, all of us here can speculate until the cows come home, and your best course of action is to contact researchers who specialise in measuing the pH of seawater.

A quick google search finds that the journal Marine Chemistry is littered with articles on seawater pH, and you should contact some of those authors, especially Byrne, as appears to be prolific on the subject.

I'm sure their email address will be on some publications.

Example,

Impurities in indicators used for spectrophotometric seawater pH measurements: Assessment and remedies
Wensheng Yao, Xuewu Liu and Robert H. Byrne, ,
Marine Chemistry
Volume 107, Issue 2, 30 October 2007, Pages 167-172
College of Marine Science, University of South Florida, 140 7th Avenue South, St. Petersburg, FL 33701, USA

Abstract

Spectrophotometric pH measurements have been widely used in oceanic CO2−system research. However, impurities in indicator dyes from different manufacturers may cause uncertainty in measured pH values. In this work we report initial findings surrounding the potential significance of indicator sources (manufacturers) on pH measurements obtained with the sulfonephthalein indicator m-cresol purple. HPLC analyses were used for comparative assessments of impurities in m-cresol purple obtained from a range of manufacturers. Results indicate that m-cresol purple from different manufacturers, and perhaps from different batches of the same manufacturer, have different types and quantities of light-absorbing impurities. Impurities can contribute to pH offsets as large as 0.01 pH units. The pH offset caused by impurities decreases with decreasing sample pH. Indicators can be purified using preparative HPLC. Until purified indicators and refined indicator calibrations become available, we suggest that investigators should reserve some portion of each indicator batch for comparison with the properties of purified indicators.

******

Spectrophotometric determination of pH in seawater off Taiaroa Head, Otago, New Zealand: Full-spectrum modelling and prediction of pCO2 levels
Shane M. Ohline, Malcolm R. Reid, Shamus L.G. Husheer, 1, Kim I. Currie(b) and Keith A. Hunter(a)
a = Department of Chemistry, University of Otago, Dunedin, New Zealand
b = National Institute of Water and Atmospheric Research (NIWA), Dunedin, New Zealand
Marine Chemistry
Volume 107, Issue 2, 30 October 2007, Pages 143-155

Accurate measurement of seawater pH has long been sought by marine chemists (for example: [Dickson, A.G. 1993a. The measurement of sea water pH. Marine Chemistry, 44, 131–142, Dickson, A.G. 1993b. pH buffers for sea water media based on the total hydrogen ion concentration scale. Deep-Sea Research, 40, 107–118; Zhang, 1996; Tapp, M., Hunter, K.A., Currie, K. and Macaskill, B. 2000. Apparatus of continuous-flow underway spectrophotometric measurement of surface water pH. Marine Chemistry 72(2–4), 193–202; Friis, K., Koetzinger, A., Wallace, D.W.R. 2004. Spectrophotometric pH measurement in the ocean: Requirements, design and testing of an autonomous charge-coupled device detector system. Limnology and Oceanography: Methods 2, 126–136]. Recently, such attempts have taken on greater significance as anthropogenic carbon dioxide emissions may create rapidly changing oceanic pH. Spectrophotometric techniques have been accepted generally as the best for determination of seawater pH. Here we report a new technique using thymol blue as the indicator dye and fitting the entire spectrum from 400 to 900 nm rather than measuring the absorbance values at only two or three points in the spectrum. This full-spectrum modelling enables a reduction in signal to noise over other techniques. In the laboratory, we find with seawater samples a pH precision increase of five-fold “withinâ€