In particular for pH, the measurement should be made INSTU.

What is the time-frame of your preservation? If you are analyzing the samples within a day after taking them, some researchers prefer not to add a preservation agent. This is especially for low-alkalinity or freshwater (e.g., S < 10) samples, as it can perturb alkalinity analyses. However, I'm not aware of a presently accepted alternative to HgCl2 for sample preservation.

I agree that it is preferable to make the measurements as the samples are taken, with out preservation. However, the titrator is not available (for example at field stations) and the samples have to be transported back to the lab. The storage time would be less than one month.

In that case the samples do need to be preserved. The point of preservation in general is to halt biological activity, typically by poisoning any organisms in the sample that will contribute to production or respiration processes and thereby give false carbon readings. So whatever alternative preservation agent is chosen, it must have some toxicity in order to be effective. Aside from immediate sample analysis and using no preservation agent, I'm not aware of a method to preserve a sample in a non-toxic way.

We have been told that ZnCl2 is an alternative for these measurements by the isotope facility at UC Davis. We have asked them for any literature, but we have not received any. We used this method last year on samples that were stored up to 3+months before analysis, and the results (radiocarbon) were not noticeably different from previous years' samples. Does anyone know of any literature comparisons of ZnCl2 vs. HgCl2? More debate on this would be much appreciated!

Thanks Brad! I have also been looking for such literature and not finding it. That was the motivation for my post initially. More recently, I have heard that a couple of labs have started to preserve samples in various ways and testing for stability of the carbonate chemistry over time.

I would like to affirm this question because I am also looking for an alternative method as mercuric chloride is illegal to purchase in the state of Rhode Island and thus, very hard for me to get ahold of.  Does anyone else work in a state/country that bans HgCl2 and have a practical and legal solution to getting ahold of it?

We're at sea right now, and we're taking some replicate samples for DIC, alkalinity, d13C of DIC, and D14C of DIC with both ZnCl2 and HgCl2.  Hopefully we will have enough replication to publish a small comparison paper in the near future, but we are only comparing inorganic carbon between the two poisons.

Brad, I look forward to seeing your results. Let me know when and where you publish them. In a pilot study, here at CSUN, ZnCl2 was found to have a large effect on alkalinity that made it difficult to calculate pH in the preserved samples. We are currently using ZnCl2 for stable isotope samples and continuing with HgCl2 if we have to preserve samples for carbonate chemistry.   

I agree that it is preferable to make the measurements as the samples are taken, without preservation. Currently I am working with the lake water samples and I use CuSO4 as preservation agent, it protects effectively organic compounds against destruction.

Since in-situ is the way to go for pH measurements, what instrument would you recommend for this job? With high sensibility (in order to replace the in laboratory Spectrophotometric pH analysis using the m-cresol dye). 

Regarding CuSO4:  We tried CuSO4 with seawater samples.  It produced a precipitate that re-dissolved during alkalinity titrations prior to reaching the critical 3.5 pH where measurement begins, but the precipitate prevented acceptable determinations of other carbonate parameters.  

Regarding in situ measurement of total pH:  A commonly used approach is based on the solid state "ISFET" technology.  For example, see the Honeywell DuraFET.  Also see papers by Martz et al, Bresnahan et al., and references therein.  One of these has been commercialized as the Satlantic SeaFET, but I can't speak to whether this would work in your setting.  Biofouling can be a significant issue in coastal areas.  There are other commercialized field instruments, including some that use spectrophotometry.  And within the research community, there are many, many non-commercialized variations.  

Hi,

Have any of you heard of a good irradiation system for preserving seawater samples? Maybe there is such a thing on the market?

Cheers,

Marcin

Hi Brad E. Rosenheim ,

Has this study been published? I'm working on a project involving CH4 sampling and would love to find an alternative for mercuric if I can.