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I've been researching ocean acidification, and the way that ocean pH is typically measured is by using sensors placed throughout all of the oceans. This is costly and time-consuming, and being able to measure ocean pH from space would be much more efficient as a global map could be created in a relatively short time and updated regularly.

However, when I researched pH measurement techniques, they all involve an instrument physically touching a solution, which would obviously not be possible from space. Ocean pH maps that have been created using satellite data measure pH indirectly by measuring ocean salinity, temperature, and other conditions, and they point out areas where it is "likely" that pH levels are low enough to harm ocean life. Is is possible to directly measure pH from a large distance? Would it be necessary to do so, that is to say, is measuring ocean pH levels indirectly accurate enough for environmental monitoring?

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Interesting analytical chemistry problem. Currently, it is not possible to accurately determine pH in a remote fashion. Spectroscopy is the technique of choice for remote sensing, but the H$^+$ is not that spectroscopy friendly i.e., it is a quiet ion. It will respond to radiofrequency in a powerful magnetic field but the sensitivity is quite poor in an NMR experiment by current standards. You can Google "measurement of pH by NMR". In general what appears to be pure to an organic chemist by NMR experiment, analytical chemists call it a dirty sample because there a lot of minor impurities.

A quick literature search shows that that satellite data can be used to make an only an educated guess about the ocean pH. It not accurate but good enough for a ball-park estimate.

Remote sensing of ocean pH by R. Sabia, D. Fernández-Prieto, J. Shutler, C. Donlon, P. Land and N. Reul, "Remote sensing of surface ocean pH exploiting sea surface salinity satellite observations," 2015 IEEE International Geoscience and Remote Sensing Symposium (IGARSS), Milan, 2015, pp. 106-109.

Within this context, the purpose of the ESA “Pathfinders-OA” project is to quantitatively and routinely estimate surface ocean pH by means of satellite observations in several ocean regions. Satellite Ocean Colour, Sea Surface Temperature and Sea Surface Salinity data (with an emphasis on the latter) will be exploited. A proper merging of these different datasets will allow to compute at least two independent proxies among the seawater carbonate system parameters and therefore obtain the best educated guess of the surface ocean pH.

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  • $\begingroup$ I'd remark that “educated guess” makes it sound worse than it perhaps is. Lots of good science relies on similar methods. The important thing is that the proxy's validity can be experimentally verified independently of the result the proxy is used to obtain. In this case, the problem seems to be not that proxies are involved but that that the error bars on the final result are huge – but as long as they're known, those results can still be useful. $\endgroup$
    – leftaroundabout
    Commented Apr 12, 2020 at 20:39
  • $\begingroup$ Right, Feynman was famous for making good educated guesses in physics. I agree that educated guesses are good and even necessary for doing science but in analytical chemistry the typically "permitted" error is less than 5%. When we develop new chemical analysis methods, we would like to have the error as low as possible. It is just a narrow perspective of analytical chemistry where guesses are not very welcome. $\endgroup$
    – ACR
    Commented Apr 12, 2020 at 22:06
  • $\begingroup$ Yeah. In physics there are some methods with error 0.000001% and some methods with error 70% or so... $\endgroup$
    – leftaroundabout
    Commented Apr 13, 2020 at 8:53

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