[scientizscht (OP)]

Is there a remote sensor for protons or pH?

[Bored chemist]

Litmus paper and a telescope?

[yor_on]

What do you mean by a 'remote sensor'?

[alancalverd]

Nuclear magnetic resonance will quantify protons without touching them. Not sure about pH, though, because it won't distinguish free protons from hydrogen atoms.

[Bored chemist]

If you are looking at biological tissue, 31P NMR can distinguish pH.
https://pubs.acs.org/doi/abs/10.1021/bi00522a006

[alancalverd]

Interesting idea but the paper seems to be pointing out that there are so many confounding factors in the case where the chemical shift of a phosphate depends on the ambient pH that it cannot be used as a reliable estimate!

So I guess the answer is yes, if you add an inorganic phosphate to your sample and assume or demonstrate an absence of Mg and K ions, you might be able to estimate its pH to within +/- 0.1. I think this rather stretches the definition of "remote" - by the time you have added your indicator and eliminated (or not) the interfering ions, then indeed measured the NMR spectral shift from just outside the rotating test tube, I would have dunked my gardener's  pH probe (£5 from most tool shops) in it, written down the answer, and done the Times crossword (available in all good operating theatres and cockpits).

Not that I have any objection to chemists playing with expensive toys (see "operating theatres and cockpits" above).   

[Bored chemist]

The really clever bit is that you can use NMR to measure pH inside a patient.
https://www.nature.com/news/1999/991111/full/news991111-2.html

You can't really do that with a garden pH probe.

[alancalverd]

That's neat. Changes in MRI contrast at low MRI fields (< 3T) are probably measurable in  vivo to a useful accuracy, though they don't say they have actually done it.

All you need is a £1M MRI machine, a patient who can tolerate the experience, some modified Gd contrast, and a hypodermic needle to inject it. ThermoScientific offer a 16-gauge pH probe for measuring pH in microliter samples - a bit more expensive than a garden tool, admittedly.

[Bored chemist]

By the time you have stuck that needle into the patient millions of times to build up an image, they will probably be dead.

Also, the far end of a hypodermic needle is not what most people would consider "remote".
It would help if the OP clarified what he was looking for.

[alancalverd]

Probably not a medical question after all.

IIRC if blood pH varies by more than about ± 0.2 (though I can't recall which is the cause and which the effect here) you get very sick and die very quickly , so the clinical requirement has got to be for much better resolution, much quicker - say ±0.05 overall precision within a couple of minutes - if it is to be of any use. Probably better to look for other signs and symptoms. Postmortem pH is all over the place anyway, but postmortem MRI is dead easy.

[Bored chemist]

It's true that the precision of the pH measurement is  not great. But mapping areas of different pH can spot tumours etc.

I wonder if this discussion is anything to do with what the OP wanted.

[alancalverd]

Unlikely, but it's interesting anyway.

I'm an advocate of using ultrasound, x-ray and MRI to find anatomical pathology, and radionuclide imaging to identify physiological anomalies, so I'm not sure what mapping pH distribution is likely to add to anyone's diagnosis or treatment. But curiosity is always welcome.

[scientizscht (OP)]

So at the end can we spot maps of different pH?

[Bored chemist]

Heather doesn't grow well on alkaline soils, so here's a very rough map of pH of the UK
http://www.moorlandassociation.org/wp-content/uploads/2015/03/Heather-coverage-UK.png