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I am looking to test soil pH in a remote location. The test results don't need to be perfectly precise, and a pH test strip should provide sufficient accuracy. However generally this kind of testing uses boiled distilled water - and in this location it will be very difficult to buy distilled water.

The location is off grid, and getting specialized equipment to the location is not an option. (This is for field experimentation with farmers in rural Africa)

The process I have seen used for this is:

  • Add 1 part boiled distilled water and 1part soil by volume, in a clean container
  • stir and leave to settle.
  • Test the water pH with a pH test strip

Would it be possible to use boiled rain water for this process, or would this be likely to contain impurities which would interfere with the test results?

If there are likely to be impurities but there are approaches that can mitigate for them, please explain.

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  • $\begingroup$ Have you considered getting a portable distiller? It would require access either to a socket or a car with a transformer, and they can cost a bit, but it would probably do the job. $\endgroup$
    – rotaredom
    Commented Nov 23, 2017 at 16:22
  • $\begingroup$ That is a possibility; however the cost and logistical challenge of getting and running one is quite high (For example there is no grid electricity). If it comes to that it may be worth buying a full soil testing kit with it's required equipment, rather than just using pH strips. I am also considering the possibility of setting up a DIY distiller- suitable parts may be locally available. $\endgroup$
    – Ben
    Commented Nov 24, 2017 at 9:39
  • $\begingroup$ I don't quite get your problem. If you have to take the probes by hand, you have to be there yourself. Guessing your weight somewhere between 50 to 100kg, whatever takes you to that location should be able to take 50l of distilled water along with you to that location, too. $\endgroup$
    – user21844
    Commented Apr 11, 2019 at 18:02
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    $\begingroup$ @Ben What about commercially available solar distillers? Of course the cost may be more than pH strips but I doubt that is your only source of costs in the first place. I'm not too familiar with soil testing but what sort of size of sample and number of samples are you talking about here? $\endgroup$
    – Bryan Krause ♦
    Commented Apr 11, 2019 at 19:10
  • $\begingroup$ Have you considered making a single stage distillation unit by placing an inverted bowl above boiling water and capturing the condensate then maybe boil the water once more to kill off any air contaminants? $\endgroup$
    – user40950
    Commented Apr 11, 2019 at 21:08

2 Answers 2

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The h2co3 content of unadulterated rain is 15 micromoles of H+/Kg at room temp. The pHBC of average field soil from a 3000km transect in asia varied from 10 to 188 - mmol-kg-ph unit... from 40 localities. This other research found soil values of 45-1000 pHBC. https://www.science.gov/topicpages/s/spiked+soil+samples.html

That means that H2CO3 would affect arid meadow soil by 0.08 to 1.0 pH points if 1kg of water reacted completely with 1kg soil in a sealed environment. And 0.33 to 0.015 pH points for the referenced research.

However, if you boil the water, the gases expand and leave the water even prior to boiling point, which can be seen as bubbles previous to 100'C, boiled water error becomes 0.2 to 0.003 pH points. This is the only ref i can find:

https://www.engineeringtoolbox.com/docs/documents/1000/solubility_CO_water.png

ttps://www-engineeringtoolbox-com.cdn.ampproject.org/ii/w1200/s/www.engineeringtoolbox.com/docs/documents/1148/solubility-co2-water.png

That means that if you have unadulterated rainwater at pH 5.5, and you boil it and jar it, you will have water with a pH near 7 after sealing and cooling.

Pure water is not natural or biological and it is aggressive to get to equilibrium. Upon contact with the atmosphere, it will immediately begin absorbing CO2 and the pH will drop and settle in at about 5.5 after about two hours.

Carbonic acidification is a background effect of the athmosphere aerated soil and rain to similar degrees.

Strong acid - pKa < 2 Weak acid - 7 > pKa > 2 Weak base - 10 >pKa > 7 Strong base- pKa > 10

oxalic acid and citric acid have a pKa in the range of 1 to 3, they are fairly strong, whereas CO2 has a pKa of about 6.4.

If your rainwater has a pH below 5.3, something is wrong with it and if the rainwater is around 5.5, it's nearly the same as distilled water.

I micromole of HCl rects completely and gives distilled water a pH of 6, whereas 1 umol of CO2 causes [a pH of 6,997][6], because only 0.3% of it converts to H2CO3.

[6]: https://books.google.fr/books?id=yRMgYc-8mTIC&pg=PA124&lpg=PA124&dq=hcl%20micromole%20co2%20distilled%20water%20concentration&source=bl&ots=OFKb4CB3MI&sig=ACfU3U0a1XQ_GSIZ5E-nj7JgPsaeDv7SbA&hl=en&sa=X&ved=2ahUKEwib-vuMkc_hAhVEThoKHYIcDtQQ6AEwCXoECFAQAQ#v=onepage&q=hcl%20micromole%20co2%20distilled%20water%20concentrationhcl&f=falseenter image description hereenter image description here

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  • $\begingroup$ I don't think the issue is the reactivity of CO2 with the soil, but rather that the rain water will have a pH below 6 and therefore would impact any pH tests that incorporate use of that water. This impact is significant if the distilled water being used for tests normally would have a pH significantly higher than the rain water. What the OP could do is test the pH of both water sources to see if they differ, and if so, what effect using the two water sources has on the soil analyses. I would think that water pH would have varying degrees of effect depending on the soil makeup. $\endgroup$
    – theforestecologist ♦
    Commented Apr 11, 2019 at 22:12
  • $\begingroup$ However, your point about any water source absorbing CO2 to equilibrate with the atmosphere within 2 hours might very well be true and nullify this issue (though presence of N or S acids from air pollution would impact the rain and not the water source held by the OP). Regardless, please provide a source to back up this claim and the various others that you make in your post. I'll be happy to upvote with proper support :). $\endgroup$
    – theforestecologist ♦
    Commented Apr 11, 2019 at 22:14
  • $\begingroup$ For agricultural purposes in rural Africa, rain is also part of the agricultural equation. High precipitation does change soil pH and leaches minerals, especially if it acid, and distilled water does too. distilled water is probably more important to test soil where for an arid region where it doesn't rain. For the purposes of the test, the dissolved elements will be present in the sample, and not leached. For agriculture, a good process is to use the tools that the farmers have, the local water and soil amendments and find the best soil mix. references would use pHBC. $\endgroup$
    – bandybabboon
    Commented Apr 11, 2019 at 22:55
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    $\begingroup$ IF the OP truly doesn't care about precision, then perhaps you're right that the pH difference in rainwater might not be significant. But to go into a study with that known potential source of error is not very good science. Again, I recommend that if rainwater is to be used, that the OP at least tests to confirm that it doesn't skew results. $\endgroup$
    – theforestecologist ♦
    Commented Apr 12, 2019 at 14:48
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    $\begingroup$ I want to accept this answer, and give it the bounty, but clearer references would be good. $\endgroup$
    – Ben
    Commented Apr 14, 2019 at 16:53
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Rainwater might not be the best option.

Rain water is naturally acidic due to the reaction of CO2 forming some weak carbonic acid [see here for explanation].

enter image description here

Source: Wikipedia

As a result, pH of "pure" rainwater is usually between 5-6; usually around pH 5.6. See here.

Most rainwater has a pH of 5.6 to 5.8, simply due to the pressence of carbonic acid (H2CO3).

Presence of any sulfur or nitrogen oxides in the air (perhaps from burning coal plants or city traffic from 100s of km away) would lead to rain becoming even more acidic. From environment.co.za:

Sulphur dioxide reacts with water vapour and sunlight to form sulphuric acid. Likewise NOX form nitric acid in the air. These reactions takes hours, or even days, during which polluted air may move hundreds of kilometres. Thus acid rain can fall far from the source of pollution.

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  • $\begingroup$ I had hoped that boiling the water would be sufficient to bring the levels of dissolved CO2 to a consistent low level. I am fairly convinced that this would not work reliably - after looking further into sulfuric and nitric acids as described in your answer, so I think this answer is correct. I think this answer would be even better if it addressed what if any effect boiling would have on the amounts of these impurities. $\endgroup$
    – Ben
    Commented Apr 11, 2019 at 20:54
  • $\begingroup$ @Ben, a discussion about the impacts boiling has on water pH is really more appropriate for Chemistry.StackExchange, so I'd advise you to ask that question there. $\endgroup$
    – theforestecologist ♦
    Commented Apr 11, 2019 at 20:59
  • $\begingroup$ @Ben: To hypothesize (because I don't know the answer): I would guess any solutes (if any) dissolved in the rain water would increase in relative concentration and as a result the water's pH would shift in relation to the acidity/alkalinity of the solutes. This is totally conjectural, and I'm not really sure there would be enough solutes to make a difference. You can read here for a discussion about the impact of boiling on water pH, though I don't think it quite answers your question. $\endgroup$
    – theforestecologist ♦
    Commented Apr 11, 2019 at 21:04

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