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.
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.
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:
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.