you only need to use distilled water to help calibrate the test(if using strips etc)

its pretty cheap to get distill water 89 cents a gallon at Walmart, if you just boil it most likely wont change your ph of the tap water much as its not really removing anything

The impurities become more concentrated when water is boiled...more "pure" water escapes, impurities remain.

If you can rig a system (that won't add impurities) to capture the steam and drop it into a sterile container that could work.

Distilled water is sensitive to pH changes even in it's purest state. Once you pop the top on commercially prepared distilled water the pH will change from atmospheric "impurities" over time...even days to a week if left sitting around. Proper storage is very important if you choose to use a bottle of distilled water over a long period of time.

nc-crn wrote "The impurities become more concentrated when water is boiled...more "pure" water escapes, impurities remain.

If you can rig a system (that won't add impurities) to capture the steam and drop it into a sterile container that could work."

Right after I posted this, I thought that exact same thing...LOL

I think I'll rig something up to my tea kettle to obtain some "pure" water.

You want to use distilled water since it has no ions/buffers in it that will screw up your pH test. For example, some tap water is very high in calcium carbonate which if used in a pH test would strongly bias the results to alkaline, even if your soil is in fact acidic. Bicarbonate ions, sulfate, sodium, etc, they will all mess up your pH test if using tap water. Distilled water has all those removed so it is looking specifically at what is extracted from your soil.

So yes, distilled water for soil tests, otherwise the results are completely unreliable.

It really is cheaper and easier to just get distilled water from your grocery store.