Verify the accuracy of ph test strips

Question

In the summer I keep a "fast set" pool setup in the garden and hence have to concern myself with chemicals etc. to keep it clean.

I use the "test strips" to monitor the ph and Chlorine levels in the pool.

I have reason to believe that the current batch of test strips that I'm using are not accurately reporting the ph level for the pool.

How can I independently measure the ph of the pool water to verify the accuracy of the test strips?

The test strips read a narrow band around neutral: 6.6 to 7.4.

Answer 1

This is a tough one. Typically you would need a "control solution" with a known pH in the narrow range of your test strips, but it is surprisingly difficult to come up with a reliable solution in a home setting without buying a specific calibration solution from a lab supply like Fisher Scientific. (rejecting that solution)

Distilled water is only pH 7 the instant it is made, so with no buffering capability, its pH will swing wildly from just the CO₂ in the air. Unless you have a milligram scale and lab-grade materials, even common household products will vary wildly based on brand, formulation… even the water and precise measurements you'll need to turn it into standard solution. (rejecting that solution)

Other homemade "standard solutions" (e.g. calcium hydroxide used by reef aquarium experts) are too far outside your range to be useful. Hardware stores (in the lawn and garden section) have bottles of 7.0 buffer solution for calibrating pH meters for testing soil. I'll assume you don't have any of that on hand so… (rejecting that solution)

So what's left?

Well… oddly enough — human blood.

Turns out that human blood has a tightly regulated pH of 7.35 - 7.45. If it goes 0.2 either up or down you die. Human blood only maintains that pH for a very short time outside the body, so measure quickly.

How's that for a life hack?

Answer 2

Do you know someone else with a pool? Even a well-maintained aquarium will do in a pinch.

Test their water and see if your results match whatever they come up with their equipment. Your local aquarium shop may be able to help; they have test kits on hand to do water testing for their customers. That way you are not buying a lot of unnecessary chemicals or duplicate test strips which will likely expire before you get to use them.

Answer 3

Buy 3 different brands of test strips. (You already have one set, so just buy 2 new sets of different brands.) When you test, use 3 strips, one from each brand.

• If the results of all 3 strips are the same, then you can be nearly certain that that is correct. (They could all show the same wrong result, though that would be highly unlikely.)
• If 2 strips show the same result, and 1 shows a different result, chances are that the 2 matching strips are the correct result and you should discard the 3rd strip.
• If the 3 strips show different results, then Houston, we have a problem.