What is most accurate PH test

[DavidCampen]

Originally Posted by TonyT

I've been using this 4 in1 Soil Tester for Moisture, PH, Temperature, and Sunlight....I know there is a lot of Tester out there much better and much more expensive than this one....but this worked for me just fine with just $30....

This one?
Amazon.com: Luster Leaf 1818 Rapitest Mini 4-in-1 Soil Tester: Patio, Lawn & Garden

Have you ever tested it with solutions of known pH to see if it actually works?

[gladwrap]

Thank you Guys for you information,the consensus seems to be a Hanna around about 35bucks ,I was thinking to get anything decent I would have to spend a fair bit more but if you guys think they will do the job,that suits me,I can buy a few more plants instead.

[larkinvalley]

Did anyone have experience with RePotMe test-stripes? they seem to have convenient range (3.5-7.5 pH) but a description on their website says they had RO water test give 4.5 pH, which makes me question its accuracy. Is it at all possible, that RO water out of a can could be so acidic?

[samarak]

Although the pH of absolutely pure water should be 7.0, really pure water that's left open to the air will wind up with a pH of about 5.65 because it will absorb ("equilibrate with") gases from the air that are acid anhydrides. Mainly CO2, but trace amount of other things like NO2, SO3, etc.

(5.65 is a theoretical value I saw somewhere, based on 335 ppm CO2 in the atmosphere if I remember right, but it's pretty close to what I see when I do the experiment.)

That said, I've used indicator papers before - my wife was a chemistry teacher for years so we have lots of Hydrion paper lying around - but I don't consider them to be accurate for more than a general idea (more or less neutral, acid, alkaline, very acid, very alkaline).

Steve

[larkinvalley]

Thanks, Steve - well, 5.45 is different from 4.5, I guess

[DavidCampen]

Originally Posted by larkinvalley

Did anyone have experience with RePotMe test-stripes? they seem to have convenient range (3.5-7.5 pH) but a description on their website says they had RO water test give 4.5 pH, which makes me question its accuracy. Is it at all possible, that RO water out of a can could be so acidic?

These strips? They are 4.5-7.5.
PH Test Strip Kit - 15 feet

I have that product though I bought it from Amazon. This is a good pH test paper. RO water cannot be used for testing the accuracy of a pH paper since even trace amounts of other materials in the water can cause the pH to vary from the theoretical value of 7.0 by as much as +- 2pH units.

These pHydrion brand, 4.5-7.5 pH papers can be used to determine the pH of a solution between pH 5 and pH 7 to about +-0.5 pH unit which is good enough for a quick check on your fertilizer solution. If you want better then you need to spend about $40 for a pH meter with 2 point calibration and also buy calibration solutions at 2 different pH values.