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Re: strange Ph readings

>Date: Wed, 4 Feb 1998 09:15:06 +0100
>From: "Pierluigi e Simone Vicini" <psvicini at mdnet_it>
>My Dupla CO2 tester is giving me different reading from my Ph and KH
>One week ago everything was ok but now my PH is 6.5, KH is 4 and CO2
>reading is on 10-15 mg/l . How is this possible.
>Is there anything wich can make the tester readings wrong? Can P or K or
>other chemicals do this?

First consider any possible errors in reading the test kits.  What is the 
resolution of each kit?  If the pH kit is +/- 0.2 and the KH kit is +/- 0.5 and 
the CO2 kit is +/- 3, consider the possible ranges of readings possible.  The 
numbers above are way off, so I don't think reading errors are the main problem. 

Note that other sources of alkalinity will make your KH test read too high.  A 
pH of 6.5 and a CO2 of 10-15 mg/l indicates a KH of 1 to 2 degrees.  Have you 
done anything to alter alkalinity besides adding carbonates?  If the only 
buffering in the water is from sodium bicarbonate or calcium carbonate, the KH 
kit will read true.  If you have added phosphate-based buffers, the KH test will 
NOT read true.  

Phosphate will also give false high readings with CO2 tests but you are getting 
too low of a value based on pH and KH (CO2 should be about 38 mg/l for those 

Which of the values changed from when things were OK?

George Booth in Ft. Collins, Colorado (booth at frii_com)
Need Info?  http://www.frii.com/~booth/AquaticConcepts.htm