How Can I Measure the "pH" of Deionized Water?
The Purer the Water, the More Difficult the Task
Conductivity
With a decrease in ionic substances comes a decrease in conductivity, since ions are charge carriers. Unfortunately, pH meter electrodes utilize ions in making their measurements. The purer the water, the less the ionic content, the lower the conductivity, and the more difficult it is to determine pH by ordinary methods.
Contaminants
Not all deionized water is of equal quality. The higher the resistance of the water, the purer it is rated; however, traces remain of sodium chloride, calcium sulfate, and other ionic substances. Among non-ionic contaminants are bacteria, organic (carbon-containing) substances, dissolved gases, etc.
Specialty Electrodes
The Cole-Parmer website says, that for a pH electrode best suited to measure the pH of pure water, "The best choice is a refillable, liquid-filled electrode, ideally made of low resistance glass. A flowing reference junction has a higher flow rate to minimize junction potentials." This is because such an electrode enables a pH potential to be rapidly attained.
Modification of Procedure
Another means of improving testing results is to add a small amount of very high purity potassium chloride to increase ionic content of the water. If very pure and added in small quantity, the pH will not change. It is important to limit exposure of the water to the atmosphere, as exposed water quickly begins to absorb carbon dioxide gas, gradually lowering the pH.
Vernier Software and Technology - "Why doesn't my pH Sensor read pH 7 in distilled or deionized water?"
Cole Parmer - Measuring the pH of Pure Water
Apswater - How to Measure the pH of Pure Water
"The Essentials of pH Measurement Design, Installation, Maintenance, and Improvement"
Published by Vincent Summers
My secular expertise includes 23 years of experience at the National Radio Astronomy Observatory, with a share in NASA's extended Voyager 2 effort. I formerly wrote for Demand Studios, Bukisa, Suite 101, Exa... View profile
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8 Comments
Post a CommentI've used ph electrode in my lab. It's all right I think. It can certainly fluctuate while titrating.
Phew! So much for the litmus paper that turned blue or red!
I had no idea that it would have to be done differently with deionized water. Thanks for the information!
This one was easy to understand. Thanks for the explanation.
Yikes. I like reading your articles but I don't fully understand the concepts. :-)
I never knew that measuring PH would be a problem with dionized water. Thank goodness, the fish tanks I had growing up all took regular tap water!
You are such a smart teacher.
Okay.........Thanks!