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Methods for pH measurement in D.I. water

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The pH measurement of Deionized water is influenced by the atmospheric CO2. When a specification asks for a pH 5.5 to 7, can you possibly advice if this measurement shall be taken after equilibrium?

Please provide us with methods for pH measurement in D.I. water.

Zissis Vassilis
- Athens Greece


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Dear Zissis, This is actually a hard question to answer but really easy to understand. First off, pH is weird especially in water. Any slight change to the water will make your pH lower or raise because there is nothing to buffer it. Even by taking out the CO2 from the atmosphere. You must remember that as soon as DI is exposed to the atmosphere, it will go to 7.0pH. If the water were left in a pipe, it would stay at 5.5pH because no air can get to it.

As for testing the water, you will probably be testing after equilibrium and if you are using mixed bed or really clean DI water, you will need to get a resistivity meter. pH meters require the electrode to measure water that has conductivity in it. DI water if cleaned properly has very little conductivity in it. There is an ASTM method of added some salt STANDARDS to your water then testing it. Many companies try and test without these salts and end up with erroneous readings from their meters. They think the meter is not working when in fact, the meter just can't function with this water cleanliness. Bob Utech


Bob Utech
Benson, Minnesota

 
Editor's note: 
Mr. Utech is
the author of -->


A Guide to High Performance Powder Coating
[link is to this book on Amazon]



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At a pH of 5.5, virtually all of the CO2 will be in that form and can be measured by a carbon dioxide electrode such as Orion's model 95-02.

At a pH of 7, very nearly half will be in the form of the bicarbonate radical, HCO3 (minus one valence). You will still be able to get a reading from the CO2 electrode, but it will be rather meaningless as the line is near vertical at pH7.

Boiling the water is a classic way to remove CO2. Bubbling Helium thru it will also remove a lot of the reactive gasses that are in air.

Equilibrium is a variable depending on the exposure to air, the temperature, the pH and what is influencing the pH such as the bicarbonate. and possibly a few other factors.

James Watts
- Navarre, Florida


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As you know, pH is really a measurement of the equivalent hydrogen ion concentration in a solution. The negative log (base 10) of the molar concentration of hydrogen ion per liter of solution will give the pH. A pH of 7.0 will have a hydrogen concentration of 1 x 10^-7 moles per liter. A pH of 5.5 will have a hydrogen concentration of 3.2 X 10^-6 moles per liter. Therefore to drop from a pH of 7.0 to a pH of 5.5 all you need to add is 3.1 x 10^-6 moles of Hydrogen ion or approximately 3/100 of a millilter of a 0.1 N Hydrochloric acid solution - which would be impossible to accurately measure.

D.I. water theoretically should have a pH of 7.0 - giving it an equilibrium of 10^-7 hydrogen (H+) ions and 10^-7 hydroxide (OH-) ions. However, as it sits in open air Carbon Dioxide will absorbed resulting in the formation of Carbonic Acid. It would take very little absorption of CO(2) to drop the pH from 7.0 to 5.5. In fact you can literally watch (on a pH meter) the pH drop from 7.0 in D.I. water as it absorbs Carbon Dioxide.

Once reaching a pH of ~5.5 an equilibrium will be established with the atmosphere and the pH should stabilize at around this point.

Several commercial pH meters are precise enough to measure accurately within this pH range assuming the meter is properly calibrated. However, don't worry too much if you see the pH drop in the D.I. water as you measure it - this is natural as explained above. If your initial readings are around 7 then drop to no lower than ~ 5.5 - you have good water.

Perhaps a better way to test your D.I. water is to measure the conductivity. You can also measure for total dissolved solids (tds), chlorides, and alkalinity in the water. Electronic meters are available commercially to measure conductivity and tds. Test procedures exist for checking chlorides and alkalinity.

Greg Haataja
helicopters - Fort Worth, Texas


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What should be conductivity of Deionised Water used for cleaning of PWA (Electronic PCB assemble i.e computer mother boards etc)in aqueous cleaning m/c.

Regards,

R P Shukla
- Gurgaon, Haryana, India


March 11, 2008

Even though we get the pH value of water after Anion exchanger as 7.0 this value decreases to 6.5 after treating the output of Anion Exchanger in Mix Bed Ion exchanger which is meant for further removal of silica.

Nataraj Das
employee of power plant - Dhaligaon, Assam, India

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