Methods for pH measurement in D.I. water

A. Hi Bharat. Who says the pH is 4.5 to 5.5? :-)

If it's a pH meter, forget about it and efore changing anything, please read the rest of this thread to make sure you are understanding that anything short of highly specialized equipment & methodology cannot offer a meaningful pH reading for water which has passed through a mixed bed. pH meters, which measure the relative number of cations and anions, require the water to have good conductivity in order to conduct the measurement, and your water has essentially no conductivity.

Luck & Regards,

A. 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 7.0 pH. If the water were left in a pipe, it would stay at 5.5 pH 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 Benson, Minnesota

Ed. note: Bob is the author of: "High Performance powder Coating" on eBay or AbeBooks or Amazon [affil links]

A. 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 ^-. 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 pH 7.

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.

A. 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 milliliter of a 0.1N 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.

A. It is extremely difficult to measure the pH of Deionized water (with a resistivity of > 0.5 meg-ohm-cm) accurately. Commercial pH meters are not designed for this purpose and are only accurate in a solution that has a significant ionic content, and possibly some buffering. For this reason, in DI water, it is more customary to use the resistivity as a measurement and then use theoretical calculations to determine the pH range.

However, if for some reason you must have a pH of very nearly 7.0, you may want to consider a mixed bed polishing tank. Typical water after a mixed bed tank of ion exchange resin has a resistivity of > 10 meg-ohm-cm and a pH of 6.8-7.2.

Q. Sir!

Thank you for answering by inquiries. But I have some more questions. I hope you can give me answers again.

1. what is the best type of water that should be used for resin regeneration? Water from the reverse osmosis (RO)or a deionized (DI) water.

2. Will using RO water have an effect in regeneration time? How will DI water perform when it comes to regeneration particularly with regards to time of regen.

I am a process engineer of the facilities group of Texas Instruments, Philippines and I am currently conducting an investigation on the correct way to regenerate our mixed bed resins.

Hope for your reply the soonest.

Thank you!

A. Dear Friend

I Would like you to know that all product water of Reverse Osmosis, is filtered about 0.012 ppm, because of this all mineral content of water has been removed, so the PH will be lower to 6.3 - 6 or maybe 5, my suggestion to raise this PH up, add caustic soda ⇦liquid caustic soda in bulk on Amazon [affil link] until the reading becomes pH 7.2

A. Greetings All - I worked in the pharmaceutical industry for 25 years making ultra-high purity water. The reason that deionized water is always below 7 pH is well-known in the industry. Since pure water has a perfect balance of Hydrogen and Hydroxyl ions...it's absolutely sound logic that it MUST be neutral pH. However, what most folks do not know is that water at this level of purity has a very rapid affinity for carbon dioxide. (CO2) Once deionized water is exposed to the atmosphere, it immediately draws CO2 from the air and, consequently, lowers the pH very slightly to the range of 6.8 or 6.9. That's why it's really impossible to take the pH of high-purity deionized water. For this reason...it is often stored under a nitrogen blanket to prevent exposure to the atmosphere until the point of use. I hopes this clarifies the issue for you.

Ramil B - We always used deionized water to regenerate our pharmaceutical ion-exchange resins. We did this to avoid any contaminating of...or reactions with...the regenerant chemicals. Contaminants introduced during regeneration can potentially leech out during the service cycle. Hope this helps. -OD

A. I have a handy chart, at work, that shows what the maximum/minimum pH ranges are for various megohm/microsiemens (conductivity) readings on DI water.

Here is the link I was referring to. Its a PDF file.
http://www.cal-water.com/pdf/ConversionChart.pdf

Re. the link given earlier as http://www.cal-water.com/pdf/ConversionChart.pdf The above link for pH vs. conductivity and the underlying assumptions are not quite correct. Regards, C A Awalkar - Mumbai, India March 4, 2015 March 2015 Thanks Mr. Awalkar. But no scientific theory is exactly correct; all of science is a simplification and a manmade metaphor. Please expound on what is wrong with the chart, and any practical implications that come to mind. Thanks. Regards, Ted Mooney, P.E. Striving to live Aloha finishing.com - Pine Beach, New Jersey

A. Hi, Subramanian. I think the point of many of the responses on this thread is that you can't measure the pH of ultrapure water with a pH meter at all, and it's pretty meaningless anyway. So when you ask how to calibrate your meter for accurate pH readings of ultrapure water, I think you need to start at the top of the page again and read slower :-)

Luck and Regards,

A. pH will be fairly close to 7 as it comes out of the resin bed, particularly if it is a mixed resin type. This pH will slowly drop as it absorbs CO2 from the air.

Note that you cannot use conventional pH meters on really pure DI water. It requires a modified approach that can be given to you by any manufacturer of research grade equipment.

A. It is normal for the pH of DI water to drop on storage. This is due to CO2 being taken up and the formation of very, very dilute carbonic acid. A nitrogen blanket may prevent this.

A. Hi Sil,

Did you measure your starting pH before adding the dextrose? I suspect that is where your difference comes from. It may, as you suspect, be dissolved carbon dioxide. Try gently warming the demineralised water for a few minutes, allow the water to cool and re-check the pH -- has there been an increase in pH?

 Hello Brian
Thank you for your suggestion. We correct measurement errors and the results are now consistent. With deionized and distilled, it gives the same pH, as expected.

A. pH cannot be measured in Distilled or deionized water because distilled and deionized water do not have enough ions present for the electrode to function properly. As soon as the deionized water hits the atmosphere the pH will drop below 6 because of the Carbon dioxide dissolution in the water water reacting with it to form carbonic acid.