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Methods for pH measurement in D.I. water

(hopefully helping readers more quickly understand the Q&A's)

SWRO = seawater reverse osmosis pH = measurement of acidity or alkalinity DM = demineralizer mixed bed = a demineralizer containing a mixture of anion & cation resin, which serves essentially as a nearly infinite series of alternating cation & anion exchangers.

Q. Sir,
SWRO water (pH range- 5 to 6) as a feed to DM plant mix bed, Rohm & Hass resin (Thermax & Purolite resin bed also available) --but always facing problem in final DM bed outlet product that pH is low ( 4.5 to 5.5)

Sir please provide solutions. By additional loss of anion resin make up pH can be increased, or by addition of cation resin pH will increase?

Please suggest which resin give high pH value for SWRO water as a feed.

Bharat Patel
- Jamnagar Gujarat India
April 11, 2022

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,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey
April 2022

⇩ Related postings, oldest first ⇩

Q. 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


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
Bob Utech
Benson, Minnesota

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.

James Watts
- Navarre, Florida

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.

Greg Haataja
helicopters - Fort Worth, Texas

Q. Hi,

I am aware that the reason for the pH of DI water to be ~5.5 is due to the carbonate system that gives off protons. Therefore, to eliminate CO2 in DI water, I bubbled N2 gas into DI water. As I continued to bubble N2 gas into DI water, I observed the pH of the water go past 7. What is making the pH of DI water to surpass pH of 7?

Cindy Lee
Colorado State University - Fort Collins, Colorado, U.S.A

Q. 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.


R P Shukla
- Gurgaon, Haryana, India

Multiple threads merged: please forgive chronology errors :-)

How to hold pH after DI polisher

Q. Sir,

What is the best way to keep the pH of DI water at 7 after the polisher? Is there a resin that will keep the pH at 7?


Ramil B [last name deleted for privacy by Editor]
- Baguio City, Philippines


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.

Lyle Kirman
consultant - Cleveland Heights, Ohio


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!

Ramil B [returning]
- Baguio City, Philippines


Q. Hi

I am a student in Marmara University in Turkey. I study in environmental engineering.

Few days ago in the laboratory we had an experiment which is about distilled water ph we calibrate the ph of distilled water 5.17 this is not possible but we find.

Then the teacher asked that why pH of distilled water is not 7 what can cause this?

I am searching for this question and I find your comment about this but it is little.

So could you help me for my research, please thank you.

Mustafa H [last name deleted for privacy by Editor]
Marmara University - Istanbul, Göztepe, TURKEY

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 [affil links] until the reading becomes pH 7.2

Renato D [last name deleted for privacy by Editor]
- Batangas City, Philippines

Q. What should the pH level be in drinking water that has been distilled?

Rock Porras
owner - San Jose, California, USA

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'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 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

Oscar Darwin
- Cainta, Rizal, Philippines
February 16, 2012

Multiple threads merged: please forgive chronology errors :-)

pH of ultrapure water?

Q. How is pH related to resistivity in pure water? I am running a system with DI water that runs between 14.4 megohms to 17.91 megohms. I have an aerospace customer that requires the cleaning water have a pH between 5.0 and 8.0. I track resistivity, but do not track pH levels. Please advise.

Ryan N [last name deleted for privacy by Editor]
Project engineer - Springfield, Massachusetts

simultaneous replies

A. That's an interesting question, to which I await the answer too! If you measured the pH your salt bridge would destroy the ultra purity I believe. And if there are virtually no ions, it doesn't seem that you could have much H+ or OH- anyway. I suspect that the pH of ultrapure water is intrinsically 7.0 but I don't know and would appreciate an education.

Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey


A. You cannot measure pH of ultrapure water with a pH sensor. There is not enough conductivity for the sensor to work. If your resistivity is in the range that you mentioned, then you will also certainly be within the pH range that is required. You can't say, however, what the pH is from the resistivity. The reason for this is that pH is the measure of a specific ion- H+ (and indirectly OH-). The presence of H+ will reduce resistivity in the water, but so will any other ionic species.

Here is some theory- H2O molecules have infinite resistance. It is ions that cause solutions to be conductive. Even perfectly pure water has some ions because of the equilibrium between H2O, H+ (ions), and OH- (ions). The concentration of the ions H+ and OH- in perfect water is 0.0000001 molar at 25° C. The pH of this water is exactly 7. Because of the difficulty (impossibility) in the accurate measurement of pH in ultrapure water, ASTM has at least removed the requirement from the specification for Type I and Type II deionized water. Perhaps you could point your Aerospace customer to that specification and convince them to remove the pH limit or to replace it with a resistivity limit.

Jon Barrows
Jon Barrows, MSF, EHSSC
GOAD Company
supporting advertiser
Independence, Missouri

A. First, pH of ultra pure water is difficult to do. There are some special techniques that can be used. Ted is correct about the contamination, which is why you test a sample and not the tank/vessel. Personally, I would contact Thermo Orion which I think had the best tech services of all the probe manufacturers that I worked with. I have no idea of any changes that might have been made since Thermo Electron bought them a couple of years ago. Finally, even tho it seems impossible, the pH will vary. I have seen it at 6 in 5 megohm water. I think that you are safe in saying that 15 meg water will always be between 5 and 8 as an imbalance of the H and OH will increase conductivity.

James Watts
- Navarre, Florida

simultaneous replies

A. The pH of pure water is 7.0. However, it won't remain so for long. With the water being very free of ions, it tends to absorb anything it can, including CO2 from the air which dissolves to form carbonic acid. As the CO2 is dissolved into the water, the pH will drop significantly. Getting to a pH below 5.0 is very possible, especially if the rinse tank is agitated.

Chad Dannemann
- Houston, Texas

A. It is nearly impossible to measure the pH of ultrapure water. It takes both a special probe and a specially designed cell to even have a chance of getting an accurate result,and just because your pH meter gives you a reading doesn't mean it is accurate. A lot of people have considered this problem,and there are many articles on the subject. One of the earliest and best was in the July, 1972 Issue of "American Laboratory", written by Gerard Otten. This may be the source of the curve that was referred to by an earlier respondent. In it he shoes that at a Resistivity of 2.0 Meg-Ohm-cm, the maximum pH range is from 6to 8.2, and this range gets narrower as the resistivity increases.

The other problem in measurements with ultrapure water,is that you can't take a sample and expose it to air and get an accurate reading. The resistivity of 17 Meg-Ohm water and the pH will drop within just seconds after exposure to air due to CO2 dissolution and absolutely no buffering capacity in the water. To be accurate, any measurements with water > 10 Meg-Ohm almost have to be done on-line.

Lyle Kirman
consultant - Cleveland Heights, Ohio

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.

Marc Green
Marc Green
anodizer - Boise, Idaho


Re. the link given earlier as

The above link for pH vs. conductivity and the underlying assumptions are not quite correct.


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.


Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

Q. So, how do I calibrate a pH sensor, used in Ultra pure water application? When I calibrate it with standard buffers, it is perfect, but on the process sample, it is reading between 5.8 to 6.0 pH, (on a continuously flowing sample), But this reading is very much below the expected reading of 7.5 to 8.0 pH.
Can I just standardize the reading to the expected value? Or can I prepare buffer solutions by adding buffer tablets in the same ultrapure water sample and use this for calibration? Will this make it more accurate on the process sample?

SUBRAMANIAN [last name deleted for privacy by Editor]
- Singapore
March 2, 2009

March 4, 2009

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,

Multiple threads merged: please forgive chronology errors :-)

Q. I just want to know the standard pH of deionized water if there is.

Thank you very much.

Ghani Gonzales

Isagani Gonzales
King Faisal Specialist Hospital & Research Center - Riyadh, Saudi Arabia

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.

James Watts
- Navarre, Florida

Q. 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
March 11, 2008

pH of electro-deionised water output decreases in storage

Ed. note: We found the proprietary parts of this presentation a bit annoying, but if you start at 0:56 and stop at 4:50, it seems to offer a very good explanation of Electro-deionization :-)

Q. Dear experts, I will like to seek some help. I am using an electrodeionised water system(CEDI). The output of CEDI pH is 7. I have a bladder tank which is used to hold the deionised water from the CEDI before pumping to production. As deionised water does not comes in contact with the air, I do not understand why the pH drops to a range of 4-5, although the resistivity maintains. All pipings are of stainless steel material. Can anyone enlighten me if its normal ? Is any form of chemical dosing required to maintain the pH over the pipelines ?

Steven Lau
- Malaysia
June 27, 2016

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.

Lyle Kirman
consultant - Cleveland Heights, Ohio
August 26, 2016

Q. I have a 40% dextrose aqueous solution
I have to measure the pH of this at 10% solution.
If I dissolve it in distilled water, it gives pH = 6.95.
If I dissolve it with deionized water, it gives pH = 4.5.
I understand the incidence of CO2, the different origins of the distilled and the deionized but I do not come to an explanation.

sil danor
- Montevideo, Uruguay
July 20, 2017

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?

Brian Terry
Aerospace - Yeovil, Somerset, UK
July 21, 2017

thumbs up sign 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.

sil danor [returning]
- Montevideo, Montevideo,Uruguay
July 25, 2017

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.

David Lubiner
- Lincoln Rhode Island
December 14, 2017

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