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pH of DM water is a mystery

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Q. Sir,
How to check Incoming DM Water?
What is the test for DM water?
Difference between Raw water & DM water?

Faruk Hussain
- Chennai, Tamil Nadu & India
March 13, 2015

Final DM water quality (pH & conductivity) suddenly basic & high conductivity

Q. 6 M3/day DM water plant. Final DM water quality in storage tank suddenly increased, i.e., pH to 10.6 and conductivity 4000 µs within one day span. We have Ultrafiltration unit, Reverse osmosis unit and mixed bed unit, and finally water goes to DM storage tank. All the units discharge within specified limits. Our final pH and conductivity should be 6-8 and 1 µs.

sumeet Nagtode
Senior Engineer.. Automotive plant - Pune, Maharashtra, India
July 27, 2015

Q. Hi Harry,

I purchased DM Water and added 10 drops of indicator and passed CO2 by blowing manually and later from CO2 extinguisher cylinder. There is no change in DM Water colour. As per your previous statement in string above, "If this does not happen then you are not dealing with DM water!"

So now can you suggest what water may this be? Why I am doing the investigation: I am losing pH of the DM water very fast. I use this DM water for Rinse tanks of my Phosphating Line.

Subhajyoti Sen
- Bangalore, Karnataka, India
July 27, 2015

A. Hi,

On reading your submission I started looking for this new boy on the block who calls himself Harry! I found that it was me back in January 2014.

Please supply some additional information without which too much unnecessary guesswork is required.
1) what was the name of the indicator?
2) what pH range does it cover?
3) You say that there was no change when you bubbled carbon dioxide though the water. What was the unchanging pH value that you got?


Harry Parkes
- Birmingham, UK

Q. Dear Sir
My question is why pH is going down in ion exchanger (SAC, SBA, MB) and also resin is regenerated and brine treatment also done.

Abhishek jain
power plant - chittorgarh rajasthan India
October 28, 2015

A. As the anion resin in the Mixed bed becomes exhausted, the pH will drop and silica will leak out of the resin.

Lyle Kirman
consultant - Cleveland Heights, Ohio

Q. Our DM plant SAC, SBA, MB is performing well in conductivity (ranges from 0.1 to 0.6), but the pH of mixed bed outlet is 5.7. Our client insists to use DM water with pH 6-6.5 to manufacture their product. Anything can be done to boost pH from 5.7 to 6.2 without adding additives?

Manu Parameswaran
- Kochi, Kerala, India
December 18, 2015

A. Hi Manu,
I had a similar problem a few years ago. What was happening in my case was that the water was being pumped to a header tank and was dissolving atmospheric carbon dioxide, making a very weak acidic solution. Gentle heating to remove the carbon dioxide worked for me, moving the pH from around 5.5 to around 6.5.

This may not be practical for you; if not try to reduce the amount of churning of the water as it goes to the holding tank.

Brian Terry
Aerospace - Yeovil, Somerset, UK

A. Hi manu,
In this case you firstly check whether there is any open space in the pipe line to allow the passage of carbon dioxide. If not then use morpholine which is an organic material used to maintain the pH only. And it will increase the conductivity but don't be scared about that; it will be in your desired value. Good luck.

Anandh Kumar
Process Engineer - Thiruvarur, Tamil Nadu, India
August 31, 2016

A. Dear Manu Parameswaran,
After complete regeneration of MB when doing air mixing sudden settlement is necessary. If 2 types of resin don't mix well then heavy weight cation resin placed on bed. So, after air mixing sudden settlement is very important. You can try this I think your problem will be solved.

Thanking you,
With best regards,

Subhadeep Bhadra
- Kharagpur, west Bengal, India
November 29, 2016

Q. Sir, I have dm plant. Capacity is 2000 liters per hour. I have a small problem here, the problem is after regeneration. Time output of dm water pH is 10 and conductivity is more than 100 µS. Up to 1 hour. After one hour decrease the pH and conductivity. One time regeneration only 20000 liters coming. After 20000 liters the pH is coming down 4. What is the problem tell me?

Mahes reddy andhra pradesh
Victoria - Hyderabad telagana India
September 9, 2016

Q. We have been dosing a pH booster in the product of Mixed Bed Polisher. This booster, is intended to increase the pH to the desired level, however, it is increasing the conductivity of the water.
Can anyone suggest any chemical that will increase the pH but not the conductivity? I don't think so ... what is your opinion?

Muhammad Tayyab
- Karachi, Sindh, Pakistan
February 3, 2017

simultaneous replies

A. I don't think so, either.

You could minimize it by bubbling gaseous NH3 through the water, I think. You'd have to try it.

dave wichern
Dave Wichern
Consultant - The Bronx, New York

When the water resistivity is > 1 Megohm, a typical pH meter is not accurate and can usually be ignored.

The only chemicals that would change the pH will also increase the conductivity. You could try a polishing mixed bed resin tank and this should result in a pH of 6.8-7.2 (but may not be measurable) and a resistivity of > 10 Megohm.

Lyle Kirman
consultant - Cleveland Heights, Ohio

Q. My organisation is having a DM water plant to produce purified water. On line DM water plant pH meter has been calibrated with pH 7.0 and 9.20 and verified by pH 4.0. The laboratory pH meter has been calibrated and verified by the same solution, Still there is a difference of 2.0 in the pH measured by online pH meter and laboratory pH meter. I am not getting why. Please suggest.

Navinkumar Pandey
pharma - Palghar, India
February 14, 2017

A. It takes a special technique to accurately measure the pH of ultra pure water.

I believe one must add some KCl so that the water has some conductivity. Various companies sell kits with the needed solutions.

This might be of help:

Got lots of good service from Cole-Parmer.

dave wichern
Dave Wichern
Consultant - The Bronx, New York

Q. I have a DM water tank I have to recirculate the tank daily, but the outlet quality of water is acidic after regeneration for few days it comes correct value then after 2 weeks again its comes to acidic so please suggest me what to do.

Prem kumar
- Chennai, Tamil nadu, India
April 18, 2018

A. As Dave mentioned above, you've got to take extra care in measuring the pH of extremely low ionic strength solutions.
One phenomenon that you may be seeing the effects of is that of carbonic acid going into your water from exposure to CO2 in the air, and giving you a pH reading closer to 6 than to 7.

Rachel Mackintosh
- Greenfield, Vermont

Q. Dear all, we have DM plant in which we are not able to achieve design parameters at MB outlet. Conductivity at Mixed bed settled at 0.5 and 5.95 pH ... till SBA all parameters within range. SBA outlet conductivity is 0.9 and pH 8.9 still MB conductivity and pH not in range. Please suggest.

Rohan Shinde
- Chhbra, rajasthan, India
July 10, 2018

Q. Our DM plant SAC, SBA, MB is performing well in conductivity (ranges from 0.1 to 0.6), but the pH of mixed bed outlet is 5.7. Our client insists to use DM water with pH 6-6.5 to manufacture their product. Anything can be done to boost pH from 5.7 to 6.2 without adding additives?

Dheeraj KUMAR Dwivedi
Employee - Kolar Road , Bhopal
May 13, 2019

A. Hi Dheeraj,
As Brian Terry mentioned in 2015, you can heat the water gently to remove CO2 and get the pH up 6,0 to 6,5. Without adding anything, that's the only thing you can do.
Best of luck!

Daniel Montañés
N. Ferraris S.A. - Cañuelas, Buenos Aires, Argentina

A. A nitrogen blanket & purge to remove dissolved CO2 may help. It is very difficult to measure the pH of high purity water with any accuracy because there is no buffering. Specialized cells are needed. In most cases the resistivity is sufficient to establish that the pH is very near neutral (6.5-7.5 at 20 °C).

Lyle Kirman
consultant - Cleveland Heights, Ohio

Q. Even after rinsing for hours pH of water coming out from regenerated anion column is not coming below 8.7. Please can anybody gives probable causes and solutions to it. Thanks.

Uday Deshpande
- Enugu state of Nigeria
July 13, 2019

A. Hi cousin Uday. Several people on this long thread gave the most probable cause, i.e., that measuring the pH of deionized water is difficult and requires specialized equipment. If you are using specialized equipment, please describe what equipment you are using.

With a generic pH meter, please recognize that conventional pH measuring equipment relies on the availability of large numbers of ions to conduct the chemical "current", and when you remove those ions in de-ionizing columns but still try to read those currents accurately it's like trying to read millivolts on a 0-1000 volt meter or trying to judge how fast a snail is moving by pacing it with your car and watching the speedometer. Please study the page carefully. Good luck!


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

A. Dear Uday,
If you rinse with DG water for about 1 to 1-1/2 hours, you can get 8.2 pH in anion bed outlet.

dinakaran mg
Water chemist - Tuticorin, Tamilnadu, India

Q. We have demineralizer plant having counter current flow. All the way we found pH at the outlet of cation tower close to 3. But now we are getting the same above 7 though the sodium and calcium are found nil at the outlet of cation. Why?

Quddus AHMA
- Dhaka, Bangladesh
June 6, 2020

A. In a cation bed having Strong cation resin it adsorbed the hardness ions like calcium, magnesium (positive ions). Only these two ions determine the water hardness. Once raw water comes out from cation bed outlet, due to hydrogen ions the water is acidic -- the pH is low like 3 to 4.

Selva raj. R
- Kalpakkam
August 29, 2023

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