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topic 32443

pH of DM water is a mystery

A discussion started in 2004 but continuing through 2018


Q. I am generating demineralized [DM] water every alternative days. At that time we are maintaining pH is above 7 by adding pH boosting chemical, it has recommended by Thermax company. After adding this pH booster pH is coming above 7, next day of addition pH boosting chemical, pH is coming down, why?

Uday Bhaskar Rao
copper - Tuticorin, Tamil Nadu, India

simultaneous 2004

A. As the de-ionized [DI] water is free of minerals, there is nothing to buffer the pH. Over time, the water is absorbing carbon dioxide [CO2] from the air to form carbonic acid which drops the pH. In order to prevent this, you must prevent the water from coming in contact with the air by using a nitrogen blanket or some other means.

Chad Dannemann
- Houston, Texas


A. Unless I am mistaken, it is no mystery. It is absorbing carbon dioxide out of the air which becomes carbonic acid in water. This will not happen if you can keep a small amount of nitrogen [N2] contained above the water. This can be a pain, however. Another method would be to slowly bubble nitrogen thru an aquarium air stone on the bottom of the tank. N2 is rather cheap in the USA.

James Watts
- Navarre, Florida


A. Uday, I assume you are talking about demineralized/deionized water. Not sure what is going on with you stabilizer chemicals. I would think that would defeat the idea of demineralizing. You could be experiencing the uptake of carbon dioxide from the atmosphere. DI water is so very pure that it takes very little to change it's properties. CO2 from the air will lower the pH. Try an experiment. Put a little phenolphthalein [PHT] in a glass of water and blow air through a straw into the water and see if the indicator changes color (I think it was PHT. Chemistry 101 was a while ago).

Trent Kaufman
Trent Kaufman
Galva, Illinois


Q. Dear sir,

Here we have a DM plant, it was commissioned in June, 2004. From the commissioning time we are getting dm water only 50% of the plant capacity. It has designed for 600 m3, but we are getting only 300 m3. What is the actual problem, whether raw water or resin? And what is the resin life? This is new plant, so we should get as per design. Can you say what is the problem actually?


Uday Bhaskar Rao [returning]
copper - Tuticorin, Tamil Nadu, India

simultaneous 2004

A. Uday,

Not enough information given. When the plant was designed, it was rated on a given quality of input water. If your water is of lower quality than the design standard, you will not get as much output between recharges. If you are using chlorinated water, you need to put an activated carbon bed in front of the treatment tanks to remove the chlorine and other organics.

James Watts
- Navarre, Florida


A. Dear Sir

Your DM consists of cationate and anionate; if either of them is not sufficient or weak you may have higher or lower pH.

If your water output is not as calculated --

1- your resin is not enough

2- water minerals too high 3-water not passing through all the resin bed, backwash may help resin can work for years, if it was not damaged by excessive heat or cold.

Good luck

Cair Shishani
Cair Shishani
aircraft maintenance
- Al Ain, UAE



SBA = strong base anion
SAC = strong acid cation
MB = mixed bed

Q. Dear sir,

First, I would like to say thanks to everybody. In our DM plant, mixed bed has capacity to run 3 times more than SBA and SAC. Normally we are doing regeneration to MIX BED after three times of SBA and SAC regeneration only. Frequently MB is failing. Plant capacity is 600 cubic meter and Plant running capacity is 30 cubic meter. I checked raw water parameters, all parameters are within designed range. Why MB is failing always? We are following same regeneration procedure. Please tell me what is happing with MIX BED.


Uday Bhaskara Rao [returning]
- Tuticorin, Tamil Nadu, India


Q. We have installed DM plant
Currently we are facing the problem psudomonas in DM plant?
What is the solution?
What are the resin properties?
Can we do the sanitization with 85 water?

- Nasik, Maharashtra, India



OBR = output between regenerations

January 2, 2010

Q. Hello sir.

I want to know how we can calculate OBR of the DM Plant and how we will identify that which bed have discharge.

Arun Tripathi
water treatment technology - Gorakhpur Uttar Prades

November 1, 2011

Q. How to calculate OBR Ion Load in DM Plant.

Dilip Raja
- Hyderabad

Hi Arun. Hi Dilip.
Topic 30932, "Ion Exchange / DM plant calculations" is a long and detailed thread on that topic. Let's keep this thread about operational problems and that one about how to do such calculations. Good luck.


Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

January 16, 2010

Q. Dear Sir,

We are facing problem of frequent regeneration of mixed bed's resulting in excessive consumption we are also having 2 ACF & the chlorides for the raw water is in the range of 30-60 mg/lt.


Amol Bhalerao
- Aurangabad,Maharashtra India

November 7, 2011

Q. Dear sir,
We are manufacturing DM water for drinking purposes and keeping the pH at 7.9 prima facie. The water is clear but on keeping it for 2 weeks it turns green and on the top it shows a white thin film why is it so? Please clarify and tell me the remedy.

D. Chandran
minihosp - Cochin, Kerala, India

May 18, 2012

Q. Dear Sir,
We are having a DM Plant having capacity of 10M3/Hr, Stream contains Strong Acid Cation-Degasser-Strong Base Anion- Mixed Bed Unit, now pH leaving the Mixed Bed unit is around 6-7, and Conductivity is around <1 Microsimens, we want to boost this pH up to 9 by using Morpholine, but our worry is that by adding adding 1 PPM of Morpholine how much PPM of TDS/Conductivity will increases as well as by adding 1 PPM of Morpholine what will be the range of pH increased.

Ashish R. Davale
- Mumbai, Maharashtra, India

December 13, 2012

Q. Hi..To all,

I Have a small DM Plant (3m3/hr). I am facing a problem that after regeneration of anion column we are unable to maintain the pH. After 3 hour rinsing it's not coming <11.

So I request to all please tell me what can the problem be?

Arun Tripathi
- Delhi, India

January 7, 2014 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Actually, my DM Plant will have the pH valve ranging from 6.5 to 7.5 which I'm using for my Pretreatment process and after 3 years of Plant running with 40 KL output for every alternative day. Now, after regeneration the pH valve obtained is 6 and after sometime it drop to 3.5. I need to know the reason for this.

Arunkumar Ramalingam
- Chennai, India

January 12, 2014

A. Hi,

The pH change which you describe it absolutely typical of DM water. The water absorbs carbon dioxide from the air becomes acidic. If this does not happen then you are not dealing with DM water! A far better guide to quality is the measurement of conductivity. Can you measure conductivity and if so what reading do you get?


Harry Parkes
- Birmingham, UK

January 15, 2014

A. The previous answer is correct in that conductivity or resistivity is the best way to measure DM water quality. Standard pH meters are not able to measure the pH of high purity water accurately. There are specialized cells and probes that can measure pH in these situations. However, if the water has a conductivity of >5 uS, then most pH meters are fairly accurate.

While it is true that high purity water will absorb CO2 with a drop in pH, it is unlikely that this will result in a pH <5. This would only occur if the CO2 is super saturated. If the pH is actually 3.5, it is probably due to something else.

Lyle Kirman
consultant - Cleveland, Ohio

April 28, 2014

Q. Hello All,

I want to know DM water parameters and its limits. How to test and control parameters? Suggest me.

Surya Narayana
Process Engineer - Tumkur, Karnataka, INDIA

Ion Exchange Materials
from Abe Books


May 2014

A. Hi Surya. You have answered a number of questions from other readers recently and we would like to return the favor by answering you, but your question is very broad...

The basic principle of DM/ion exchange is that a column full of resin is loaded up with acid, and another is loaded up with caustic, and then water is passed through them in sequence. The H+ from the acid column replaces the cations in the water, the OH- from the alkali column replaces the anions in the water and you are left with purer water. If the water is still not pure enough for the particular application it can then be passed through a mixed bed column which contains both cation and anion resins, and which works as if it were thousands of cation-anion-cation-anion-cation-anion columns in series.

Important issues are the flow rate which must be sustained, how much minerals are in the incoming water, and how pure the outgoing water needs to be. With these factors a system can be designed. Your city library will have an entire shelf of books on water treatment, each of which will probably have at least several chapters on demineralization. Good luck.


Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

May 23, 2014

Q. Ted, thank you so much......

My question is DM/DI water parameter limits for plating process.

1. pH
2. Chlorides
3. Total Dissolved Solids
4. Conductivity

I want to know Upper limit & Lower limit of above parameters for good results.


Surya Narayana [returning]
Process Engineer - Tumkur, Karanataka, INDIA

May 28, 2014

A. The parameters vary, depending upon the application. For most general electroplating applications, ASTM Grade IV specifications are more than adequate. However, in many more critical, microelectronic and semiconductor applications, more stringent parameters are needed.

Try a web search on ASTM Grade water specification.

Lyle Kirman
- Cleveland, Ohio USA

March 13, 2015

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

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

July 27, 2015

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 29, 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

October 28, 2015

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 29, 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
- Cleveland, Ohio USA

December 18, 2015

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 21, 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

August 31, 2016

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

November 29, 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

September 9, 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

February 3, 2017

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

simultaneous February 14, 2017

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

February 14, 2017

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
- Cleveland, Ohio USA

February 14, 2017

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

April 18, 2018

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
Plating Solutions Control Specialist / Industrial Metals Waste Treatment - Brattleboro, Vermont

July 10, 2018

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 & hellip; 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

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