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pH of DM water is a mystery

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 replies

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 [on eBay & Amazon (adv.)] [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
electroplater - 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 replies

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
Khair 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

sidebar notes


OBR = output between regenerations

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
January 2, 2010

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

Dilip Raja
- Hyderabad
November 1, 2011

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

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
January 16, 2010

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
November 7, 2011

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 Microsiemens, 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
May 18, 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
December 13, 2012

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 7, 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

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 Heights, Ohio

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
April 28, 2014

"Ion Exchange Materials"

on AbeBooks

or Amazon

(affil links)

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

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 23, 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
consultant - Cleveland Heights, Ohio
May 28, 2014

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