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"Factors affecting the Quality of Ultra Pure DI water"



 

Q. This is a question concerning about the quality of DI water. I have few doubts,

1. What is the quantity of ions required to bring down the resistivity of a 18 M Ohm DI water by 1 Mohm (17 ohms) / 18 to 10 M ohms / 18 to 1 Mohms. Is there any general thumb rule?

2. Does different ion have different conductivity? How will they affect? Does 1 ppm of Na+ bring down the conductivity as 1 ppm of K+?

3. How much does the resistivity go down when 18 M Ohm water comes in contact with air? (At what point does it stabilize.

4. Can we measure the resistivity using bench top Resistivity meter or Pocket resistivity meter. Do we need a simple electrode or a flow cell is required? How to collect sample for resistivity measurement?

5. We do contamination testing on the parts washed with High purity DI water. (IC, FTIR, NVR) the test shows no presence of ions (Anions and Cations). The result is always below detectable limit. (less than 0.1 ppb). But when we monitor my online resistivity meter the changes are much it varies from 7-8 Mohms to 2 Mohms. What is that which is causing the variation? If it is ions why is it not detected in the contamination test(our machine is calibrated). (Or) Ions in very low concentration can cause the variation? 0.1 ppb?

Please give your expertise on this, this will also provide a base for knowing more about Ultra pure DI water behavior. Please contribute as much as possible to this question. Finishing.com should consider bringing a separate page or topic for DI water / Ultrapure water.

Thank you very much and looking forward to hear from all experts soon.

Thambidurai Karthik
- Singapore
^


 

A. Sounds like you are using an RO/EDI setup. Are you checking for silica as a contaminant. 18 megohm water will typically read about 1 ppb of silica. Silica is the main contaminant left after RO. Resistivity will change upon contact with air. An instrument, such as a Myron L Ultrameter 4P can be used to measure it. To get an accurate reading, you must let the sample source flow directly into the instrument sample cup with minimal air exposure. The water can not be expected to maintain a high resistivity after being released from the purification process.

Phil Pace
- Tulsa, Oklahoma
^


 

A. Karthik,

I don't have quantitative data for you, but I can verify the concerns you have on maintaining the purity of the ultra-pure DI water. Deionized water is an extremely aggressive solvent. When water is purified to the level you describe, it will dissolve anything it possibly can. Ultra-pure DI water will dissolve carbon dioxide and other substances from contact with air, and it will begin to attack the container material - even steel or plastic. I suggest you use a benchtop resistivity meter, not a pocket-version. Your application requires higher-than-normal precision. I would also advise trying to prevent your ultra-pure water from having contact with air. You will need to also research what type of material you should use to hold this water in.

tim neveau
Tim Neveau
Rochester Hills, Michigan
^


 

Q. Hi Phil / Tim,

Thank you very much for your valuable reply. We do have online measurement (flow cell) for measuring the resistivity of DI water and have instruments to estimate the trace ionics in the DI water. I just want to compare if the info we get from the instrument is correct. That is the reason I was looking for quantitative data. Also I want to know more about DI water (in depth) and its performance when trace contaminants are present. Hope some one can provide with the data so it will be valuable reference for anyone who wish to know about this.

Thanks,

Thambidurai Karthik
- Singapore
^


 

A. Hi Karthik,

I just want to set the record straight re Tim's statement about plastics being attacked. In the field of the chemical resistant plastics (which sure doesn't include nylon nor styrene type products) there is a vast plethora of suitable plastics for D.I. water.

ABS, yes, even that 'lowly' material is said to be OK @ room temperature .... but then with PVC, Pe, PP, CPVC, PVDF, etc. and all the fluorocarbons, they are said to excel to their max. temp. ratings... which in the case of PVC is 140 degr. F or 60 degr. C.

18 Meg Ohm d.i. water sure looses potency fast when exposed to air! Consider, please, floating lids for round tanks, OK? Easy to make. A flat plate with a 2" turn-up and a central grab knob. For deep tanks put a hole in the grab knob and use some Polyethylene Terephalate rope, commonly known as Terylene to the boating and yacht cognoscenti.

The only rider I would add is that when one talks about PVC one means uPVC, grade l, type l, ie. the standard unplasticized pipe material. If you use vinyl tubing, ah, you could well get some leaching effect (ammonia does this nicely!) which should result in a loss of plasticity, sic. flexibility ... but that should be ALL !

Having got that off my chest I can now go and sleep peacefully !

freeman newton portrait
Freeman Newton [dec]
(It is our sad duty to advise that Freeman passed away
April 21, 2012. R.I.P. old friend).

^


 

Q. Karthik

Did you ever get quantitative answers to these questions ?

If so could you send me a copy ?

Thanks

Richard Foote

1. What is the quantity of ions required to bring down the resistivity of a 18 M Ohm DI water by 1 Mohm (17 ohms) / 18 to 10 M ohms / 18 to 1 Mohms. Is there any general thumb rule?

2. Does different ion have different conductivity? How will they affect? Does 1 ppm of Na+ bring down the conductivity as 1 ppm of K+?

3. How much does the resistivity go down when 18 M Ohm water comes in contact with air? (At what point does it stabilize.

Richard Foote
- Arlington, Texas
^


May 14, 2009

A. "1. What is the quantity of ions required to bring down the resistivity of a 18 M Ohm DI water by 1 Mohm (17 ohms) / 18 to 10 M ohms / 18 to 1 Mohms. Is there any general thumb rule?"

It's a definitive issue:
18 meg is 0.0277 mg/l TDS
17 meg is 0.0294 mg/l TDS
10 meg is 0.0500 mg/l TDS
1 meg is 0.500 mg/l TDS

"2. Does different ion have different conductivity? How will they affect? Does 1 ppm of Na+ bring down the conductivity as 1 ppm of K+?"

By those definitions, yes. By weight may give you a different answer -- but as seen above, you're not talking in 1 ppm ranges to significantly affect your results. Note also (as mentioned earlier) that CO2 dramatically affects water quality, as does anything it comes in contact with - so your method of handling can be as important as the contaminants you introduce during rinsing.

"3. How much does the resistivity go down when 18 M Ohm water comes in contact with air? (At what point does it stabilize."

Essentially, it doesn't - not for your purposes. By its' very nature, it will continue to absorb anything it can from its' surroundings. It's a function of time and contamination sources (air, containment, etc.).

George Mandry
water conditioning - San Antonio, Texas
^


November 11, 2009

Q. What parameters in the feed water (after activated carbon filter) should I send for analysis to check that might have affected the DI water resistivity of Ions exchange resins causing low resistivity (10 to 14 Mohm.cm) instead of getting 17 to 18 Mohm.cm? Please advise.

Goh Zhao Hua
- Singapore
^


April 12, 2010

A. "What parameters in the feed water (after activated carbon filter) should I send for analysis to check that might have affected the DI water resistivity of Ions exchange resins causing low resistivity (10 to 14 Mohm.cm) instead of getting 17 to 18 Mohm.cm? Please advise."

Really need to know more specifics re. your water system design and operations. Can you ever attain 18 meg? Have you in the past? Please note that simply putting deionization resin in a water stream isn't necessarily going to result in an 18 meg water output - there are any number of variables involved! This is usually something where you need someone that KNOWS DI systems in to look over what (and how) you are doing things.

George Mandry
- San Antonio, Texas
^


June 1, 2012

Hi there!

A1. Resistivity decreases proportionally with TDS and ions present in D.I water.
A2. Different ions have different conductivity, usually we use 0.67 as a common factor whenever converting conductivity to TDS.
A2. Air greatly affects D.I water due to CO2 which leads to carbonic acid (reversible reaction)
A4. Choose bench top or online
A5. Please see calibrations

Mark Anthony B. Omadto
- Baguio City,CAR,Philippines
^

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