Factors affecting the Quality of Ultra Pure DI water

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.

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.

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 °F or 60 *deg;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 !

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.).

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.

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.
A3. 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