Deionized Water

The maximum resistivity of water is related to the presence of ions produced by the simple equilibrium dissociation of water into H+ and OH-, and the theoretical total absence of any other ions. To be concise, resistivity varies with temperature, so it is correct to place a temperature alongside the resistivity reading for it to have a true meaning. Resistivity increases with falling temperature.

Absolutely pure water, a theoretical concept known as Kohlrausch Water, is not achievable in practice, though there are a number of plants across the globe producing water at around 18.2 MΩ-cm at 25degC, 24h/day, 365d/year completely automatically, principally for the semiconductor manufacturing industries. This purity level is required as anything less causes silicon wafer yield to drop away. Water at this level of purity begins to degrade rapidly as, being a polar solvent practically with no other ions in it other than H+ and OH-, the inadvertent presence of any ion reduces the resistivity. Also, having no resilience against bacteria in itself, it is an ideal medium within which these may multiply.

Probably the best material for piping for water at this level of purity is polyvinylidene difluorde [PVDF] and there is a close relationship between quality and value.

A typical ultrapure water plant could have as many as 17 stages of processing between town mains and product. Design and construction of production and circulating systems for this grade of water requires a certain level of specialism, investment and risk, the correct balance for which is probably best achieved by discussing with a specialist systems supplier and entering into a contractual arrangement to supply. Watercare International can certainly help, and other suppliers are available.

Interesting...tricky stuff water.

Thanks.

But I'm still curious on how the figure 18.31 is derived.....any idea?

In terms of DI water being a corrosive liquid, i have gotten different feedbacks from different specialist about this. Some claimed that copper piping wouldn't really post any problem but others are against using it.

We have a detergent dispenser system running on metering pumps that uses copper piping that carries DI water to flush the detergent. I was against this design because as far as i know, DI water would degrade the piping and probably leach impurities into the water. Our main lines are made of Sch 80 PVC pipes (to save cost). We have the system running for a week now and we are still able to achieve a reading of 17.6 Megaohm at the point of use.

Detergents, being part polar and part non-polar, will have a low resistivity when diluted as the molecule ionises, so the absolute purity of a dilution water supplied for it would not be critical as far as resistivity downstream of the dilution point is concerned. So 17.6MΩ-cm at the point of dilution is probably of little significance in this particular application, as is the use of copper piping, unless the process downstream of the unit has a particular requirement for high purity that has not been posted here.

PVC has the ability to leach plasticisers into the process stream over time, and would not be the best choice of piping material to maintain semiconductor ultrapure water quality. As indicated here, it is probably satisfactory for the application posted.

Yes, DI water is extremely corrosive. This is because water naturally tends to dissociate into hydrogen and hydroxyl ions, and the ions in turn tend to exchange with any polar substances in contact with the water. That is why you shouldn't use copper pipes for the purpose.

Hello,

pl.note that during dimeneralisation ( means mineral free )procedure ,all chemical impurity of feed water are absorb by the cationic/anionic resin .thus treated water called DI (De ionise means ions free)water. And thus water become hungry ,so DI water always try to get content from out side i.e.from container ,pipe line ,atmosperic gases etc. when asmosperic co2 absorb by water ,than same water converated in to waek carbonic acid ,as a result its corrosive .

narendra patel

for aquaion technology inc

Theoretical resistivity of ultrapure water is 18.18 Megaohm at 25 Celsius degree. Please, always write both (resistivity/conductivity & temperature) together.

Ultrapure water is easily available with current technology even in an industrial environment. Actually, we have our own ultrapure water loop for calibration purposes and it was not so difficult to build.

To get valuable information on the theory of resistivity of ultrapure water, please see this website:

http://glo.mt.com/mt/filters/applications_process-analytics-applications_process-analytics-purification/Water_Purification_application_browse_0x0002498300028705000435a6.jsp?als=thornton

2. Is it true that although DI water have a theoretical pH of 7.00 but they are in fact corrosive by nature? People have advised me not to use copper piping for DI flow because of corrosion issue.

Don't confuse pH (acidic-caustic) with corrosion. A pH level of 7.0 indicates the water is neutral, neither acidic nor caustic, but it doesn't indicate purity. Perfectly pure water will be neutral because there is nothing disolved in it to affect the pH. I have a swimming pool that I strive to have at @ 7.2 to 7.4 pH. It fluctuates with rain, time, use, sunlight, etc, so I have to add acid to lower the pH or baking soda to raise it toward the target. I gurantee that if the pool water has a 7.0 pH it isn't pure!

As has been noted "pure water becomes hungry" or ion hungry so when it contacts material that has a relatively loose hold on its ions the water pulls them away from the material causing "corrosion". Same thing happens, for a different reason, when water flows through a buried metal pipe or a ship moves through water. This is why cathodic protection systems are used to negate the ion exchange and the corrosion.

DI water is corrosive by nature like all the water. Water is corrosive, at least if there is no acid or other oxidant, 'cause there is oxygen dissolved that act as a cathodic reaction. Is not important that the pH is 7. If you can remove all the oxygen from the water you will stop corrosion.

I've found a table regarding corrosion resistance of Cu and Cu alloys, that has data regarding Distilled water: http://www.cda.org.uk/megab2/corr_rs/corrosio.htm . you can see that @20 °C there is no corrosion if there is no Oxygen or CO2 dissolved.

hope this help

regards