pH of MilliQ and De-ionised Water - isn't it supposed to be 7 (Sep/13/2007 )

Out of curiosity, I measured the pH of milliQ Water from our older MilliQ machine (don't know what they are called). It showed around 6.3 with the pH strip.

We have a new machine and this also shows 6.3.

Isn't it supposed to be 7?

Has anyone checked pH of MilliQ they are using?

-Bungalow Boy-

The pH of high purity water is very poorly controlled. Any trivial amount of any contaminant will shift it many pH units. This is why you use buffers. pH of high quality water is often controlled by the dissolved carbon dioxide from the air, which leads to an acid pH. But I'll bet the pH you are really reading is the pH of the last solution someone made up and measured with your pH meter.

-phage434-

see this thread too

-fred_33-

Thank U Fred_33 and phage434, That was helpful. I am clear now. No doubts.

-Bungalow Boy-

I heard somewhere or read somewhere that MilliQ or deionized water is slightly acidic ..... but pH 5 or 6 seems more acidic

-mysterious-

QUOTE (tfitzwater @ Sep 30 2005, 06:54 AM)

MilliQ water absorbs CO2 from the air. The value of [CO2(aq)] in water at 25º C in equilibrium with air that is 350 parts per million CO2 (the current concentration of this gas in the atmosphere) is 1.146 x 10-5 moles/liter (M). The carbon dioxide dissociates partially in water to produce equal concentrations of H+ and HCO3-: pH = 5.65. (http://web.indstate.edu/thcme/mwking/ionic-equilibrium.html)

I did not attempt to do the calculation but this is from the link Fred_33 provided

http://www.protocol-online.org/forums/inde...?showtopic=9911

-Bungalow Boy-

I think the only way that you'll get pH7 MilliQ water is if you make it in a 100% inert atmosphere. Of course, as soon as you expose it to the air, your pH will start to go south. which is one reason why it's only good for a few hours, unless you are making a buffer.

-swanny-

QUOTE (swanny @ Sep 13 2007, 04:41 PM)

I agree with swanny; CO2 of air acidifies water; so degasing may reduce CO2 and carbon acid but you may not reach pH7

-The Bearer-