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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: umegard on January 17, 2007, 03:44:59 PM

Title: Can pure water have other pH than 7?
Post by: umegard on January 17, 2007, 03:44:59 PM

If I were to remove H+ ions from my destilled water, what would happen to the pH?

I other words, can you construct an acid/base that is just water with a non neutral pH?

Title: Re: Can pure water have other pH than 7?
Post by: Dan on January 17, 2007, 03:48:35 PM

If you remove the H⁺, you have to replace it with another cation, and then you don't have pure water. Eg. Replace H⁺ with Na⁺ and you have a sodium hydroxide solution.

Title: Re: Can pure water have other pH than 7?
Post by: DevaDevil on January 17, 2007, 05:29:14 PM

1 - there is no such thing as 100% pure water, even triply distilled water, or the purest Millipore water has some ions / contaminants.

2 - Kw (water equilibrium (heard of it?, then apply Le Chatelier's priciple; remove the H+ and there will be made more of it from OH-, as well as water will split back up in H+ and OH- to resupply the concentration of these 2 ions)

so answer is very easily: NO


To change pH add acid or base, like sodium as Dan said

Title: Re: Can pure water have other pH than 7?
Post by: umegard on January 19, 2007, 05:23:25 PM

Thanks for your answers!

Title: Re: Can pure water have other pH than 7?
Post by: Mitch on January 19, 2007, 06:59:21 PM

If you could selectively remove H⁺ from water the pH would go up obviously enough. But, the more interesting observation would be to see what a liquid with excess negative charge that is electrically isolated from its environment would do if left to its own devices.

Title: Re: Can pure water have other pH than 7?
Post by: maakii on January 30, 2007, 11:13:36 AM

Kw is also dependent on temperature, if temperature is increased, Kw will also increase, and there are more OH- and H+ ions. Thus, pH will change.

However i am not sure if it increases or decreases, can anyone help me with this?

Title: Re: Can pure water have other pH than 7?
Post by: Borek on January 30, 2007, 11:40:50 AM

In general Kw increases with temperature, pH of pure water goes down with temperature. I don't remember exact details and I can't locate them right now. I think CRC handbook contains table of Kw vs T.

Title: Re: Can pure water have other pH than 7?
Post by: Ψ×Ψ on January 30, 2007, 08:56:28 PM

Quote from: maakii on January 30, 2007, 11:13:36 AM

Kw is also dependent on temperature, if temperature is increased, Kw will also increase, and there are more OH- and H+ ions. Thus, pH will change.

However i am not sure if it increases or decreases, can anyone help me with this?

Excellent point!  I'd completely forgotten about that!

Title: Re: Can pure water have other pH than 7?
Post by: maakii on January 31, 2007, 04:39:22 AM

Thanks!

Hey everyone i found this,

T (°C)   Kw (mol2 dm-6)   pH
0   0.114 x 10-14   7.47
10   0.293 x 10-14   7.27
20   0.681 x 10-14   7.08
25   1.008 x 10-14   7.00
30   1.471 x 10-14   6.92
40   2.916 x 10-14   6.77
50   5.476 x 10-14   6.63
100   51.3 x 10-14   6.14

So in summary pH goes down as temperature increases, but the solution is not considered more acidic, as the number of OH- ions increases as well. The new pH value of pure water at the new temperature is now considered the new "neutral value".

To read up more go here : http://www.chemguide.co.uk/physical/acidbaseeqia/kw.html