What is the pH of water?Is it vary with Temperature.if varies how much?

pH of pure water is 7.All acids have a pH that is lower than 7 and bases have a pH that is higher than 7. As everyone know water is not considered to be an acid(or) a base it is neutral.

The potential of electric for positive Hydrogen ions or pH, can be important for a variety of reasons,especially for guessing electrolytic properties it is determined from H3O+ concentration produced from the dissociation of acid (or) base in water.

A constant ratio of H3O+ ion to OH- ion exist in water at any time .such that

[H3O+] [OH-]=1 x 10-14 Mol/L at 25°C

when an acid (or) a base is added ,it increases the value of Hydrogen ion (or) Hydroxide ion respectively.This creates an imbalance in the pH, which makes the solution either acidic (or)basic.

What is the relation between pH and Temperature

2H2O = H3O+ + OH- ΔH =+57 kJ/Mol (it is endothermic reaction)

Ionization constant Kw =[H3O+] [OH-]/H2O

As Temperature increases Kw increases with increase in [H3O+]

pH = -log10 ([H3O+])

Increase in [H3O+] there is decrease in pH.

So, Increase in Temperature there is decrease in pH

If pH falls as Temperature increases this does not mean that water becomes more acidic at higher temperature.A solution is acidic if there is an excess of Hydrogen ions over Hydroxide ions. In the case of pure water , there are always the same concentration of Hydrogen ions and Hydroxide ions and hence,the water is still neutral (pH=pOH) even if its pH changes.

The problem is that we are all familiar with 7 being the pH of Pure water, that any thing else feels really strange. Remember that to calculate the neutral value of pH from Kw.

Temperature Kw pH pOH

0°C             0.114 x 10-14          7.47             7.47

10°C 0.293 x 10-14 7.27 7.27

25°C 1.008 x 10-14 7.00 7.00

100°C 51.3 x 10-14 6.14 6.14

Even when the Temperature is varying from 0°C to 100°C the pH is going to vary from 7.47 to 6.14. and it is based on ionization constant.

Previously I used to think pH of pure water varies from 4 to 9 based on the place and Temperature

An expert is someone who knows some of the worst mistakes that can be made in his subject and how to avoid them

-Werner Heisenberg

pH plays a critical role in Chromatography, method transfer between laboratories and Column chemistry.