pH adjustment of 3.5% NaCl solution - (Jul/02/2015 )

I have made a 3.5% NaCl solution using distilled water. I am trying to raise the pH of this NaCl solution from its actual pH 6.8 to 7.8 using 0.1 M NaOH solution. The problem I am facing is that once the pH value reached ~7.8 then after one day it drops back to its original pH of 6.8. I have also tried to adjust the pH using 1 M Na2HPO4 buffer solution having  pH ~9.0. But the problem remain same. In this case after one day the pH drops down to 7.5.

Can you give some suggestions about how can I increase the pH value of this solution to 7.8 so that it remains fix and does not change. Looking forward for your help. Thanks a lot.

The problem is that you are not buffering the pH, and there is CO2 in the atmosphere which acidifies solutions. You need to buffer your solution at 7.8. Instead of adjusting the pH with NaOH, you should instead add 10 mM of Tris-HCl adjusted to pH 7.8 (assuming your application allows Tris as a buffer).