Imran Ahmad As far as increasing pH is concern, the use of buffer is recommended. Use inorganic buffers, one best example will be phosphate with pKa at 6.8 at room temperature. low range would be 15 to 20 mM and high range will be from 50 to 200 mM. you can adjust the pH +/- 1 pH unit meaning from 5.8 to 7.8, beyond this range it would not be useful. If you digesting for heavy metal determination, check the metal content of the buffering species 1st as it normally contained very low amount of heavy metals as impurities.

You should follow inorganic chemistry practical book

thanks a lot for the guidance Dr Shukor and Dr farid