preparation of solutions that need pH adjustment - (Apr/08/2005 )
I generally use the method below when to prepare solutions that need pH adjustment.
Weigh the chemicals, put them in a beaker and add distilled water to a volume below the final volume (e.g. 450ml water for a final volume 0f 500ml). Dissolve the chemicals and adjust the pH with NaOH or HCI then complete the volume to the final volume (e.g. 500ml)
However the volume at which I adjust the pH is below the final volume and I think the pH may change when I add more water to reach the final volume.
Is there another method to prepare such solutions ?
It depends on how accurate you want your concentrations of chemicals in solution. I usually just make up the full volume with dd.H2O and then pH adjust. I dont think distilled water will really have that much of an impact if your only adding a small amount.
quite a few of the solutions that need to pH'ed are buffers, so if you make up the solution to the final volume the pH will not drastically change.
assuming that ddH2O does not contain other than "H2O" molecule, it theorically does not change pH after addition. But if you prepare your solutions in order to have few ddH2O to add in final your pH will be the required one.