Hi Mohamed, ammonium oxalate is used to prevent precipitation of the dye (the so-called Hucker modification). If you don't intend to store the solution for longer than a few weeks, you can leave the oxalate out but need to keep an eye out for precipitation (perhaps filter before use). The solution for Gram staining is made as follows: dissolve 2 grams of crystal violet in 20 ml ethanol (>95%), add 80 ml of distilled water and mix well until fully dissolved. Filter this solution through coarse filter paper and store in a dark glass bottle.