It is possible to do it with bleach and lye, but I don't really like that method.
First of all, you shouldn't have reacted the chrome alum with an excess of NaOH because Cr(OH)3 dissolves in excess NaOH as Cr(OH)6 3- ions.
My favorite method is combining stoichiometric quantities of KOH and Cr(OH)3 with a 10-20% excess of H2O2 (3% stuff from the pharmacy works, but
afterwards you need to boil a lot of liquid) after the reaction: Cr(OH)3 + 3 H2O2 + 4 KOH = 2 K2CrO4 + 8 H2O
After doing this you need to boil the liquid until only a very small amount of liquid remains (this contains the KOH that might remain) and you'll get
pure K2CrO4 powder.
I prefer using potassium salts because sodium chromate is quite hygroscopic, but you most probably can also make Na2CrO4 this way (I didn't test that
reaction though).
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