Basic chromium sulfate is produced from chromate salts by reduction with sulfur dioxide, although other methods exist. The reduction could formally be written: Na2Cr2O7 + 3 SO2 + H2O Cr2(SO4)3 + 2NaOH Since 33% of the anion charges are due to hydroxy ions the basicity is 33% (but in tanning jargon it is known as 33% reduced). Products with higher basicities, e.g. 42 or 50% may be obtained by the addition of sodium carbonate, these are often used in combination with sodium formate. The sodium sulfate is often left in the technical product since it is inert with respect to the tanning process. It is important to fully reduce the hexavalent chromium to trivalent since the hexavalent is more likely to cause health problems for tanners and leather consumers.