Reaction of NaOH with Chromic Acid

[sanubala]

I want to know how dilute solution of NaOH reacts with ppm levels of Chromic Acid. I would guess its a straight acid base reaction. Am I right?

 

[kenvlach]

In dilute aqueous solution, at neutral and alkaline pH, chromic acid is present as the yellow chromate ion, CrO₄^-2. The neutralization may be written

H₂CrO₄ +2 NaOH = Na₂CrO₄ + 2 H₂O

If your initial solution is acidic, you will observe a color change upon neutralization, as the orange-red dichromate ion Cr₂O₇^-2 reacts to form yellow chromate:

Cr₂O₇^-2 + OH^- = 2 CrO₄^-2 + H⁺

The neutralization reaction for an initially mildly acidic solution* is therefore

H₂Cr₂O₇ +4 NaOH = 2 Na₂CrO₄ + 3 H₂O

Both sodium chromate Na₂CrO₄ and sodium dichromate Na₂Cr₂O₇ (which can be formed at strongly acidic pH*) are highly soluble. For removal of hexavalent Cr as part of a wastewater treatment process, it is best to either remove it via ion exchange or reduce it to trivalent Cr prior to neutralization.