# Chromyl chloride test side products

I have read the test for chloride ions (chromyl chloride test). The reaction is: $$\ce{K2Cr2O7 + 4NaCl + 6H2SO4 -> 2CrO2Cl2 + 2KHSO4 + 4NaHSO4 +3H2O}$$ My doubt here is if chlorine gas is evolved in a side reaction. Is it evolved in detectable amount? The side reaction is: $$\ce{6Cl- + (Cr2O7)^{2-} + 14 H+ → 3Cl2 + 2Cr^{3+} + 7H2O}$$ Can someone please confirm this?

• Yes, in a very good old book about preparative chemistry it is stated that chlorine is a side product in the synthesis of chromyl chloride. – vapid Oct 24 '16 at 13:28
• Can you give the reference of the book? I am interested to check it out, assuming it's still in print. – Black Jack 21 Oct 24 '16 at 16:37
• Sure, I can give you the reference, but it is in Polish... – vapid Oct 25 '16 at 7:06
• In short, potassium dichromate (VI) is not strong enough to oxidse chloride ions. However, if potassium permanganate (VII) is used, the chloride ions will be oxidse because potassium permanganate (VII) is strong enough to oxidse it. For more explanation, I write my explanation in the answer section below. – Simon-Nail-It Oct 25 '16 at 11:38
• Chlorine gas will be evolved if there is water in the reaction mixture. In completely dry condition, the amount of chlorine evolved will probably be very small. – Shoubhik R Maiti Jan 28 '18 at 9:09

## 1 Answer

If you write the ionic equation, it seems that chlorine gas will produce. However, we see their half equation and standard potential.

$$\ce{ Cr2O7^2- + 14H+ + 6e- -> 2Cr^3+ + 7H2O}$$ Standard potential: $$E^\circ = \pu{+1.33V}$$

$$\ce{ 6Cl- -> 3Cl2 + 6e-}$$ Standard potential: $$E^\circ = \pu{-1.36V}$$

Thus, the standard potential cell for this ionic you say is equal to $$\pu{+1.33V} + (\pu{-1.36V}) = \pu{-0.03V}$$

Because the standard potential cell is negative, so, it is call not energetically feasible or this reaction will not happen.

P.S.: The explanation only work under standard conditions.

• As you noted the difference in potentials is very small, therefore some of the chloride can get oxidized as practice shows (some sources give 1.36 V value for both chlorine and Cr(VI) reduction). – vapid Oct 25 '16 at 14:46
• This is not correct. Potassium dichromate does oxidise chloride ion in solution in presence of acid. Unless someone specifically measures the reactants, they are not in standard condition( 1 molar) so, the general method of adding potentials does not work. – Shoubhik R Maiti Jan 28 '18 at 9:07