chromic acid test and aldehydes

[jmarsh]

i was working on an identification of functional groups lab and was doing a chromic acid test on an unknown - it was soluble in water, had a neutral pH and bright orange precipitate for the 2,4 D test - all was fine UNTIL - i did a chromic acid test to determine whether it was an aldehyde or ketone and it turned dark murky brown and got pretty hot - not what my lab mannual said it was going to do and not the reaction caused by my known compounds - i tried again, thinking the error was my own - the second test produced the same result - i have surfed around trying to find an answer - no luck - my prof identified it as an aldehyde and now i'm wondering why not green - i cannot say that something did not get mixed up the reagents were under the hood and many people used them that evening, the reagents were for bromination and lucas as well

[John Cuthber]

If you add too much reagent you might get a mess of mixed chrome(III) +Chrome(VI) compounds.