I’m familiar with that test - it’s not very reliable, but can be useful if you know it’s limitations and how to interpret it.
This is from an old article by Phil Davis:
There's a well-known test that will reveal the presence of ferrous oxalate in the sensitizer: simply add a drop of weak potassium ferricyanide solution to a few drops of the sensitizer solution. No color change, or a very slight one, indicate that the solution is essentially ferrous-free; a pronounced blue color will result if there is ferrous oxalate present.Although this is an interesting and enlightening chemical test it’s almost too sensitive to be
practical. I certainly don’t want to encourage you to be careless or sloppy but you should be
aware that a trace of ferrous oxalate in the sensitizer (certainly more than enough to fail the
ferricyanide test) is not going to have any visible effect on your prints. If you discard your
solutions as soon as they react visibly with the ferricyanide you’re wasting money.As a rule of thumb, ferric oxalate solutions are probably usable (but not necessarily in first class
condition) as long as they remain completely clear and free from any trace of precipitation. But
don’t interpret this as license to use sensitizers that are truly spoiled. Any suggestion of
cloudiness or sediment is cause for real concern.Ferrous oxalate is certainly a serious contaminant but its gradual build-up in the sensitizer
solution worries me less than its much more obvious presence as a dry sage-green crust that’s
very likely to form around the mouth of the sensitizer storage bottle and on the inside of the
dropper cap. Flakes of this material that may fall back into the solution, or onto your freshlysensitized
paper, are genuinely worth worrying about. So are traces of ferrous oxalate that will
surely contaminate your coating brush if you don’t wash it immediately and thoroughly after
each coating operation.