[leemix]

I have been doing gravimetric determination of nickel(in coins) using dimethylglyoxime as a project but all my trial have proved fruitless.A red precipitate is mean to form but i have got a dark green precipitate in every trial of mine.My question is what am i doing wrong.

This is my procedure
1st i dissolve the coin in 6mol/l nitric acid and then transfer the solution into a 400ml beaker.
Make the solution up to 200ml using distilled water.
Put the solution on a hot plate temperature 70-80 degrees
Add 50ml of dimethylglyoxime with constant stirring
Add ammonium hydroxide dropwise with constant stirring.
but the solution turns dark green.

I did this procedure for my standard one using nickel chloride and it turned red but with coins its always turning dark green.Please help me

[Borek]

What are other metals present in the coin?

[leemix]

copper and nickel.i have been using citric acid so to stop copper from forming a complex with the dimethylglyoxime.

[Borek]

My bet is that you have too much copper or other contaminants. If it works for the standard, your DMG is OK. If memory serves me well masking with citric acid works as long as the amount of other ions is relatively small.