I have a bit of a problem I'm stumped with regards to the Gravimetric Determination of Sulfate. So here's the process & problem
Start by precipitating out the sulfate with a barium chloride solution, over heat. The precipitate is then filtered out into an ashless filter paper, put into a crucible, and the paper is burnt off. The mass of the barium sulfate is then taken (minus crucible).
The lab is very similar to this one http://www.csudh.edu/oliver/che230/labmanual/gravsulf.htm
and is pretty standard first year stuff.
Over three lab sections, the amount of sulfate determined was on average 15-20% below what we know is in the sample.
I've checked all the stock solutions, Na2S04, BaCl2, HCL - and ensured they are at the right concentrations.
What could be causing such a loss of sulfate? Any suggestions would be appreciated!