Why did I get a white precipitate with BaCl2 in sulfate testing after AgNO3 treatment?

[bc89]

hi, so we had a general anion unknown to do that may contain 5 of the following: carbonate, sulfite, chloride, bromide, iodide, oxalate, chromate, sulfide, sulfate, phosphate, nitrate, nitrite, chlorate, acetate, and thiocyanate. I am positive that my unkown had phosphate, iodide, and thiocyanate, and when i tested for sulfate i removed iodide and thiocyanate, removed excess silver ion from past steps by precipitating it out with NaCl, made the soln acidic, added CaCl2, then i added BaCl2 and got NO PRECIPITATE. But then i used another method which was add AgNO3 to my unknown, centrifuge, add HNO3 to the soln, then add BaCl2, and i got a white fluffy precipitate that was insoluble in 6M HNO3, 12M HCl and i heated it in a water bath and it did not dissolve. so my question is did i have sulfate? or could i have reduced/oxidized thiocyanate to sulfate in that step? but i don't think i used any oxidizing/reducing agents in that step, or could it have been some other ppt..??

 

[chemisttree]

In your second analysis you added Ag and then added a source of chloride. You ppd AgCl. Try dissolving that fluffy stuff in ammonium hydroxide. My guess is that it will be soluble. Not sulfate unless you have some undissolved stuff after you add the ammonium hydroxide.

 

[PhaseShifter]

You'll notice that you identify which cations are present by combining them with various anions and observing the results, likewise you identify the anions present by reacting them with various cations.
If you try to run both sets of tests on one test tube of solution, the second set of tests will invariably detect all the reagents you used in the first set.

The way around this is to split your unknown into two samples, then test one sample for cations only and the other sample for anions only.