# Finding sulfate contamination in crystallized silver nitrate

What is the best way to check for a small amount of sulfate contamination (if any) in silver nitrate? Usually, I would use dilute barium chloride to precipitate out the resulting insoluble white barium sulfate in an unknown aqueous solution that contained sulfates. I tried this and got a white precipitate, but now I believe the precipitate I'm getting is insoluble silver chloride.

Is there a better/different method to check for sulfates in silver nitrate? I was researching for a better solvent to use, but haven't been able to find any conclusive answers so far.

• When you suggest "sulfate contamination" how much sulfate would you expect in wt%? // Barium nitrate wouldn't ppt AgCl, but would ppt BaSO4 (assuming a reasonable amount of sulfate). – MaxW May 26 '17 at 16:25
• In this case a very small amount of contamination. – Qst May 27 '17 at 7:29

Your choices are restrained as the precipitation of $\ce{SO4^{2-}}$ in $\ce{BaSO4}$ is the classical way to quantify the former and an electrochemical determination (in aqueous solution) is not practical.

• Electing $\ce{Ba(OH)2}$ may lead to the formation of silver hydroxyde, equally poorly soluble in water.

• $\ce{Ba(PO3)2}$ itself is very poorly soluble, as were the $\ce{Ag3PO4}$, too.

• As correctly stated by you, an aqueous solution of $\ce{BaCl2}$ is not suitable, as $\ce{Ag+}$ will form the precipitate of $\ce{AgCl}$.

• Hence I suggest to give $\ce{Ba(NO3)2}$ a try. The anion is the same as in $\ce{AgNO3}$. The solubility of this salt in water is reported to equal $\pu{4.95 g / 100 mL }$ ($\pu{0 ^\circ{}C}$); or even $\pu{10.5 g/ 100 mL}$ ($\pu{25 ^\circ{}C}$), respectively, according to the English entry in wikipedia.

Addition: Please note, the solubility of $\ce{Ag2SO4}$ ($\pu{0.83 g / 100 mL}$ water) at $\pu{25 ^\circ{}C}$ (ref.) is substantially lower than the one of $\ce{AgNO3}$ ($\pu{256 g / 100 mL}$ water) at $\pu{25 ^\circ{}C}$ (ref).

Barium nitrate has a water solubility of $\pu{10.5g/100mL}$ at $\pu{25^oC}$. It isn't specified in the question what concentration of sulfate you suspect might be present, but given that you are trying to check for sulfate by precipitating with barium, barium nitrate should be the way to go.

• Interesting, I had thought about using barium nitrate instead of the chloride, but did not have the reagent on hand and was hesitant to prepare any because I was unsure if that would work. I'll try that out and see what happens. In this particular case I suspect a very small amount of sulfate contamination. – Qst May 27 '17 at 7:27
• If you have enough of your unknown solution and a good microbalance, you can reasonably quantify down to 10's of mg/L sulfate by precipitation with barium. The limiting factor becomes the solubility of barium sulfate, which is ~2-3 mg/L at room temperature. Anyway, best of luck ;) – airhuff May 27 '17 at 15:52