Why is silver nitrate used for cyanide titration? Or more to the point, since silver isn't exactly cheap: Why can't cuprous nitrate ($\ce{CuNO3}$) substitute for the pricier ($\ce{AgNO3}$) in cyanide titration?

I assume that free cyanides bind eagerly with all Group 9-11 metals, since they're used industrially to leach the heavier ones.

I'm guessing that there's something relatively quirky about the Period 4 metals Co, Ni, and Cu that prevents their use as a titrant of aqueous cyanide: E.g., it appears that nickel nitrates are very hygroscopic.
Due to the hardness of copper ions relative to silver ions, copper ions will bind hydroxide more readily in the presence of cyanide than silver ions do. Once ligated by $\ce{OH^-}$ ions, $\ce{Cu^{1+}}$ will oxidize to $\ce{Cu}^{2+}.$

The softer acid, $\ce{Ag^{1+}}$, doesn't do this nearly as quickly, especially not in the presence of a softer ligand like cyanide.