Lou said:
Truth be told, any PGC salts I ever did that are opened NEVER EVER ran close to assay, hell some weren't even consistent out of the same bottle. Don't know why. I think some of it was pre-mixed with KCN and or K2CO3 or something but it always ran way less than theory would have you believe. I'm not even sure if it's hygroscopic or not.
When I worked for Sel-Rex, they sold 3 types of PGC, sealed, in either 1 or 5 oz (gold content) bottles. All 3 were labeled differently. They were all used primarily to replenish the gold in existing gold plating baths.
(1) Pure PGC - 68.3% gold. No additives. Small, bright white, very heavy crystals. This worked in any bath originally made up with PGC, whether acid or alkaline. For the pure 999.9 gold baths (pH=6-6.5, usually), only pure PGC could be used.
(2) Pure PGC + a small amount of either nickel or cobalt sulfate. Used to replenish the gold and the Ni or Co in hard acid gold baths (pH=4-4.5). The Ni/PGC salts were light green and the Co/PGC salts were light pink. Overall, the purity was only a little less than 68.3% because very little Ni or Co was added.
(3) Pure PGC + KCN. Only for replenishing alkaline baths which were in a KCN matrix. Overall gold purity of 40%, if I remember right. In a cyanide gold bath, the cyanide tends to break down to form K2CO3. The extra KCN mixed with the gold acts to replenish the KCN. I would imagine that the KAu(CN)2 and the KCN weren't well-mixed.
I'm thinking that the few times the gold content was questioned and an assay was required, I had to recover and refine all the gold contained in one or two bottles. I seem to remember that it was always right on.
Other manufacturers probably package their salts differently.