Question

What is the limit of impurities that 24K gold can contain for it to still be an element and not an alloy.

Answer 1

Caratage is the only measurement of purity of gold alloyed with other metals. 24 carat is pure gold with no other metals. Lower caratages contain other metals. These are alloys. 18 carat gold contains 75% gold and 25% other metals, usually copper or silver.

However it is practically not possible to attain 100% purity, so, the highest purity is called 999.9 parts in 1000 parts gold. So the impurity tolerance is 0.1/1000 = 0.01% impurity.

in some countries however, 999 is also accepted as pure gold in which the impurity level is 1/1000 = 0.1%