Firstly to those who are unfamiliar with the terminology carat simply means simply a level of purity for gold alloys, the purity is measured as 24 times the purity by mass. In lay terms 24 carat is 99.99% gold (none or minimal alloy) and 22 carat is 91.66% (22 parts gold and 2 parts alloy). Understanding this leads to the question why use an alloy? The primary reason is due to gold’s inherent softness, pure gold is extremely soft and malleable, in jewellery or coinage this is often unsuitable. Unsuitable because coins in distribution are susceptible to wear and tear, the alloy toughens the coin and copper is much harder. The alloy(typically copper but sometimes silver) that is added changes the colour of the coin, copper adds a red-ish brown colour(as evidenced in the Krugerrand) and silver being a less dominant colour softens the gold colour(as evidenced in the American eagle).
Many buyers of gold coins believe that they are getting an inferior product when they are buying 22k gold coins, however this is not the case. 22k coins(Kruger, eagle, sovereign) weigh more than their equivalent 24k coins, the alloy is an extra weight difference that the buyer is not paying for. The gold is what the customer is paying for and when dealers present Krugerrands etc, they do call them 1 troy oz coins but this is simply because the alloy is disregarded.
24k coins in mint condition do have an aesthetic appeal that 22k does not have, this is because pure gold has a sparkle and fineness that dissipates when an alloy is added. However 24k coins are a relatively new phenomenon and are not minted to be handled/circulated. An example being the exquisite Aussie Kangaroo it certainly looks more attractive than the 22k equivalent.
However what many customers fail to understand regarding 24k coins is the fact that these coins are soft and scratch easily. The 24k coins often come in cases for good reason as a damaged or scratched 24k coin loses its premium. From experience many older 24k coins have not been maintained properly. On the other hand sovereigns from as early as the 19th century are often in a perfectly saleable condition. Most Krugerrands from the 1970’s again are in good condition and not as susceptible to scratching.
Good advice to buyers when buying gold coins is: only buy 24k if you are prepared to look after them (i.e. keep them encased). For those of you who like to handle your gold coins 22k is perhaps more suitable.