This does not work — and if it would, might cause other problems
To add to Someone_evils answer, the full optional rule for Alchemical Crafting with Alchemist Supplies from Xanathar's Guide to Everything reads:
Alchemist's supplies enable a character to produce useful concoctions, such as acid or alchemist's fire.
Alchemical Crafting. You can use this tool proficiency
to create alchemical items. A character can spend
money to collect raw materials, which weigh 1 pound
for every 50 gp spent. The DM can allow a character to
make a check using the indicated skill with advantage.
As part of a long rest, you can use alchemist's supplies
to make one dose of acid, alchemist's fire, antitoxin,
oil, perfume, or soap. Subtract half the value of the created item from the total gp worth of raw materials you are carrying.
So first, you buy raw materials in town, which costs you 50 gp per pound of them (chemical substances such as bases, acids, oils, buffers etc.). This might include some platinum powder as a catalyst, but obviously does not include only platinum, as first, a pound of platinum costs 500 gp, and no sane merchant is going to sell you a pound of platinum for 50 gp, and second, a pound of platinum as your only raw material would be rather useless as raw material to create things like oils, acids and alchemist's fire. The rules here are kept abstract -- these materials are not supposed to be used for anything else than for alchemical crafting, so it does not matter what they exactly are.
Then you tranform these raw materials into an alchemical substance. For example, you can create one dose of acid, which has a market price of 25 gp. This will consume 12.5 gp of raw materials, or a quarter pound. You probably also will need a vial to put it in, unless the DM handwaves such details.
The game actually makes a statement on the price for which you can sell any kind of equipment. It's on p. 144 PHB:
As a general rule, undamaged weapons, armor, and other equipment fetch half their cost when sold in a market
The items you can create with this, acid, alchemist’s fire, antitoxin, oil, perfume, or soap are all on the Adventuring Gear table in the Equipment chapter on page 150 PHB and thus clearly are "other equipment". As it costs you half their value in raw materials to create them, and you can get half their value by selling them, you can make exactly zero money from creating and selling them.
What if you could create platinum?
It is not unreasonable to think you could try to create gold or platinum by alchemical means -- after all that is the main thing alechemists in Earth's history were trying to achieve, turning lead into gold. The useful things that came of this, like an understanding of basic chemistry, were more of a by-product.
Platin or gold would count as a trade good as it is listed on the trade goods table on page 157 PHB, with a value of 500 gp per pound of platin, and you could fetch the full value, as p. 144 PHB also explains:
Trade Goods. On the borderlands, many people conduct transactions through barter. Like gems and art objects, trade goods—bars of iron, bags of salt, livestock, and so on—retain their full value in the market and can be used as currency.
There is a reason alchemists always sought the philosopher's stone that permanently can turn one substance into a more valuable one: it would be a way to get rich easily. It still would not be infinite: you only can make one long rest every 24 hours, so you would be limited to making at best 250 gp per day this way, as you have to subtract half the value in raw materials. It would be a lot of money for the early levels, and you can do it „for free“ during your long rest using this optional rule, so it would be just an additional pick-up.
You might be able to make more by spending additional hours during the day to craft, but that would only be free if your time has no value. That's not the case in most campaigns for other reasons — to make the depletion of daily powers and hit points matter, there often is some kind of urgency or ticking clock.
Moreover, there may be other issues with it. The entry on trade goods in the PHB p. 157 says:
Guilds, nobles, and royalty regulate trade. Chartered companies are granted rights to conduct trade along certain routes, to send merchant ships to various ports, or to buy or sell specific goods. Guilds set prices for the goods or services that they control, and determine who may or may not offer those goods and services.
If you could spin platinum from gold or other raw materials, the local noble or royalty might take an interest and conclude that you have to hand off a large chunk of what you can make in taxes to them — if they don't lock you up somewhere safe to work exclusively for them as the "royal alchemist".