Go to a decent tile shop and get one of the products by HG to suit your tiles/type of grout. They how have a computer on their shop displays that you can enter the problem into and it will tell you what you need. Helps to know the tile type -ceramic, porcelain, natural stone as well as the grout used - premixed, powdered etc.
Basically its a hydrocloric acid, bit like brick acid but there are a few variations depending on tile and type of grout as the acid can attack marble & limestone.
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