English translation: close to and classified as 100 % pure
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Explanation: "purity classified as 100%" sounds good to me. This means that the chemist knows that this is not an absolutely pure acid, but, taking into account the amount of unpurities compared to the overall quantities, these unpurities are unsignificant. It's a little bit like rounding a value, depending upon its context of use: e.g. you don't care about mg when you buy a salad, but you care if you buy gold.