What Pure Means: 99% of What, Measured How?

**Purity by weight.** The fraction of the sample that is the wanted substance. This is what a supplier's 99.5% normally means, and it says nothing about what the other 0.5% is — which is often exactly what matters. **Purity by a specific assay.** The fraction measured by one named method: titration, chromatography, spectroscopy. A figure like "99.5% by titration" is a much stronger claim, because it says how it was established — and a much narrower one, because a titration only sees what reacts with the titrant. **Absence of a named impurity.** "Iron under 5 ppm." This is often the useful specification and it says nothing about overall purity at all. A reagent can be 95% pure and still be the right one, if the 5% is water and the reaction does not care. The practical rule: **ask what the impurity is, not how much there is.** One percent of water is usually harmless and occasionally fatal; one percent of a catalyst poison is fatal every time. This is also why a substance is sold in grades — technical, reagent, analytical, food, pharmaceutical — that are not simply ascending purities but different specifications about different impurities.

Four tests need no instrument beyond a scale and some glass, and between them they catch most of what goes wrong. **Melting point.** A sharp melting point at the literature value is strong evidence of purity, and a depressed, broadened one is strong evidence against. It has its own rung. **Thin-layer chromatography.** One spot means one substance, within the limits of the system. Several spots means several substances. This is the fastest purity check there is and the plates can be made at home — the catalogue already has that blueprint. **Drying to constant weight.** Weigh, dry, weigh again, repeat until the weight stops changing. The difference is water, and water is the commonest impurity in anything crystalline. **Appearance and colour.** Weak evidence, and worth recording anyway. A white solid that has gone faintly pink or grey has something in it, even if the melting point has not moved. What a maker cannot do is put a number on overall purity. That needs an instrument. So record the METHOD and the OBSERVATION — "single spot on TLC, melting point 156 to 157 C against a literature 157" — rather than inventing a percentage.

Knowing the source tells you which purification will work, which is the whole point of asking. **Unreacted starting material.** Usually similar to the product, which makes it the hardest to remove. Chromatography or a careful recrystallisation. **Side products.** Often quite different, so often easy — a different solubility or a different boiling point. **The solvent.** Trapped in crystals or left in an oil. Drying removes it. **Water.** From the air, from the reagents, from the washing. Drying agents and desiccators, and it comes back if the product is left out. **The reagents' own impurities.** Whatever was in the starting materials is still there unless something removed it. This is why a synthesis from technical grade material cannot give a reagent grade product. **The apparatus.** Grease from joints, metal from a spatula, fibres from filter paper, plasticiser from tubing. Small, real, and the reason trace analysis is done in glass and PTFE. Write down what the crude product is expected to contain before purifying it. That prediction is what chooses the method.

Every purification throws material away. That is how it works — the impurity leaves, and some of the product leaves with it. **Recrystallisation** leaves product dissolved in the mother liquor. A good recrystallisation recovers 60 to 80%; a fussy one recovers far less. **Distillation** leaves product in the pot and in the fore-run. **Extraction** leaves product in the layer being discarded. **Chromatography** leaves product on the column and in the fractions either side of the ones you kept. **Filtration** leaves product wetting the filter cake and the paper. So the question is never "how pure can I make it" but **"how pure does it need to be for what comes next"**. A pigment going into paint needs to be free of grit and of soluble salts, and does not need to be analytically pure. An intermediate going into another reaction usually needs to be free of one specific thing. Recrystallising three times to chase the last half percent, and ending with a tenth of the material, is the commonest way an otherwise good preparation is wasted.