Residue on Ignition

ignition is not to be confused with setting it alight. What you do is raise the temperature gradually to evaporate what will evaporate. then you drive off all water of crystallization and hydration and finally burn off all organics. care should be taken to avoid loss of fine ash with and rapid process.

pure water = zero.

dissolved salts will show up. dissolved organics will not

Water is "burnt hydrogen", and cannot be oxidised further (hydrogen peroxide, H₂O₂, is a peculiar material that decomposes into water and oxygen over time), though it can be evaporated or boiled to dryness. Evaporating the water away produces a residue of solids, and its measurement as a proportion of the total material before evaporation started gives a value for the total dissolved solids [TDS] of the water sample and a valuable indication of its purity and potential for either use or further treatment.

Typical values: seawater anywhere from 31000 to 39000 parts per million [ppm], average 35000ppm. Dead Sea, around ten times that figure. "Brackish" water anything from 1200ppm upwards. World Health Organisation guidelines for "potable" water, no more than 1000ppm and "wholesome". Typical domestic potable in the UK ~500ppm though it varies according to source. High pressure boiler feed water typically 1-5ppm and will be specified as a limit by the boiler manufacturer; most low pressure boilers "blow down" at around 2500-3000ppm to reduce the risk of "priming", i.e. water droplets being carried over into the steam, and a lower feed TDS will reduce the frequency of blowdown, enhancing the economy of the boiler operation. "Water For Injection", typically lower than 5ppm though the water must be bacteria- and pyrogen-free (see Wikipaedia), achievable principally by distillation and membrane separation processes, like Reverse Osmosis.

And the purest water of all? That used to wash silicon chips in the wafer fabrication industries, where impurities are measured in fractions of a part per billion (10⁹). A plant built recently involved 17 stages of processing beginning with a town-main potable water starting point. At these levels of purity the water will no longer conduct electricity to any useful extent. Static electricity build-up may be expected for several pipe-diameters downstream of any centrifugal pump, though it decreases rapidly with distance from the pump. About the only flow measurement technique that works on this stuff is ultrasonic time-of-flight, and consumption of "ultrapure water" will have seriously bad effects on anyone drinking it.

Bottled water has to satisfy 9 criteria before it is allowed on the shelves for purchase for consumption in the UK. Domestic potable water at the tap/faucet has to satisfy 42.....