The degree of water purity necessary will vary with the analyte and the sensitivity required, but in general must be roughly 10 times lower in concentration than the lowest concentration to be measured or the lowest standard. Often it is advantageous to use a combination of these to get the best attributes of each. Distillation Laboratory stills are typically either of stainless steel or glass construction. The former generally have copper elements, while the latter have glass coated elements. Glass stills produce cleaner water, particularly for trace metal analysis, but produce much less water and are more expensive.
Stills work by boiling water in a large chamber, producing steam which enters a condenser.
In high capacity stills, the initial cool feed water goes into the condenser first, where it causes condensation of the steam and is itself pre-warmed. It then enters a split valve which maintains the water level in the still, while the excess goes to waste. Inside the still, two metal rods are attached to a voltage source and extend down into the water. As steam is produced, the remaining water in the still becomes progressively more concentrated. The same processes occur in many hot water heaters using hard water, keeping lots of plumbers employed doing acid flushing.