Double distilled water has been through the distillation cycle twice, not once. That second pass strips out the residual dissolved gases, ionic traces, and organic carryover that a single distillation run tends to leave behind. It’s produced on a <\/span>water distillation unit<\/span><\/a> or, where trace-metal purity matters most, an all glass distillation apparatus\u2014the “all glass” part matters because it keeps metal out of the water-contact path entirely. Conductivity on properly produced double distilled water usually sits below 2 \u00b5S\/cm, with resistivity in the 1\u20135 megohm-cm range. Labs reach for it when single-distilled or deionized water simply isn’t clean enough\u2014pharmaceutical formulation, electrochemistry, trace analysis, cell culture work.<\/span><\/p>\n Run water through a still once, and you get rid of most dissolved salts, heavy metals, and suspended particulate. That’s single distillation, and for a lot of lab work it’s fine. But it doesn’t touch everything. Dissolved CO\u2082 still gets absorbed back in. Some organics share a boiling point close enough to water’s that they ride along in the vapor. And if the still itself has metal parts in the water path, you can pick up trace ions from the equipment instead of the feed water.<\/span><\/p>\n A second distillation pass \u2014 ideally through a glass distillation unit rather than a metal-bodied one \u2014 knocks most of that down further. This is why pharmacopoeial monographs, electrochemical reference work, and injectable-grade manufacturing specify double distilled water rather than settling for a single pass: the purity floor is measurably lower.<\/span><\/p>\n The principle isn’t complicated. Feed water goes into a boiling chamber\u2014borosilicate glass in most lab-grade units, sometimes stainless steel in larger industrial ones\u2014and a heating element, usually an immersion coil or a silica sheath heater, brings it to boiling point. As it vaporizes, the dissolved minerals and non-volatile impurities stay behind in the chamber. Only the vapor moves on.<\/span><\/p>\n That vapor travels through a condenser (coil-type or Liebig-type, cooled by circulating water) and comes back out as liquid on the other side\u2014your distillate. For double-distilled water, this distillate becomes the feed for a second, identical run, ideally on a dedicated glass unit so no metal component gets reintroduced into the water at this stage.<\/span><\/p>\n Bionics Scientific builds these units with borosilicate 3.3 glass columns, automatic water-level cutoff, and anti-bumping design\u2014the last of which matters more than it sounds, since bumping (violent, irregular boiling) can carry liquid droplets straight into the distillate and quietly ruin an otherwise clean batch. It’s a common weak point in cheaper stills.<\/span><\/p>\n These two terms get used almost interchangeably, but they’re not quite the same thing.<\/span><\/p>\n A <\/span>glass distillation unit<\/b> has a glass boiling chamber and glass condenser, but fittings, supports, or the heater housing outside the direct water path may still be metal or PTFE.<\/span><\/p>\n An <\/span>all glass distillation apparatus<\/b> goes further\u2014every wetted surface, from the boiler through the vapor path to the collection point, is glass. Nothing in the flow touches metal. This is the specification you want when even trace metal leaching would compromise results, such as ICP-MS sample prep or electrochemical reference solutions where a few parts per billion of contamination can throw off a reading.<\/span><\/p>\n Which one you need comes down to how sensitive your application is to trace metals, not just how much water you need per hour. Worth confirming the wetted-surface material against your actual use case before ordering rather than assuming “glass” means the same thing across every model.<\/span><\/p>\n Pharmaceutical injectable and ophthalmic formulation. Trace metal and analytical chemistry. Electrochemistry and battery research, where ion contamination skews measurements. Semiconductor rinsing. Cell culture media prep. Calibrating pH and conductivity meters, where even minor contamination throws off the baseline. Basically anywhere the water itself can’t be allowed to introduce variables into the result.<\/span><\/p>\n Exact capacity and heater wattage vary by model\u2014check the specific unit’s datasheet before planning installation around a particular throughput.<\/span><\/p>\nWhat Double Distillation Actually Does<\/b><\/h2>\n
How a Water Distillation Unit Works<\/b><\/h2>\n
Glass Distillation Unit vs. All Glass Distillation Apparatus<\/b><\/h2>\n
Where Double Distilled Water Gets Used<\/b><\/h2>\n
Specification Table<\/b><\/h2>\n
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\n Parameter<\/b><\/td>\n Typical Value<\/b><\/td>\n<\/tr>\n \n Distillation stages<\/span><\/td>\n 2 (double distillation)<\/span><\/td>\n<\/tr>\n \n Output conductivity<\/span><\/td>\n Below 2 \u00b5S\/cm<\/span><\/td>\n<\/tr>\n \n Resistivity<\/span><\/td>\n 1\u20135 megohm-cm<\/span><\/td>\n<\/tr>\n \n Wetted material<\/span><\/td>\n Borosilicate 3.3 glass (all-glass models)<\/span><\/td>\n<\/tr>\n \n Heating type<\/span><\/td>\n Immersion coil \/ silica sheath heater<\/span><\/td>\n<\/tr>\n \n Condenser type<\/span><\/td>\n Coil or Liebig-type, water-cooled<\/span><\/td>\n<\/tr>\n \n Output capacity<\/span><\/td>\n 2 L\/hr to 20+ L\/hr, model-dependent<\/span><\/td>\n<\/tr>\n \n Feed water requirement<\/span><\/td>\n Pre-treated, single-distilled, or RO water<\/span><\/td>\n<\/tr>\n \n Storage vessel<\/span><\/td>\n Borosilicate glass or PP carboy (metal-free)<\/span><\/td>\n<\/tr>\n \n Power supply<\/span><\/td>\n 220V\/50Hz single-phase standard; 3-phase on high-capacity units<\/span><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n Temperature Chart: Distillation Stages<\/b><\/h2>\n