Water purity and lab glassware washers: Just how "pure" does it have to be?

Hands dispensing RO or DI water

The purity of rinse water supplied to a laboratory glassware washer to produce clean glassware has been the subject of debate. To begin with, it’s important to define what the term deionized water refers to. Deionized water is water that has ions removed from it, such as sodium (Na+) and calcium (Ca2+), and does not refer to any specific water purity level.

DI water washing factors

The deionization process can produce water within a wide range of purity levels. Deionized water can be used in applications that call for medium grade water (10-20 microsiemens), or it can be used in a polishing station to produce type 1 water (16-18 microsiemens). Below are four points to consider when selecting the purity level of the water used for rinsing in a laboratory glassware washer:

  1. Laboratory grade glassware washers are constructed with stainless steel interior components. Washer rinse cycles start with contaminated tap water. Using ultra-pure water for rinsing will not significantly reduce this contaminant load. This is because ultra-pure water will contaminate quickly as it fills a glassware washer’s stainless-steel sump, resulting in rinse water that will never approach the 18 megohms purity target.
  2. Laboratory water polishing systems actively produce 18 megohm water, storing and maintaining this type of water at that purity level is extremely difficult and costly. For example, even carbon dioxide in normal air, will contaminate exposed 18 megohm water, thus reducing its purity.
  3. Deionization filters used in laboratory polishing systems are designed for producing type I water for analytical applications. Type I water will quickly be exhausted due to the higher water usage rate during multiple rinse cycles available with laboratory glassware washers.
  4. It should be noted that at very high temperatures and over a long period of time, 18 megohm water can corrode the stainless interior of a glassware washer.

Given the initial limitations of glassware washer water cleanliness levels, as well as cost of making and storing the volumes needed for type I water, it is far more economical to use RO or deionized water for washer rinse cycles. RO water systems can make and store larger volumes of water at lower costs than type I water systems. Laboratory glassware cleaned in a glassware washer with RO filtered water is acceptable for most wet bench chemical analysis.

Other factors that can have an impact on water purity

Aggressive acid baths, which are not recommended in a glassware washer, and subsequent rinsing with type I water can clean glassware to roughly the same ionic concentration level found in type I water. For organic chemical removal, a usual first step is to bake the labware in an oven. Labconco has completed third party testing on laboratory glassware cleaning efficiency using 1 microsiemen purified water rinses in our FlaskScrubber Glassware Washer. Contaminant carry-over tests have been completed using analytical test methods EPA 200 series metals, EPA 524.2 volatile organics, EPA 525.1 semi-volatile organics and EPA 8270 semi-volatile organic compounds. The results measure the remaining contaminants at the parts per billion (ppb) level.


For Labconco’s SteamScrubber® and FlaskScrubber Glassware Washers, we recommend using our WaterPro® RO Station. The internal 17 Liter storage tank supplies up to 2 out of the potential 8 RO water rinses. If the washer will be programmed for several rinses in pure water, we recommend the addition of one of our water storage Bladder Tanks. Labconco’s glassware washers use 12.9 liters of water for each fill. Two fills of tap water are required for detergent washes and up to 6 rinses with tap water or pure water are available, requiring 12.9 liters each. 


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