GMP 7 – 2019 Page 1 of 3

This publication is available free of charge from: https://doi.org/10.6028/NIST.IR.7383 -2019

GMP 7

Good Measurement Practice

for

Cleaning Precision Glassware

1 Cleanliness requirements

The volume contained in or delivered from precision glassware depends on the cleanliness 

of the container. Glassware must be sufficiently clean to ensure uniform wetting of the 

entire internal surface of the standard. When clean, the walls will be uniformly wetted, and 

the water will adhere to the surface in a continuous film. If films of dirt or grease are 

present, the meniscus may crinkle at the edges and liquids will not drain properly and will 

leave water drops on the internal walls. Lack of cleanliness can cause errors in setting a 

meniscus and incomplete wetting/drainage of the calibration liquid (generally water).

Glassware that is submitted for calibration should be cleaned (and autoclaved if needed) 

prior to submission to the laboratory to ensure removal of all chemical, biological, 

radiological, or other contaminants. 

2 General cleaning options

If glassware is cleaned by the laboratory, appropriate inquiries should be made to determine 

prior and subsequent uses along with implications of certain cleaning methods. Some 

methods of cleaning glassware prior to calibration or use are ineffective or detrimental and 

may leave chemical contamination deposits that can be detected in some chemical analysis 

operations. 

There are several suitable detergents (both liquid and powder), available from laboratory 

supply houses that do not contain phosphates. The catalog descriptions usually indicate 

whether they contain phosphates. Laboratory detergents that contain phosphates can leave 

a deposit on the glass that will cause water to "bead" on the surface making it appear to be 

dirty and making it difficult to properly set a meniscus. Once a phosphate deposit has 

occurred, it is very difficult to remove. It may be removed with hot (approx. 65 °C) sodium 

dichromate-sulfuric acid cleaning solution. However, use of this hot solution is hazardous, 

is a problem for safe disposal, and is not recommended unless proper safety equipment is 

available (see the next section). 

If acetone is used to remove oil or grease film, always follow with alcohol (ethanol) before 

drying or rinsing with water. Denatured methanol can leave an oily residue due to the 

denaturing process. Acetone, if allowed to dry, may also leave a film deposit.

The above solvents need not be used if the glass does not have oil or grease film. 

Mechanical shaking of water and suitable non-phosphate detergents is usually adequate for 

cleaning glassware. 

These cleaning agents do not preclude the use of other suitable methods, of which there are 

several. Some have special applications that would not normally be encountered by 

metrologists or field inspectors.
GMP 7 – 2019 Page 2 of 3

This publication is available free of charge from: https://doi.org/10.6028/NIST.IR.7383 -2019

3 Advanced cleaning methods1

Safety and residue considerations 

Safety data sheets (SDS) must be available, studied, and carefully followed, with 

all appropriate personal protective equipment (PPE), before using any of the 

following cleaning methods. Chromic acid solutions are not recommended for 

routine use because it is a hazardous waste and hazardous to health! Suitable 

education or training in the handling of chemicals is highly recommended.

Care should be exercised when using most cleaning solutions because they can 

cause skin irritations or severe burns on contact. Dilute solutions become 

concentrated as the water evaporates; therefore, always flush the exposed area 

immediately with large quantities of water.

Suitable chemical-specific-resistant goggles or a face mask should be worn to 

protect the eyes from splashes and rubber gloves should be worn to protect the 

hands. It is advisable to wear an acid resistant laboratory coat or a rubber apron to 

protect clothing when using strong acids for cleaning. The glassware should be 

handled gently to avoid breakage and to prevent spilling acids and other cleaning 

fluids. All cleaning should be done in a laboratory sink or on an acid-proof 

laboratory bench, preferably within a fume hood, to the extent possible.

Some of the cleaning materials mentioned leave minute traces or residues unless 

the rinsing process is carried out thoroughly. While such traces may not be harmful 

if the purpose of cleaning is to prepare the glassware for calibration, they can give 

trouble when the glassware is used in certain laboratory operations. For example, 

manganese and chromium compounds, even in extreme dilution, may retard or 

inhibit growth of micro-organisms, and traces of phosphorus may interfere with 

delicate tests for this element. When glassware is to be calibrated, final rinsing must 

be with pure distilled or deionized water.

Solvents

Frequently it is desirable to give glassware a preliminary rinse or soak with an 

organic solvent such as xylene or acetone to remove grease, followed by a water 

rinse. The rinsing with water must be done thoroughly if acid will be used later to 

clean the glassware.

Unless autoclaving is necessary, glassware should be cleaned as soon as possible 

after use to avoid setting and caking of residues. Pipets, for example, may be placed 

in a jar containing a weak antiseptic solution immediately after use. Autoclaving is 

necessary to disinfect glassware that may have been used to contain potentially 

dangerous biological fluids. 

 1 While the metrologist is not ordinarily faced with the problems for which these procedures are necessary, they are 

given here if they are needed.
GMP 7 – 2019 Page 3 of 3

This publication is available free of charge from: https://doi.org/10.6028/NIST.IR.7383 -2019

Chemical cleaning optional methods 

When a piece of glassware is badly contaminated with stopcock grease (except 

silicone grease), it may be necessary to rinse with acetone once or twice before 

using one of the methods below. For silicone grease, the acetone can be omitted,

and the piece soaked for 30 min in fuming sulfuric acid. Warm decahydronaphthalene (decalin) also has been suggested as a solvent for silicone grease. In this 

case, let the piece soak for 2 h, drain, and rinse once or twice with acetone, followed 

by a water rinse. 

Fill with sulfuric acid-dichromate mixture and let stand. After removal of 

the mixture, rinse with distilled water at least six times. To make the 

cleaning mixture, dissolve 60 g to 65 g of sodium- or potassium-dichromate 

by heating in 30 mL to 35 mL of water, cool and slowly add concentrated 

sulfuric acid to make one liter of solution. This solution is available from 

laboratory supply companies. Note: Extreme care should be exercised in 

handling acidic solutions. 

Scrub with a 1 % to 2 % hot solution of a detergent. Rinse well after 

brushing. Several suitable, commercial washing compounds are available. 

Fuming sulfuric acid (very hazardous material) is an excellent cleaning 

agent. Usually, cleaning can be accomplished by use of a comparatively

small amount of acid, manipulating the vessel so that the acid contacts the 

entire surface, and immediately emptying and rinsing. 

4 Drying and protection

See GLP 13, Good Laboratory Practice for Drying Containers for appropriate drying 

techniques. It is not necessary to dry any container marked “to deliver.” If an article is to 

be dried after cleaning, as is necessary for all vessels marked "To Contain", acetone, 

followed by ethyl alcohol may be used. American Chemical Society, ACS, grading

specification for reagents may be selected based on glassware use. Where ACS, reagent, 

or USP grades are selected the quality is suitable for use with food or medical processes 

and will meet or exceed other application requirements. Drying may be hastened by 

blowing clean, dry air into the vessel (or drawing the air through the vessel). Be sure not 

to mix acetone with alcohol. 

Efficient air filters must be provided to remove any particles of oil or dirt from compressed 

air used for drying purposes.
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This publication is available free of charge from: https://doi.org/10.6028/NIST.IR.7383-2019