**GMP 7**

**Good Measurement Practice**

**for**

**Cleaning Precision Glassware**

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.

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 a number of suitable detergents (both liquid and powder),
available from laboratory supply houses that do not contain phosphates.
The catalog descriptions usually indicate whether or not 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 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, not methanol) before drying or rinsing with water.
Acetone, if allowed to dry, also leaves a film deposit.

The above solvents need not be used if the glass does not have an 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.

See GLP 13, Good Laboratory Practice for Drying Containers for
appropriate drying techniques. It is not necessary to dry any container
marked "to deliver."

[Cleaning Methods]{.underline} [^1]

*Safety Note: Material safety data sheets (MSDS) should be available,
studied, and carefully followed 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.*

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.

A selection of general cleaning methods follows:

1\. Fill with sulphuric 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 to 65) g of sodium- or
potassium-dichromate by heating in (30 to 35) mL of water, cool and
slowly add concentrated sulphuric 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.

2\. Scrub with a 1 % to 2 % hot solution of a detergent. Rinse well
after brushing. A number of suitable, commercial washing compounds are
available.

3\. Fuming sulphuric 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 comes in contact with all of the surface, and immediately emptying
and rinsing.

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 above methods. For silicone grease, the
acetone can be omitted and the piece soaked for 30 min in fuming
sulphuric 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.

[Cautions]{.underline}

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 also 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.

If an article is to be dried after cleaning, as is necessary for all
vessels marked \"To Contain\", acetone, followed by ethyl alcohol
(American Chemical Society Specification) may be used. Drying may be
hastened by blowing clean, dry air into the vessel (or sucking the air
through the vessel).

Efficient air filters must be provided to remove any particles of oil or
dirt from compressed air used for drying purposes.

[^1]: While the metrologist is not ordinarily faced with the problems
    for which these procedures are necessary, they are given here in the
    event that they are needed.