How can I test the accuracy of my volumetric glassware?
07 January 2015
A huge range of tests in the laboratory require the use of volumetric glassware such as bulb or volumetric pipettes, volumetric flasks, graduated cylinders and burettes, to accurately measure larger volumes of liquid.
As this volume measurement is a critical part of the overall test the glassware should be considered for regular monitoring.Volumetric Glassware is used in the laboratory for high accuracy measurement of volumes – but how often do you monitor this?
Only HF, hydrofluoric acid, significantly attacks glass. Concentrated phosphoric acid and concentrated alkaline solutions will also cause degradation at high temperatures (over 70°C). Glass is highly resistant to most other chemicals including most common acids, bases and organic solvents, but a small amount of corrosion can occur over time. Because of this corrosion volumetric glassware should be monitored to ensure results are reliable.
Typical corrosion can occur over a period of 2-5 years, though will vary with application, cleaning procedures, temperatures of use etc. Due to gradual erosion of the inner surface of the vessels volumes will gradually increase. There can also be inaccuracies caused by cracks or physical damage or wearing of the visible graduation markings.
The standard DIN EN ISO 4787 defines calibration (monitoring) of volumetric instruments with values of 0.1 to 10,000ml and states that this should be conducted gravimetrically. There are 4 main steps to this process;
1. Preparation
Identify the type, volume, batch number, serial number of the glassware. Conduct an initial visual check to see if graduations are legible and if there is any visible damage. Clean as required.
2. Conduct the Gravimetric Monitoring
Measure each instrument in turn, once for volumetric instruments calibrated to contain (TC; In) and 3 times for volumetric instruments calibrated to deliver (TD; Ex). Use distilled water of quality class 3 as the calibration liquid. The DIN standard states full requirements for the ancillary equipment which should be used.
For (TC; In) volumetric instruments weigh the empty instrument (IE) before filling with calibration liquid slightly above the graduation mark. The meniscus should be correctly set, then the instrument weighed again (IL)
For (TC; Ex) volumetric instruments fill to slightly above the graduation mark then set the meniscus to the required mark (read at eye level). Afterwards the calibration liquid should be delivered to a glass vessel to weigh the delivered liquid (IL – IE). The waiting times for pipettes (5 seconds) and burettes (30 seconds) should be adhered to, as liquid flows down the walls of the vessels during this time – measuring too soon could give a lower volume result.
3. Calculation
Use the basic equation of;
V20 = (IL – IE) x Z
Where Z is calculated to include common correction factors as below;
Z = (1 / ρW - ρA) (1 – [ρA / ρB]) [1 - γ (t - 20°C)]
ρW = density of water (dependent on temperature)
ρA = density of air (dependent on air pressure, temperature and humidity)
ρB = density of the balance weight used for calibration of the balance
γ = coefficient of cubical thermal expansion of the material
t = temperature of the calibration liquid
4. Evaluate the result
Compare the result with admissable limits for the glassware and previous documentation on the unit.
Companies and laboratories should define their processes for products to analyse and so set admissible limits for volumetric instruments. For example the wear limit of glassware might be set to 1.5 times the instrinsic error limit of the instrument. If the result is round to exceed this wear limit then the user would consider replacing it for a new one.
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