Lab Analyst Good Practice: Use & Calibration of Volumetric Apparatus

Volumetric apparatus is essential requirement for quantitative estimations. Accuracy and precision of your results is greatly dependent on correct use and regular calibration of volumetric apparatus.

Types of Glassware

Laboratory glassware is made from borosilicate glassware which is resistant to thermal shock and chemical attack.
Two categories of glassware are commonly used in laboratories:

Type A

Type A glassware is used for work requiring highest accuracy of results. It complies with requirements ASTM E694 standard. The glassware bears the letter A and is calibrated to half the tolerance level of Class B glassware. It come with a calibration certificate bearing a unique serial number for establishing traceability. Type A glassware does not require further calibration.

Type B

Double the tolerance limit of class A glassware. It is used for routine work and requires calibration before putting to use.

Cleaning of Glassware

Volumetric glassware must be washed and dried before use as unclean glassware can introduce contamination errors.
  • Soak used glassware in water and drain. Clean with a cleaning detergent.
  • Rinse with dilute potassium dichromate solution in sulphuric acid.
  • New glassware is slightly alkaline in nature so soak in slightly acidic solution e.g. in 1% HNO_3 or HCl for an hour, then rinse with distilled water
Cleaning machines save time when bulk cleaning is required. They accommodate a wide range of glassware shapes and sizes and permit a range of cycles including washing, rinsing and hot dry air cycles. Suppliers of laboratory glassware washing machine include:

Care in Handling of Volumetric Glassware

  • When washing take special care so that it does not break on striking wash basin walls
  • When shaking a volumetric flask, hold both neck and bottom using both hands to prevent breakage
  • Remove the stopper from volumetric flask before oven drying. Do not raise temperature above 60^0C and cool to room temperature before use.

Calibration of Glassware

  • Calibration of volumetric apparatus at specified intervals is an important task.
  • Volume, weight and delivery time at calibration temperature are common parameters for calibrations.
  • Maintain constant temperature(25 +/- 2^0C) during calibration and use calibrated weights
  • Read lower level of meniscus for clear liquid readings and upper level for colored solutions readings, keeping your eye level horizontal in line with the liquid level

Calibration of graduated pipette

  • At time of calibration, ensure no air bubbles are sticking to the glass walls and no water droplets exist above the graduation mark
  • Weigh a clean dry beaker (W_1)
  • Deliver water from calibrated mark till it is emptied or in case of graduated pipette till the calibrated mark.
  • Repeat 3-4 times till consistent reading is obtained for weight (W_2), volume and delivery time.
  • Volume collected = (W_2 - W_1) / D,
    where D is density of water at 25^0C .

Calibration of Burette

  • Weigh a clean dry beaker (W_1)
  • Deliver water from 0 mark to the point of calibration with tip touching the beaker wall.
  • Weight of the collected water + beaker (W_2)
  • Take note of the water temperature with a calibrated thermometer
  • Repeat process at least 3 times
  • Volume collected = (W_2 - W_1) / D where D is density of water at recorded temperature.
  • Record the delivery time and volume for consistent results.

Calibration of Volumetric Flask

  • Dry the flask after rinsing with acetone
  • Take weight (W_1)
  • Fill the flask up to the graduation mark with distilled water and weigh the filled flask (W2)
  • Note the temperature of water using the calibrated thermometer
  • Volume = (W_2 - W_1) / D , Where D is density of water at recorded temperature
  • Repeat 3 – 4 times to ensure consistency of volumes

Micropipettes for Highest Accuracy

Micropipettes are used to accurately measure and dispense small volumes of liquids in µl to ml range i.e., 1µl to 1000µl (1ml)

Advantages of using micropipette

  • Fast
  • Accurate
  • Reproducible
  • Disposable tips
  • Do not require washing or rinsing
  • Reduction of pipetting errors associated with normal pipettes

Precautions while using Micropipettes

  • Never use the micropipette without a tip.
  • Never lay or invert a loaded micropipette down or tilt it as this could allow liquid to run into the pipette barrel.
  • Return the button to the top position smoothly and DO NOT snap back.
  • Use the micropipette for only the specified pipetting range.
  • Wipe droplets if any from the sides of tip but never wipe the tip opening.
  • Never use a micropipette without a tip as it will contaminate the shaft assembly.
  • Store micropipette set to its maximum volume. This releases pressure in the spring inside the micropipette.
  • If trying to pipette organic solvents make sure they are compatible with the tip and also allow solvents to come to room temperature before pipetting.

Calibration of Micropipettes

  • Label and weigh five centrifuge tubes.
  • Using single tip to transfer same volume in each centrifuge tube with single low range volume.
  • Weigh each tube and record weight of water transferred into each tube.
  • Calculate volume of water in each tube
    • V= W/D
  • Where V = volume in µl
    • W = weight in mg
    • D = Density of water in mg/ µl at the recorded temperature
  • Calculate the standard deviation based on the five volumes.
  • Determine the accuracy of micropipette by calculating the percentage (%) error.
  • Repeat above steps with single upper range volume of micropipette.

Choice of Balance for Micropipette Calibration

Weighing of transferred volumes requires different types of balances for the required accuracies
Micropipette capacityBalance to be used
0.5 -10µlMicro Balance
10 – 200µlSemi Micro Balance
200 – 1000µlAnalytical Balance
Feel free to contact me for test methods

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