9 Chapter 9: Procedures for Weight and Volume Measurement

Accurate weight and volume measurement is essential in pharmacy compounding to ensure precision, consistency, and patient safety. Errors in measurement can result in ineffective or unsafe medications. This learning step covers weighing solids and measuring liquids using appropriate equipment and procedures.

 Weighing Procedure

1. Use balance on level and stable surface, free from drafts
2. Check the balance calibration before use.
3. Tare (zero) the balance before weighing ingredients.
4. Place labelled weighing paper or a weigh boat on the balance (middle of the pan).
5. Slowly add the ingredient until the desired weight is reached.
6. Remove and transfer the measured ingredient carefully.
7. Clean the balance after use to avoid cross-contamination.
8. Use a clean paper or boat for each ingredient.

   Common Errors in Weighing

• Not taring the balance before weighing.
  • Failing to zero the scale before adding ingredients leads to inaccurate measurements
• Using a balance that is not level or calibrated.
• Adding too much or too little ingredient due to improper handling.
• Air currents or vibrations affecting measurement accuracy
• Transferring incorrectly
  • not transferring the entire weighed amount (e.g., powder stuck to weigh paper or spatula) causes underdosing
• Incorrect container placement
  • placing the weight paper or boat off-center on the scale pan can affect the reading

**Add how to measure information here with picture

Liquid Measurement Procedure

1. Select the appropriate measuring device (graduated cylinder, pipette, syringe, etc.).
2. Pour the liquid slowly into the container.
3. When pouring liquids from a stock bottle, pour from the opposite side of the stock bottle label to avoid spillage on the label.
4. Read the liquid level at eye level, using the bottom of the meniscus.
5. Transfer the measured liquid into the formulation as needed.
6. Clean stock bottle (including threads) after measuring

Common Errors in Volume Measurement

• Not reading the meniscus at eye level
  • looking from above or below leads to incorrect volume interpretation
• Using an incorrect measuring device
  • using too large of a measuring device for too small of a volume reduces accuracy *remember 20% rule
• Pouring too quickly
  • can cause splashing, spills or excess volume.
• Using contaminated or wet measuring devices
  • residual moisture or previous compounds can affect the volume and stability of a new preparation.
• Failing to account for liquid adhesion to the container walls (especially with viscous liquids).
• Not calculating the correct sub gradations on the device.
  • **major point of error- this must be done correctly
• Not allowing time for proper drainage of graduated cylinders to minimize loss
  • may need 1 minute to allow liquid to drain, especially viscous liquids

Other Considerations:

• Weights are additive in most mixtures
• Volumes may or may not be additive (may have contraction or expansion)
• Mixing liquids requires appropriate qs procedures
  • If qs’ing with blend of vehicles, must blend them first then qs with combined vehicle
• Dilutions, fortifications, and aliquots may be employed to obtain desired amounts or concentrations

Proper use of measuring devices for weight and volume are critical to ensure patients are receiving the correct doses of their medication. Please watch the provided videos to learn more.

How to use graduate cylinders:

https://www.youtube.com/watch?v=SE-nUsrbELE

How to use an electronic scale:

https://www.youtube.com/watch?v=0UymyTJATLc

How to use syringes:

https://www.youtube.com/watch?v=_TnDr8cKums&t=3s