The Pharmaceutics and Compounding Laboratory
Accuracy in Measurements

Volumentric Glassware and Other Devices Intended "to Deliver"


Pipets

Pipets are recommended for the delivery of all volumes <5 ml and are required for delivering volumes <1 ml (in the absence of an appropriate syringe). There are two basic types of pipets:

  • The single volume or transfer pipet is the most accurate and simplest to use type, but is, obviously, limited to measurement of a fixed, single volume. Generally, these pipets deliver their inscribed volume by complete drainage of the pipet from an etched mark. They are normally used for the accurate transfer of 1.0, 2.0, 5.0, 10.0, and 25.0 ml of liquid.
  • Volumetric Pipets     Volumetric Pipet

    Volumetric Pipets

  • The Mohr, or graduate multiple volume pipet, is graduated from a point near the tip to the nominal capacity of the pipet. Thus, it can delivery multiple volumes of liquid with good volumetric precision. From a practical economic stand point, the Mohr pipet is generally the preferred instrument for compounding.
  • Graduated Pipets     Graduated Pipet

    Graduated Pipets

A pharmacist can accurately compound any prescription requiring relatively small volumes of liquid if he/she has just three basic sizes of Class III Mohr pipets available:

    1 ml Mohr pipet, subdivided in 1/100 ml
    2 ml Mohr pipet, subdivided in 1/10 ml
    5 ml Mohr pipet, subdivided in 1/10 ml

Measuring Techniques

1. Single volume pipets

  • Using a respirator (rubber bulb) for suction, draw the liquid into the pipet until it is above the graduation. Caution: Since we may be using corrosive or poisonous liquids mouth pipeting is strictly prohibited in the pharmaceutics lab.

Video View a video demonstration on how to use a pipet bulb

  • Remove the pipet from the stock solution.
  • Wipe the end of the pipet with a tissue or Kim wipe.
  • While holding the pipet in a vertical position, release pressure, allow the liquid to flow into a beaker (or other vessel) until the bottom of the meniscus coincides with the etched line. Droplets which remain suspended from the tip can be removed by touching the inside of the beaker with the tip of the pipet.
  • Allow the pipet to drain for 30 seconds (or up to 5 minutes for viscous liquids) while touching the tip of the pipet to the inner side of the receiving vessel.

2. Mohr (graduated or calibrated) pipets

The manipulations are essentially the same as with single volume pipets except that fractional volumes may be transferred by noting the meniscus level before and after delivery. Thus, 1.50 ml may be delivered after observing an initial reading of 8.50 ml by allowing the liquid to flow until the meniscus reaches 7.00 ml. When working with viscous materials, it is necessary to check the final reading after waiting approximately 1 minute. (This can be very tricky!) It should be noted that the final graduation of a Mohr pipet is usually some distance above the pipet tip so that delivery is performed from graduation to graduation and not from graduation to tip as with the single volume pipet. Make certain that you know which type of pipet you are using to ensure accurate delivery.