Liquid Measurement

Dr. Amani Elsayed

Learning Objectives

• Differentiate between volumetric/nonvolumetric and TD/TC glassware or devices.

• Select glassware/liquid measurement devices which are appropriate for a particular measurement operation.

• Describe and/or demonstrate the proper techniques for using pipettes, syringes, and graduates for measuring and delivering liquids.

Introduction

• The pharmacist is concerned with liquid measurement from two perspectives.

• She should be concerned with the ability to accurately measure the components of a prescription preparation.

• She must be concerned with how the patient will measure and deliver an accurate dose of a liquid medication.

Volumentric Glassware and Other Devices Intended "to Deliver" TD

• Volumetric Pipets• They are normally used for the

accurate transfer of 1.0, 2.0, 5.0, 10.0, and 25.0 ml of liquid.

• Mohr pipette• 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.

Volumetric Pipets

Mohr pipette

Volumentric Glassware and Other Devices Intended "to Deliver" TD

• Syringes• Hypodermic Syringes• Hypodermic syringes

come in a variety of sizes.

• They are especially useful for measuring and delivering viscous liquids. Hypodermic Syringes

Volumentric Glassware and Other Devices Intended "to Deliver" TD

• Oral Syringes• Oral syringes are also

available as a device for accurately providing a dose of liquid medication to the patient. They are especially useful with children and elderly adults who frequently require nonstandard doses, and for whom accuracy of dose is most critical.

Oral Syringes

Volumentric Glassware and Other Devices Intended "to Deliver" TD

• Graduates• Graduates are used for

measuring and transferring liquids in a manner that is less precise than with a pipette.

• As a general rule, a graduate should be used which has a capacity equal to or just exceeding the volume to be measured.

Graduated Cylinders

Conical Graduates

Volumentric Glassware and Other Devices Intended "to Deliver" TD

• Calibrated Medicine Droppers• Calibrated medicine droppers

are often used to deliver small doses of liquid medication, 1 ml or less, to the patient.

• In order to maximize accuracy of dosing of a compounded solution from a medicine dropper, the pharmacist must calibrate the dropper with the intended solution at the time of delivery.

Volumetric Glassware Intended "To Contain"

• Volumetric Flasks• Volumetric flasks are

used to prepare solutions of a specific volume and concentration.

• Because of their shape, the pharmacist would not reasonably expect them to deliver their full volume.

Nonvolumetric Glassware and Devices

• Erlenmeyer Flasks, Beakers, and Prescription Bottles

• Erlenmeyer flasks, beakers, and prescription bottles are not volumetric devices and should NOT be used to measure liquids.

• "Graduation" marks on such vessels are only approximations of liquid capacity

Erlenmeyer Flask

Beaker

Nonvolumetric Glassware and Devices

• Teaspoon• The Teaspoon and the

problem concerning the actual volume of liquid contained in a "teaspoonful" has existed for many years.

• Despite various recommendations, the problem persists.

• Patients continue to self-administer liquid doses from spoons which, in all probability, are far from standard.

Guidelines for Selecting Liquid Measurement Devices

• Always select the smallest device (graduate, pipette, syringe) that will accommodate the desired volume of liquid. This will minimize the potential for errors of measurement associated with misreading the scale.

• Use a graduated pipette, syringe, or calibrated dropper to measure/deliver volumes <1 ml.

• Remember that oily and viscous liquids will be difficult to remove from graduates and pipettes, and at best require long drainage time. Consider using a disposable syringe instead, or better yet, measuring by weight rather than volume.

Guidelines for Selecting Liquid Measurement Devices

• Never use prescription bottles, nonvolumetric flasks, beakers, or household teaspoons as measurement devices, unless you calibrate it yourself.

• When small (<5 ml) or very accurate doses are required, provide the patient with a calibrated dropper, oral syringe, or similar device to ensure proper dosing.

Quiz

1) What might you recommend to provide a parent to improve the accuracy in administering a dose of ½ tsp. of antibiotic liquid to infant

A) A 3 ml oral syringe B) A dosing cup with 2 tsp. graduation mark C) A dropper with no graduation mark D) A plastic teaspoon

Quiz

2) An ointment prescription requires 30 ml of peanut oil. What is the most efficient way to measure and deliver this viscous oil?

A) 30 ml single volume pipette B) 50 ml graduated cylinder C) Calibrated beaker D) Weight

Quiz

3) The density of peanut oil is 0.915. How much will 30 ml of peanut oil weigh?

A) 27.45 g B) 32.79 C) 29.08 D) 30.915

Quiz

4) The following volumes of teaspoon, tablespoon and dropper are Correct EXCEPT (more than one answer)

A) Tsp. 15 ml B) Tbs. 15 ml B) Dropper (30 drop/ml) D) Dropper (20

drop/ml) E) Tsp. 5 ml F) Tbs. 5 ml