The Pharmaceutics and Compounding Laboratory
Accuracy in Measurements

Syringes


Hypodermic Syringes

Hypodermic syringes come in a variety of sizes ranging from 0.5 ml (calibrated 0.01 increments) to 60 ml (calibrated in 2 ml increments). Syringes may be used to deliver a wide range of liquid volumes with a high degree of accuracy. They are especially useful for measuring and delivering viscous liquids.

Hypodermic Syringe    Hypodermic Syringe

The table below indicates that measurements made with syringes are more accurate and precise than those made with graduated cylinders. Besides being easy to operate, these syringes are unbreakable and economical. They are applicable in other experiments as long as the reagents do not react with the chemicals used to manufacture the syringes.

Volume of Water Delivered by Graduate Cylinder and Plastic Syringe
 
Actual Volume Delivered
Nominal Volume (mL) 10-mL Graduated Cylinder 10-mL Plastic Syringeb
1.0 0.94 .03a 1.01 .02b
5.0 4.94 .04 4.98 .03
10.0 9.75 .03 9.95 .02
10.0 - 9.98 .02

aStandard deviations were obtained from five trials, except for the last entry where five different syringes were used with one trial each.
bWith Luer-Lok tip, manufactured by Becton-Dickinson and Co., Rutherford, NJ.



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 Syringe

Measurement Techniques with Syringes


Anatomy of a Syringe

Fluids are pulled into the syringe by pulling back on the plunger. The tip of the cannula (needle) must be fully submerged in the fluid to prevent drawing air into the syringe.

The tip of the plunger is covered by latex. The point at which the latex presses against the wall of the syringe barrel forms a line or guide that is lined up with graduation marks on the side of the barrel to determine the volume that will be delivered when the plunger is depressed.

Generally speaking, an excess of solution is drawn into the syringe so that any air bubbles may be expelled by holding the syringe cannula end up, tapping the air bubbles up into the hub, and depressing the plunger to expel the air. This ensures that the hub and cannula will be completely filled with solution and the volume of delivery will be accurate. Serious dosage errors can occur by failure to consider the volume of solution contained in the hub and cannula of the syringe.

Syringes come in a variety of sizes. Like graduates and pipets, they provide for the most accurate measurement when the capacity of the syringe equals or barely exceeds the volume to be delivered. Cannula sizes also vary, both in length and diameter. For the purpose of measuring viscous liquids, one would chose a syringe with a short, large diameter cannula OR remove the cannula altogether.