* syringes.
In addition to these, pipettes are also used in the prep- aration of some pharmaceutical products. For more information on pipettes see Chapter 5 (page 71).
Conical measures
In general, conical measures rather than cylindrical measures are used in pharmacy practice (Figure 4.3). They have a number of advantages over cylindrical measures. For example:
* They are easier to fill without spilling liquid on the sides above the required level.
Table 4.3 Main volume units used in pharmacy
Unit Abbreviation Litre equivalent 1 litre 1 L (or 1 litre) 1 litre
1 millilitre 1 mL 0.001 litre
1 microlitre 1mL 0.000 001 litre
Key Skill 4.3 Conversion between base volume units
To convert between the base volume units, multiply or divide the figure by 1000. For conversions between more than one base unit (e.g. microlitres to litres), convert via any intermediate units (e.g. millilitres).
Conversion from litres to microlitres Conversion from microlitres to litres 1 microlitre 1 litre 1000 1000 1 millilitre 1 millilitre 1000 1000 1 litre 1 microlitre
* It is easier to drain out the preparation.
* It is easier to rinse out the residue left after draining viscous liquids into the preparation.
* They are easier to clean after use.
On the other hand, it must be borne in mind that compared to cylindrical measures, with conical measures:
* It is harder to read the meniscus accurately.
* It is more difficult to estimate volumes between graduations (although in practice, owing to the error, this would be considered poor professional practice and is never done).
It is not good practice to use a conical measure to measure a volume that is smaller than half of the total volume of the measure. Because of the shape of the measure, it is more accurate, for example, to measure 10 mL in a 10 mL conical measure than it would be to measure 10 mL in a 100 mL conical measure.
Stamped conical measures can be used to measure any marked amount but it is sensible to select the smallest measure for the desired volume. For volumes less than 1 mL, a syringe is used (see below) although a 5 mL syringe can be used to measure graduated volumes up to 5 mL.
The markings on a beaker are only approxima- tions and cannot ever be used to measure any volume accurately.
Remember, when measuring liquids, that the bot- tom of the meniscus should be in line with the desired graduation mark (Figure 4.4). When reading the vol- ume of the liquid, ensure that your eye is in line with
Figure 4.3 Selection of conical measures (250 mL, 100 mL, 25 mL, 10 mL and 5 mL). Position of eye in relation to meniscus Bottom of meniscus to line up with volume mark Volume marks
Figure 4.4 Diagram to show which edge of the meniscus is to line up with the volume mark (note – the meniscus has been exaggerated in this diagram).
the meniscus. This will avoid any parallax errors. If your eye line is not level with the meniscus (for exam- ple you are looking down onto the meniscus) errors in reading the correct volume will occur (Figure 4.5).
In addition to ensuring that your eye is in line with the meniscus, it is important that the measure remains on a flat surface. In many cases, this will involve moving your head down to the correct level. Do not lift the measure to the level of your head as it will not be possible to read the volume accurately with the liquid moving about in the measure. Even if the liquid appears stationary, it is unlikely that the measure will be level. When measuring certain dark or coloured liquids it may be difficult to see the bottom of the meniscus clearly. In these cases, placing a piece of white or coloured card or paper behind the measure may help. Finally, it is important to avoid spilling any liquid down the side of the measure as this could result in the incorrect amount of liquid being added to the prepa- ration. It is also poor pharmaceutical practice.
Example 4.11Measuring a colourless liquid: 25 mL potable water
1 Select an appropriately sized conical measure.
2 Check that the measure is clean and dry. 3 Obtain a beaker of potable water to use as
stock.
4 Measure 25 mL potable water in the conical measure. Check the volume and sign your initials in the ‘Dispensed by’ box on the product worksheet.
5 Ask for a second check from another member of staff. Once the check has been performed, the member of staff initials the ‘Checked by’ box on the product worksheet.
Extract from product worksheet:
Ingredient Batch number Quantity Dispensed by Checked by Potable water N/A 25 mL
Error of parallax
Meniscus
When measuring a viscous liquid (e.g. Syrup BP), it is important once measured that the measure is drained properly to ensure all of the measured liquid is transferred. This will involve holding the measure above the vessel that the ingredient is to be transferred to for several seconds.
Syringes
Traditionally, small volumes have been measured in pharmacy using a graduated pipette (see Chapter 5, page 71). However, more recently, 1 mL syringes have been used to measure graduated volumes less than 1 mL, and 5 mL syringes to measure graduated volumes up to 5 mL. It must be remembered that although their use is commonplace, syringes are less accurate than pipettes.
For all syringes, the following technique should be employed:
1 Select an appropriate syringe.
2 Move the plunger up and down a couple of times to check that it moves smoothly without sticking. 3 Draw up liquid in excess of the required volume. 4 Invert the syringe so the nozzle is pointing
upwards.
5 Draw the plunger back a little further to allow all the liquid in the nozzle to enter the main chamber. 6 Tap the syringe to allow all the trapped air to
come together at the top of the syringe. 7 Expel the air.
8 Re-invert the syringe over the liquid and push the plunger until the desired volume is reached. The desired volume is reached when the measuring edge of the plunger is in line with the desired volume mark in the syringe barrel (Figure 4.6).
9 Add the measured liquid to the preparation. Do NOT expel the liquid left in the nozzle. The syringe is calibrated to allow for this excess.