Balances and weighing

1. General purpose balances which weigh to the nearest 0.01 g with a capacity of about 300 g. Chemicals may be dispensed for weighing, into a suitable weighing container, directly onto these balances.
2. Analytical four-figure balances for quantitative work, which weigh to the nearest 0.0001 g (0.1 mg) and have a maximum capacity of about 100 g. Chemicals must not be transferred onto the balance at any time and analytical balances must only be used for weighing by difference.

General-purpose balances The most useful feature of this type of balance is the electronic zero facility (self-taring), which means the mass of the weighing container can be subtracted automatically before weighing chemicals. To operate a standard self-taring balance: Check that it is level, using the adjustable feet to centre the bubble in the spirit level (usually at the back of the machine). For relatively accurate work or when using in a fume cupboard, make sure that the draught shield is in place. Ensure that the balance is switched on: the display should be lit. Place an empty weighing container centrally on the balance pan and allow the reading to stabilize. If the object is larger than the pan, take care that no part rests on the body of the balance or the draught shield as this will invalidate the reading. Press the tare bar to bring the reading to zero. Place the chemical or object carefully in the weighing vessel: Solid chemicals should be dispensed with a suitably sized clean spatula. Non-volatile liquids should be dispensed using a Pasteur pipette but take the weighing container off the balance pan before dispensing; then reweigh the liquid plus container. Repeat until the desired weight is obtained. Allow the reading to stabilize and make a note of the reading. If you have added excess chemical, take great care when removing it. Remove the container from the balance, remove the solid (with a spatula) or liquid (with a Pasteur pipette) and reweigh. If you need to clean any deposit accidentally left on or around the balance, switch off the balance. Take care not to exceed the limits for the balance: while most have devices to protect against overloading, you may damage the mechanism.

Fig. 4.3 Single-pan balance: (a) general purpose, two decimal places

Analytical balances These are delicate precision instruments and as such are likely to be found away from the open laboratory in draught-free conditions on a vibrationdampened surface. Analytical balances are maintained to the highest specifications and should need no adjustments on your part, such as levelling and zero adjustment. The key points for using an analytical balance are summarized below: No chemicals must be transferred within the weighing compartment of the balance. If it has a 'locking' function, the balance pan must always be 'locked' when placing and removing objects onto and from the balance pan. The doors of the balance must always be closed when taking measurements. analytical standard solution is given below

Fig. 4.3 Single-pan balance: (b) analytical, four decimal places.

Weighing containers These come in various materials, shapes and sizes: from glass weighing boats to beakers and even special glazed paper. The weighing container to be used depends on several factors: The amount of chemical to be weighed. The properties of the chemical to be weighed: is it solid, liquid, volatile, corrosive, deliquescent, hygroscopic? How and into what type of vessel it is to be transferred. The accuracy to which it is to be weighed. Some of the common types of weighing container are shown in Fig. 4.4. For analytical procedures, only weighing boats, weighing bottles or glass or plastic sample tubes should be used. Weighing boats are used to transfer a solid directly into a volumetric flask via the neck of the weighing boat: this procedure* is recommended when the chemical is known to be totally soluble in the solvent and allows you to omit the solution preparation stage in a beaker or conical flask. You must ensure that the neck of the weighing boat will fit well inside the ground-glass joint of the neck of the volumetric flask so that all the chemical can be washed down the sides of the volumetric flask and does not stick to the ground-glass joint during the quantitative transfer. For general-purpose work, weighings can be made directly into preweighed or 'tared' conical flasks or beakers, again to avoid a transfer stage. Much more common is the use of disposable plastic weighing dishes of the appropriate size. The edges of these dishes can be squeezed together to form a 'funnel' to prevent losses when transferring the solid. Remember that plastic disposable dishes may dissolve in organic solvents such as prop anone (acetone), toluene, etc., and should not be used for low-melting organic solids or liquids. Watch- and clock-glasses should be avoided if you wish to transfer the solid into narrow-necked vessels such as conical flasks or sample tubes since it very difficult to direct the solid into the narrow opening of the vessel from the large 'flat' surface of the watch- or clock-glass. In such cases, or when large amounts of solid are to be transferred, it is advisable to use a wide-necked filter funnel called a 'powder funnel'.

Fig. 4.4 Weighing containers: (a) plastic weighing dishes; (b) weighing bottles (c) weighing boat.

Fig. 4.5 Transferring a solid using glazed paper.

Fig. 4.6 Transferring a solid to a narrownecked flask.

1. Place the clean, dry weighing boat or sample tube on a general-purpose two-decimal-place balance and zero the balance.
2. Weigh out the calculated amount of chemical within the accuracy of the balance.
3. Check the zero reading on the analytical balance by pressing the bar/button with the balance doors closed.
4. Relock the balance pan by pressing the bar/button.
5. Carefully transfer the weighing boat or sample tube to the balance pan of the analytical balance (for very acurate work use tweezers or the tongs since the sweat from your fingers will contribute to the weight recorded) and close the balance door.
6. Release the balance pan by pressing the bar/button, allow the balance to stabilize and record the weight of the chemical and container. If the last decimal place 'cycles' between two or three numbers, determine the mid-point of the 'cycle' and record this value as the weight.
7. Lock the balance pan by pressing the bar/button, remove the sample container and transfer the solid to your volumetric flask, beaker or conical flask by pouring, but do not wet the weighing boat or sample tube with solvent.
8. Replace the weighing container on the analytical balance pan, close the balance door and weigh if the last decimal place 'cycles' and record this value as the weight of the 'empty' weighing container.
9. Lock the balance pan by pressing the bar/button and remove the weighing container from the balance.
10. Subtract the weight of the 'empty' weighing cotainer from that of the weighing container plus sample and you now know the mass of chemical, to an accuracy of four decimal places, which has been transferred into your volumetric flask, beaker or conical flask.