Weighing: use of laboratory balances

Balances may be either electrically or manually operated. All types should be posi- tioned on a firm level bench away from vibrations, draughts and direct sunlight. 

The balance is used to weigh chemicals for production of reagents, and cleanliness is essential if accurate results are to be obtained: 

● Remove dust by blowing or using a soft brush. 

● Remove stains or chemicals using a soft brush. 

● Use a plastic weigh boat or filter-paper to weigh chemicals on the balance; never place chemicals directly on to the pan.

Important: If water has been used to clean the balance, make sure that it is thoroughly dry before weighing. Always set the balance to zero before weighing. Check the accuracy of the balance regularly according to the manufacturer’s recommen- dations. Handle loose weights with forceps. 

1. Sensitivity of a balance 

The sensitivity corresponds to the smallest mass that makes the pointer move over one division on the scale. For example, if the sensitivity of a balance is 1 mg, this means that a mass of at least 1 mg is needed to move the pointer. 

For routine laboratory purposes, the sensitivity of a balance can be considered to be the smallest mass that it will measure accurately. 

2. Open two-pan balance (Fig. 3.28) 

The two-pan balance has two pans supported by shafts. It may be designed for use with separate weights, as illustrated in Fig. 3.29, or may incorporate a graduated arm with a sliding weight. It is used to weigh large amounts (up to several kilo- grams) when a high degree of accuracy is not required, e.g. 22.5 g, 38 g, 8.5 g,380 g. 

Sensitivity: 0.5 g. 

If the pans are made of easily scratched or corroded material, protect them with circles cut out of strong plastic or old X-ray films; the two circles should be of equal weight. 

Instructions for use 

1. Place the bottle containing the substance to be weighed to the left of the balance. 

2. Place on the left-hand pan the receptacle (folded paper or dish) in which the substance will be weighed. 

3. Place on the right-hand pan the weights equivalent to the weight of the recep- tacle plus the amount of the substance to be weighed. 

4. To measure out the substance to be weighed, hold the bottle tilted in your left hand (label upwards) and tap the neck of the bottle gently with your right hand, so that the powder or crystals to be weighed fall little by little into the receptacle (Fig. 3.30). (Use a clean spatula to dispense small amounts of substances for weighing.) 

When the substance has been weighed, move the bottle to the right-hand side of the balance (Fig. 3.31).

Fig. 3.28 An open two-pan balance

Fig. 3.29 Set of weights (in grams) for use with an open two-pan balance

Fig. 3.30 Measuring out the substance to be weighed

Fig. 3.31 Keep weighed and unweighed substances apart to avoid confusion

Thus place: 

— the weighed substances on the right 

— the unweighed substances on the left. This avoids confusion. 

Read the label three times: 

— before taking the bottle off the shelf; 

— while weighing the substances (label facing upwards); 

— after weighing, when you move the bottle to the right of the balance. 

3. Analytical balance 

This balance has two pans suspended from a cross-beam inside a glass case. Use the balance: 

— to weigh small quantities (up to 20 or 200 g, depending on the model); 

— when great accuracy is required: e.g. 3.85 g, 0.220 g, 6.740 g. Sensitivity: 0.1–0.5 mg, depending on the model.

Components (Fig. 3.32) 

● Cross-beam (CB). This is the structure from which the pans are suspended. 

● Knife edges (KE1, KE2, KE3). These support the beam at the fulcrum during the weighing and give sensitivity to the balance. Those on the beam support the suspended pans. 

● Stirrups (S1, S2). 

● Pointer (Pt). 

● Pans (P). 

● Beam release screw (or pan lock control) (B). Locks the pan so that the sudden addition of weights or chemicals will not damage the delicate knife edges. 

● Adjusting screws (AS1, AS2). Used only for initial adjustment of the unloaded balance to a reading of zero. 

Figure 3.33 shows a set of weights for use with an analytical balance.

Instructions for use 

● Always ensure that the cross-beam is at rest (beam release screw tightened) be- fore placing the weights and the substance to be weighed on the pans.

Fig. 3.32 Components of an analytical balance AS1, AS2: adjusting screws; B: beam release screw; CB: cross-beam;

KE1, KE2, KE3: knife edges;  P: pans; Pt: pointer; S1, S2: stirrups.

Fig. 3.33 Set of weights for use with ananalytical balanceSingle pieces: 1 g, 2 g, 5 g, 10 g, 20 g,50 g, 100 g, 200 g and 500 g.Single fractional pieces: 2 mg, 5 mg,10 mg, 20 mg, 50 mg, 100 mg, 200 mgand 500 mg

● Check that the pans are balanced (after closing the glass case) by unscrewing the beam release screw. 

● Always place the substance to be weighed on a piece of paper folded in four, or in a watch glass or porcelain dish. 

● Use the adjusting screws (AS1 and AS2) to obtain a perfect balance when com- pensating for the weight of the receptacle in which the substance will be weighed. 

● Always use forceps to pick up the weights. 

● Always put the cross-beam back at rest before removing the weights and the substance that has been weighed from the pans.

4. Dispensary balance (Fig. 3.34)

This balance also has two suspended pans, but it has no glass case and no supports.

Sensitivity: 5–10 mg.

The dispensary balance is more accurate than the open two-pan balance, but weighs only up to 50 g.

After using the dispensary balance, put it away in a closed cupboard.

Fig. 3.34 A dispensary balance