Views: 36

Balances 

Weighing is an essential part of almost any analysis used for both measuring the samples and preparing the standards. Weighing must be made to 3-4 significant figures e.g. 3.000g and therefore the weighing device must be both sensitive and accurate.

Types of Balances

(a) Analytical balance

The analytical balance is the most accurate and precise instrument in any laboratory. Objects of up to 100 grams may be weighed to 6 significant figures. Analytical balances are generally used for gravimetric analyses, and for the preparation of standard solutions.

These are the most important equipment in any analytical laboratory.. There are various types of analytical balances i.e.

1. General purpose balance
2. Semi analytical balance
3. Micro-analytical balance
4. Other special purpose balance

Some general rules should be kept in mind when using an analytical balance:

1. Allow samples to reach room temperature before weighing. Samples that are too hot will set up convection currents and the apparent sample weight will be incorrect. A slowly drifting apparent sample weight is indicative of this problem.
2. Chemicals should be placed in a weighing bottle, a plastic weighing tray, or coated weighing paper. Weighing paper is best for small quantities (usually <1g); weighing trays are used for larger amounts; and stoppered weighing bottles are recommended for reactive, volatile, toxic or hygroscopic substances. In the absence of a weighing bottle, a small beaker and watch glass may be substituted. Never place chemicals directly on the pan!
3. The analytical balance should be kept clean at all times. If a solid is spilled inside the weighing chamber, carefully remove it with the balance brush. The desiccant should be replaced or regenerated when it changes color. Used weighing papers and trays should be immediately discarded.

Electronic balance

Electronic balance is based on the principle that when an electric current is passed through a wire placed between the poles of a permanent magnet, a force is generated, which moves the wires outside the magnetic air gap. In this system, the force associated with the object being weighed is coupled mechanically to a servomotor that generates the opposing magnetic force

The system contains a null detector, which checks the position of the wire in a magnetic field. This detector may be an optical device consisting of a vane attached to the beam, a small lamp and a photo detector.

When the two forces are in equilibrium, the error indicator is at the reference position and the average current in the servo motor coil is proportional to the resulting force holding the mechanism at the reference position. When the beam is displaced from its balance position, the amount of radiation reaching the detector changes and causes rapid change in current through the coil.

 An error signal is sent to the circuit that generates a correction current. These current flows through the coil attached to the base of the balance pan creating a magnetic field and restoring the indicator to its reference position. The correction current needed to restore the system is proportional to the mass of the balance pan. Calibration is performed by placing a known weight on the balance pan and adjusting the circuitry to indicate the mass of the calibrating weight.

 There are many models of electronic balances available today   with numerous optional features, but a typical electronic balance will have a way of indicating the zero setting with no load on the pan and a digital display to show the weight of the mass.

Electronic balances however still have some errors   but most of these errors can be minimized by;

1. Avoiding careless  errors in weighing e.g. spillage’s  and misreading the values of the weight , the dial  or the position of the vanier
2. Ensuring that deliquescent and hygroscopic chemicals  are weighed immediately and in a closed containers  or weighing bottles
3. Glass vessels should not be wiped with a dry cloth before weighing as these may  acquire a static charge  and cause an error in  weighing
4. The sample being weighed should be at least at the same temperature as the weighing balance. Heated crucibles  and samples  should first be cooled to room temperatures  before weighing

Electronic balance may also have other possible sources of errors i.e.

1. Interference  by ferromagnetism  or magnetized samples
2. Interference by electromagnetic radiation  around the instrument
3. dust which may  lodge between  the coil and the permanent magnet  of the servomotor
4. Interference due to buoyancy and vibrations

Installation of Balances

The correct positioning of balances depends a lot on the condition of the balance room itself. The balance room should have the following requirements

1. Proximity – the balance room should be positioned in a central location for easy access by many people
2. External environment– the balance room should not be situated on the outside wall of the laboratory.
3. This is in order to avoid drought and fluctuation in temperature. It should have only one door opening into it and ventilation should be by means of air conditioners and not windows
4. Vibration – the balance room should not be situated near any source of vibration such as moving machinery, passing traffic or movement of persons near the vicinity of the apparatus
5. Door– the door to the balance room should be fitted with a spring to avoid vibration when it is slammed or banged

 Humidity is must be maintained at 50^oc and the temperature is 25 ^oc . Constant temperature is very important in order to prevent expansion of internal parts of the balance. Balance rooms should always be kept clean and the floor should be mopped and not swept, all the corners between the floor and the walls should be covered to prevent accumulation of dust.

Effects of vibration on a balance

Vibration makes accurate reading on the balance to be impossible. It also reduces the life span of the balance by causing excessive wear on the internal parts i.e. the knife-edge.  Moving machinery, traffic, people etc. cause vibration.

The basic method of overcoming vibration is by interposing a number of dissimilar materials between the balance support and the source of vibration. The materials used must be stable. if the amount of vibration is small; the balance should be placed on a rubber stopper and supported on a wooden support.

Care of a Weighing Scale

1. The balance should be kept clean at all times
2. Chemicals and dust should be wiped away with a soft cloth
3. Always cover the balance  
4. Do not apply lubricants to the knife edges or bearings
5. Servicing must be carried out by experts only