Analytical Balances

1. Analytical Balances Weighing is a required part of almost any analysis, both for measuring the sample and for preparing standard solutions. In clinical laboratory sciences we deal with rather small weights, in the order of grams to a few milligrams or less. Standard laboratory weighing is typically made to three or four significant figures, and so the weighing device must be both accurate and sensitive. There are various sophisticated ways of achieving this, but the most useful and versatile device is the analytical balance

2. Analytical Balances Most analytical balances used today are electronic balances . The mechanical single-pan balance is still used, though, and so we will describe its operation. Both types are based on comparison of one weight against another (the electronic one for calibration ) and have in common factor such as zero-point drift and air buoyancy.

3. Analytical Balances The weight of an object is the force exert on it by the gravitational attraction. This force will differ at different locations on Earth. Mass, on the other hand is the quantity of matter of which the object is composed and invariant. We really deal with masses rather than weights

4. Analytical Balances They are two types 1- Mechanical. 2- Electrical. The mechanical analytical balances are also two types 1- The equal arm balances. 2- The single pan substitution balance.

5. Analytical Balances The equal arm balance: Consist of a knife edge supporting the exact center of the beam when L1 = L2

6. The equal arm balance The length of the two lever arms are equal, therefore the mass supported by each arm will equal for the system to be in equilibrium. If an unknown mass is added to one side the beam is deflected, the deflection is offset by adding standard weight mass until the beam return to the horizontal position so that mu L1 =ms L2

7. The single pan substitution balance It is situated asymmetrically on the central knife. It has one pan and a counter weight on the other side. Series of calibrated weights are supported. When unknown weight is placed on the weighing pan, the beam deflects in the direction of the heavier side. To return the system to equilibrium, a correspondent mass of calibrated weights is removed from the front of the beam. This process is a substitution process.

8. Quality of mechanical balances: The condition of the knife edge is critical to the performance. It is made of extremely hard material. So the area of contact must be with reduced fiction. There is considerable at the point of contact with the beam. Therefore shock can easily damage these parts. Mechanical balance are also equipped with dampers oppose the movement of the beam, allowing it to come to equilibrium rapidly. Damping is either magnetic plate or air damper (piston).

9. Advantage of substitution balance over the equal arm balance: Equal arm balance is time consuming. The sensitivity in the equal arm balance depends on the load placed on the balance, while the load is always on the substitution balance. Error in substitution balance is eliminated because comparison of unknown mass with known mass takes place on the same arm. Two knife edges can reduce friction.

10. Electrical Balances In electrical balance, an electromagnetic force is used to counter balance the object's weight and restore the beam to it's zero position. The magnetic of the force is proportional to the load. Some sort of electronic 's (digital) is used to converts the current to a digital display of the weight.

11. Electrical Balances The null detector: The position of the balance beam and is used to determine the balance point at which the system is in equilibrium. To return the system to equilibrium, electromagnetic restoring forces are used in place of standard weight. The signal from the null detector is applied directly to control the current through an electromagnetic to return the balance to equilibrium.

12. Electrical Balances When the null meter read zero, the compensating force required to bring the balance to equilibrium is proportional to the weight of pan. The current required produce the compensating force is converted to a digital display of the weight.

13. Electrical Balances Advantages of electronic balance over mechanical balance: High precision “the agree between repeated weights of the same mass”. No hand. Time to make weighing is short.

14. Electrical Balances Quality assurance and maintenance: The performance of an analytical balance should judged by:- Sensitivity, “the ratio of the charge in scale division to specific weight change. Precision. Accuracy “ The agreement between the measured weight and the calibrated weight of an object”. Readability “The smallest fraction of the scale division that can be read. The capacity of the balance must also be consider in selecting the balance.

15. Maintenance The balance should be placed away from detector vibration. Object that will over load the balance and exceed its capacity should not be placed on the pan. Plastic container could generate electrostatic forces For optimum balance performance, preventive maintenance and calibration should be done according to the manufacture instructions at least one a year. Calibration should be checked every time the balance has been moved. The warm up time for electronic balance should be verified. A calibration weight at least 60% of the capacity should be used in calibration and at different pan position.

16. The Triple Beam Balance The Triple Beam Balance is a typical mechanical balance. It has a beam which is supported by a fulcrum. On one side is a pan on which the object is placed. On the other side, the beam is split into three parallel beams , each supporting one weight. In measuring the weight of an object, rather than adding additional weights, each of the three weights can be slid along the beam to increase their lever arm.It works just like a tetter-totter. If you have two people of unequal weight, the heavier person sits closer to the fulcrum to decrease their lever arm.

17. The Triple Beam Balance The triple beam balance is a rugged, inexpensive weighing instrument. It is named for the three beams which carry weights.A. The middle beam reads only in 100 g increments.B. The far beam reads only in 10 g increments.The weights in each of these must always sit in a "notch". They cannot be placed at arbitrary points on the beam.C. The weight on the front beam can be placed to read continuously from 0 to 10 grams.