Using a Chemical Balance

Chemical balance is a delicate instrument which is used to weigh small amounts of substances accurately up to the fourth place of decimal. It is necessary for the volumetric analysis as the substances used to prepare standard solutions are weighed.

Aim:

To set the chemical balance for the preparation of standard solutions (oxalic acid and sodium carbonate) in qualitative analysis.

Theory:

The laboratory instrument built for measuring weight is called the balance. The name is derived from mechanical devices that utilise known weights to balance the object to be weighed across a fulcrum. Quantitative analysis involves weighing with great accuracy. Analytical balance in quantitative analysis can be used for weighing substances not heavier than 200g. The least count is 0.0002g.

An electronic balance is one that uses an electromagnet to balance the object to be weighed on a single pan. A chemical can be conveniently weighed on a piece of weighing paper without having to determine the weight of the paper. Taring means that the balance is simply zeroed with the weighing paper on the pan.

The balance is enclosed in a glass case which protects it from dust, air movements etc. The base of the balance rests on screws whereby the edges and agate plates on which they rest are brought into a horizontal position by means of a plumb bob attached to the balance column. The balance pans are made of light metal coated with nickel to prevent oxidation. Substances should not be placed on pans directly. It should be weighed on watch glasses or in weighing bottles.

A balance that is used to obtain four or five digits to the right of the decimal point in the analytical laboratory is called the analytical balance. The modern laboratory utilises single pan electronic analytical balance almost exclusively for precision. In the weight, box weights are arranged in the order of 50, 20, 20, 10, 5, 2, 2, 1 and it also has a rider made of thin wire. The weight of the rider is 0.0002g. The rider is placed on the beam with the help of a hook which is fitted with the balance.

Materials Required:

1. Chemical balance
2. Weight box
3. Weight tube
4. Substance to be weighed
5. Watch glass

Apparatus Setup:

Procedure:

1. Raise the beam slowly and note that the pointer swings equal decisions on both sides of the zero mark.
2. Adjust the balance before taking weights on the pan.
3. Always take the weight of the permitted load and never overload the balance.
4. If the weighing substance is wet use a watch glass or weighing bottle. Do not place wet substances directly on the pan.
5. Do not weigh if the substance is hot.
6. While weighing only use the side doors of the balance.
7. The weights, fractional weights or rider should not be touched directly with the finger.
8. Always use the same balance for weighing different substances to reduce error.

Rider:

The riders are the sliding pointers positioned on top of the beams to show the pan and beam weight in grams. It is a loop of a small aluminium wire and usually weighs 10 mg. It can be easily seated on the balance beam.

Result:

The chemical balance has been set to weigh the given substance whose standard solution has to be prepared.

Precautions:

1. Never near the thumb of the pipette. Close the index finger to it.
2. Do not knock out the solution’s last drop from the pipette’s glass jet end.
3. Always hold the pipette’s lower end in the liquid while being sucked.
4. Fill the burette with the solution carefully and see that the stopcock is not leaking.
5. Carefully handle appliances and chemicals.
6. To read, position the eye precisely on the solution’s meniscus level.