विषय सूची
THE ANALYTICAL BALANCE
Weighing is a required part of almost any analysis in chemistry and many types of balances are used in chemical laboratories. These are divided into two main classes:
ROUGH BALANCES:-
which detect a difference in weight of one-tenth of a gram (‘sensitive to one-tenth of a gram) and
ACCURATE BALANCES:-
The main parts of the chemical balance are:
1. The Beam- It is the most important part of the analytical balance. It is made from a light and rigid aluminium alloy. It is graduated like a scale in 50 or 100 equal parts on either side of the central zero mark.
2. Terminal knife edges- At equal distances from the centre of the beam are placed two terminal knife edges of agate.
3. Adjusting screws- At the extreme ends of the beam, there are two adjusting screws,which are moved inward or outward to adjust the equilibrium of the empty balance.
4. Central agate- The beam is mounted on a central agate, prism-like edge.
5. Central pillar- It supports the central agate.
6. Stirrups- The terminal knife edges support stirrups which are used to suspend pans.
7. Beam arrest- It supports the two ends of the beam, while the balance is at rest.
8. Pointer- From the centre of the beam extends a long pointer which swings over a scale below.
9. Pointer scale- It is a scale at the lower end of the central pillar.
10. Knob- It is a key or lever provided at the base of the balance. It is directy linked with central agate- By turning it clockwise or anticlockwise, the pans are set in swings or brought to rest.
11. Pan arrests- These support the pans when the balance is at rest.
12. Gravity nut- It is a brass nut movable along the pointer.By raising the gravity nut,swings of the pointer are made wider. Thus, it is used to adjust the sensitivity of the balance.
13. Levelling screws- There are three levelling screws at the base of the balance. By turning these clockwise or anticlockwise, the balance can be levelled. The correct level of the balance is checked by a plumb line or by a spirit level fixed to the balance.Before use, the balance must be levelled.
14. Glass case- The balance is properly enclosed in a glass case to protect it from air currents, dust, moisture and laboratory fumes.
15. Rider rod- This is a rod provided at the top of the balance. It carries a rider hook for the purpose of carrying a rider.
