Two Pan or Equal Arm Analytical Balance

Two pan analytical balance is a classical weighing instrument used for measuring mass of an object. It is also called equal arm analytical balance because both arms of the beam are equal in length.

It works on the principle of first class lever. The beam is supported at the centre on a sharp knife-edge. This centre point acts as the fulcrum.

Two pans are suspended from the two ends of the beam. The pans are at equal distance from the centre. The unknown sample is placed in one pan and known standard weights are placed in the other pan.

The standard weights are added until both sides become balanced. When the beam becomes horizontal, the balance is in equilibrium. The mass of the object is equal to the total mass of standard weights used.

This balance gives accurate result only when both arms are exactly equal. Even a very small difference in arm length may produce error in weighing.

The use of equal arm balance is very old. Early forms of balance were used in Indus Valley and ancient Egypt for trade and weighing of goods. Later metal weights and knife-edges were used to improve the balance.

The two pan analytical balance was developed as a scientific instrument by Joseph Black in the mid-18th century. It used rigid beam and precision knife-edges for reducing friction. This made the balance useful for accurate chemical measurement.

Later this balance was replaced in many laboratories by single pan substitution balance of Erhard Mettler. The single pan balance reduced the problem of error due to unequal arm length and made weighing faster.

Principle of Two pan or equal arm analytical balance

Principle of Two Pan or Equal Arm Analytical Balance is based on the principle of first class lever and law of moments. In this balance the fulcrum is present at the centre of the beam.

Two pans are suspended on both sides of the beam. The distance of both pans from the fulcrum is same. So the two arms are equal in length.

When unknown object is placed in one pan, it produces turning effect on that side. Standard weights are then placed in the opposite pan.

The weights are added until the beam becomes horizontal. At this position the clockwise moment and anticlockwise moment become equal.

As the arm length is same on both sides, the mass of unknown object becomes equal to the total standard weights added. Thus the mass of object is measured by balancing it with known weights.

Parts of a Two pan or equal arm analytical balance

The following are the parts of Two Pan or Equal Arm Analytical Balance–

Beam
It is the central horizontal lever of the balance. It is usually made up of brass or aluminium alloy and supports the two pans at both ends.
Knife-edges
These are sharp triangular prism like parts. They are made up of agate, steel or synthetic sapphire and helps the beam and pans to move with very less friction.
Bearing planes
These are flat surfaces on which the knife-edges rest. They are generally made up of agate and help in smooth movement of the beam.

Pointer and scale
The pointer is attached at the centre of the beam. It moves over a small graduated scale and shows whether the balance is in equilibrium or not.
Pans
These are the trays suspended from both ends of the beam. One pan holds the sample and the other pan holds standard weights.
Stirrups and hooks
These parts connect the pans with the end knife-edges of the beam. They support the pans during weighing.

Standard or pillar
It is the central supporting post of the balance. It holds the beam mechanism in proper position.
Arrestment mechanism
It is used to lift the beam and pans from the knife-edges when the balance is not in use. It prevents wear and damage of delicate parts.
Case or draft shield
It is the protective enclosure around the balance. It protects the balance from dust, air current and outside disturbance.

Rider
It is a small calibrated wire weight placed on the graduated beam. It is used for measuring very small fractions of mass.
Rider arm
It is used to move the rider along the beam. The rider is shifted to get fine adjustment during weighing.
Adjustable vane or milled heads
These are small adjustable weights present on the beam. They are used to set the balance at zero before weighing.

How to Use Two pan or equal arm analytical balance?

The following are the steps to use Two Pan or Equal Arm Analytical Balance–

The balance is first checked properly. The balance should be level and the pans should be clean.
The beam is released gently with empty pans. The pointer should swing equally on both sides of zero mark.

If the pointer does not come to zero, the balance is adjusted before weighing. Zero adjustment should be done slowly.
The beam is arrested before placing anything on the pans. This protects the knife-edges from sudden load.
The sample is prepared in a clean watch glass, weigh boat or other suitable vessel. Chemical should not be placed directly on the pan.

The sample and vessel should be at room temperature. Hot or cold material may produce air current and affect the reading.
The sample or vessel is placed on the left pan. It should be placed carefully and not dropped.
Standard weights are placed on the right pan by using forceps. Weights should not be touched by bare hand.

Heavier weight is tried first and the beam is released gently. If it is too light, smaller weights are added one by one.
If the weight is too heavy, the beam is arrested and that weight is removed. Then the next lighter weight is placed.
This process is continued until the balance comes near equilibrium. Small fractional weights are used at the end.

When the balance is nearly balanced, the glass doors are closed. This prevents air draft and breath from disturbing the reading.
The rider is moved by external arm for final adjustment. It is moved slowly along the graduated beam.
The beam is released gently and the pointer is observed. The rider is adjusted until the pointer swings equally on both sides of zero mark.

The weights present on the pan are noted before removing them. The rider value is also noted.
The total mass is calculated by adding the standard weights and rider value. This gives the apparent mass of the sample.
After weighing, the beam is arrested again. The sample is removed and all weights are returned to their proper places.

The rider is removed or placed back to zero position. The balance is kept clean and ready for next use.

Precautions Two pan or equal arm analytical balance

The following are the precautions for Two Pan or Equal Arm Analytical Balance–

The balance should be checked for level before weighing. It should be kept in vibration free place, away from direct sunlight, air draft and sudden change of temperature.

Standard weights should not be touched by bare hand. Forceps should be used for placing and removing the weights.
The standard weights should be placed on the right hand pan only. Same practice should be followed every time to avoid confusion.
The beam arrestment should be used while changing weights or moving the rider. This protects the delicate knife-edges from damage.

The beam should be arrested when pointer is near the zero mark. Sudden arrest at extreme position may disturb the balance.
All movements should be done gently. The pans, weights and rider should not be moved roughly.
Substance should not be placed directly on the pan. Clean watch glass, weighing boat or suitable vessel should be used.

Hot or cold object should not be weighed directly. The object should be brought to room temperature before weighing.
Hygroscopic substances should be weighed in closed vessel. Otherwise they absorb moisture from air and the weight increases.
The sliding door of the balance case should be closed during final reading. This prevents air current from affecting the pointer.

The weights used should be noted before removing them from the pan. The empty places in the weight box should also be checked for confirmation.
The rider should be removed or brought back after weighing. It should not be left on the beam after the work.
The pans and balance case should be kept clean. Spilled material should be removed immediately.

The balance should not be overloaded. Excess load may damage the beam and knife-edges permanently.

Some common errors to avoid when using a two-pan or equal-arm analytical balance

The following are the common errors to avoid when using Two Pan or Equal Arm Analytical Balance–

The balance should not be used without leveling. The base thumb-screws should be adjusted before weighing, otherwise wrong reading may occur.

Sudden or jerky movement should be avoided. The beam should be released slowly because rough movement may damage the knife-edges.
Standard weights should not be touched by bare fingers. Oil, moisture and heat from hand may change the mass, so forceps should be used.
Wrong pan should not be used for sample and weights. The object is kept on the left hand pan and standard weights are kept on the right hand pan.

The arrestment mechanism should not be neglected. Beam and pan arrest should be used while changing weights or moving the rider.
The beam should not be arrested while it is swinging widely. It should be arrested when the pointer is near the zero mark.
The case door should not be left open during final reading. Air draft may disturb the swing of the pointer.

Chemicals or powders should not be placed directly on the pan. Clean watch glass, weighing boat or suitable vessel should be used.
Hot or cold object should not be weighed directly. It may create convection current and give wrong reading.
Hygroscopic substance should not be weighed in open condition. It should be weighed in closed vessel because it absorbs moisture from air.

The balance should not be overloaded. Excess load may damage the beam and knife-edges permanently.
The rider value should not be forgotten during calculation. After weighing, the rider should also be removed or returned to proper position.

Factors that can affect the accuracy of a two-pan analytical balance

The following are the factors that can affect the accuracy of Two Pan Analytical Balance–

Unequal arm length affects the accuracy of balance. Both arms should be exactly equal from the centre, otherwise the same mass on two pans may not balance properly.
Beam flexing may occur when heavy load is placed on the pans. The beam may bend slightly and the effective arm length changes.
Knife-edge wear reduces the sensitivity of balance. If the knife-edges become blunt or chipped, friction increases and the beam does not move freely.

Air current affects the pan and beam. Air from door, window, ventilation or breath may disturb the swing of the pointer.
Temperature fluctuation affects the balance parts and sample. Warm or cold sample may produce convection current inside the case and reading becomes unstable.
Air buoyancy may affect the weighing result. The sample and standard weights displace air differently, especially when their densities are different.

Vibration disturbs the equilibrium of balance. Foot movement, nearby instruments and building vibration may prevent stable reading.
Static electricity gives unstable reading. Plastic or glass container may develop charge and produce attraction or repulsion.
Improper handling causes error in weighing. Touching weights, pans or sample by bare hand may add oil, moisture and heat.

Dust and chemical spill affect the result. Extra material on pan or inside case may add unwanted mass.
Humidity affects hygroscopic sample. Such sample absorbs moisture from air and its mass increases during weighing.
Improper leveling gives wrong reading. If the balance is not horizontal, the beam does not swing equally on both sides.

Magnetic interference may affect the weighing. Magnetic sample may attract metal parts of the balance and give incorrect result.

How do you calibrate a two-pan or equal-arm analytical balance?

The following are the steps for calibration of Two Pan or Equal Arm Analytical Balance–

The balance is first kept in level position. Base thumb-screws are adjusted until the balance becomes horizontal.

The beam is released gently with empty pans. The pointer swing is observed on the scale.
If the pointer does not swing equally on both sides, the pans and upper beam are cleaned with camel hair brush. Dust may disturb the balance.
If the swing is still not equal, the centre vane or end screws are moved slightly towards lighter side. This is done until the swing comes at centre.

The sensitiveness of balance is checked with empty pans. The deviation of pointer is first noted.
A 1 mg rider or weight is then added on the beam and new deviation is observed. This shows the response of balance to small mass.
The same sensitiveness test is repeated with equal load in both pans. Loads like 0.5 g or 5 g may be used.

The equality of arms is checked by placing equal heavy weights in both pans. For example, 50 g weight is kept in each pan and deviation is noted.
The weights are then interchanged between left and right pans. If the arms are equal, the deviation should be equal and opposite.
The standard weights are also calibrated. This is important because the final mass depends on the accuracy of weights used.

For weight calibration, a known reference weight is placed in right hand pan. A counterpoise weight is placed in left hand pan and balance is made equal.
The reference weight is then replaced by matching weight from the weight box. The change in pointer deviation is noted.
From this deviation, the small difference in true mass of the weight is calculated. In this way the weights are checked and corrected.

After calibration, the beam is arrested and all weights are returned to their proper place. The balance is left clean and closed.

Uses of Two pan or equal arm analytical balance

The following are the uses of Two Pan or Equal Arm Analytical Balance–

It is used for high precision mass comparison with standard weights. It is also used for calibration work where unknown mass is compared with known mass.

It is used as teaching instrument in schools and colleges. The working of beam, pans and weights helps to understand lever principle, mass and metrology.
It is used in remote field work and places where electricity is not available. It works mechanically, so no power supply is required.
It is used in chemical analysis and assaying work. Powders, chemicals, crucibles and other laboratory vessels can be weighed accurately.

It is used for density determination of solids and liquids. By using Archimedes’ principle and suitable liquid, specific gravity can be calculated.
It is used in pharmaceutical and food quality testing. Drug compounds, excipients, food samples, reagents and reference standards can be weighed.
It is used for general laboratory work like sample preparation, differential weighing, formulation, recipe calculation and percent weighing.

It is used for pipette calibration and piece counting. The mass of delivered liquid or number of similar small objects can be found by weighing.

Advantages of Two pan or equal arm analytical balance

The following are the advantages of Two Pan or Equal Arm Analytical Balance–

It does not need electricity for working. So it can be used in field place, developing area and also during power failure.

It is strong mechanical balance. If it is handled properly, it can be used for many years without major repair.
It is less costly than electronic analytical balance. Maintenance cost is also low because no electronic system is present.
It is useful for teaching purpose. Students can see the beam, pans, weights and movement of pointer directly.

It helps to understand mass measurement and lever principle. The balancing of two pans shows how known mass is compared with unknown mass.
It can be used for accurate weighing by special method. In Gauss’s transposition method, the sample and weights are interchanged between two pans to reduce error of unequal arm length.
It can reduce systematic error during careful weighing. Double transposition weighing helps to reduce error caused by slow environmental drift.

It gives reliable result when standard weights and proper technique are used. The mass is obtained by direct comparison with known weights.

Disadvantages of Two pan or equal arm analytical balance

The following are the limitations of Two Pan or Equal Arm Analytical Balance–

Its accuracy depends on equal length of both arms. If one arm is slightly longer or shorter, the mass reading becomes wrong.

It is difficult to make both arms exactly equal. Even very small difference in arm length may produce error in final weighing.
Beam flexing may occur when heavier weights are placed on the pans. The beam bends slightly and this changes the effective arm length.
It takes more time for weighing. Standard weights have to be added, removed and adjusted again and again to get equilibrium.

It needs careful handling during weighing. Rough movement may damage the knife-edges and reduce the sensitivity.
It depends on operator skill. Wrong weight selection, wrong rider reading or wrong observation of pointer may give error.
Air current and vibration may affect the pointer movement. So the balance needs stable and protected place for use.

It is slower than single pan balance and electronic balance. Direct weighing by matching weights is a tedious process.

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