Weighing in on the topic of analytical balances.

FORMULATING accurately is the key to creating and duplicating a product, and balances serve as basic tools for the measurement of mass.

Simple balances were used as long ago as 5000 B.C. In fact, the word "balance" is derived from the Latin word bilanx, which means "two pans." The modern analytical balance originated during the mid-18th century when a Scottish chemist developed the technique of using a lightweight, rigid beam supported on a knife-edged fulcrum. The accuracy achieved by this innovation surpassed all other devices.

The original instrument operated as a seesaw. A sample was placed on one pan and a known weight was added to the other pan until equilibrium was established. To avoid handling extremely small reference weights, balances were equipped with a rider that could be moved along the beam to make small torque changes. Since the operation of such balances was affected by air currents and humidity, the working parts were enclosed in a glass case. It also requires a solid, vibration-free bench.

Hundreds of variations were made to the above balance, including the use of aluminum for the beams, but the fundamental design served as the mainstay in laboratories from 1750 to about 1950. Some lab rats even collect these double pan balances! In 1947, Ehrhard Mettler, a Swiss firm, began commercialization of the single pan weighing balance.

In the single pan substitution balance, the opposite side of the short beam carries a continuous load of weights which are mechanically removed until an approximate balance with the sample in the pan is reached. The remaining weight within one gram can be estimated using a dialable optical scale.

Electronic Options

By the 1950s, electronic balances were developed that had beams made of quartz rods with a central quartz fiber. The electronic balance has this rod held in position so that the pan does not move. Instead, a permanent magnet generates an electromagnetic field with current regulated by a position sensor and measured by an analog-digital converter. This electromagnetic field compensates to balance the force pressing downward onto the pan from the sample being weighed. When calibrated, the amount of current needed to keep the pan balanced can determine the weight of the sample.

The advantage of an electronic balance includes the elimination of mechanical stresses that occur in a moving balance, the measurement sensitivity is high, and obviously, the speed and ease of use. Some weighing tips to ensure accurate results are:

* Weigh powders on paper or dishes.

* Handle objects with tongs, tweezers or gloves to prevent fingerprints;

* Let hot samples cool before weighting;

* Weigh hygroscopic materials rapidly to prevent water absorption and

* When making repetitive weighings, always use the same procedure.

Balances must be calibrated through the use of standard masses (kilogram and gram weights), or by using built-in calibration masses.

Some modern balances are capable of calibrating automatically for differences in ambient temperature, humidty, gravity or atmospheric pressure. Some instruments include static electricity elimination features and a configurable draft-shield system.

Another instrument has an audible alarm that sounds when the temperature rises or falls outside a predetermined limit. These features help to eliminate potential problems during the weighing process.

Well-known analytical balance manufacturers include Denver Instruments, NRD, Scientech, Ohaus, Mettler Toledo and Sartorius. Prices range from about $1000 to as much as $16,000 for a microbalance that measures two grams to an accuracy of 0.0001mg.

HARVEY FISHMAN HAS A CONSULTING FIRM AT 34 CHICASAW DRIVE, OAKLAND, NJ 07436, [e-mail], SPECIALIZING IN COSMETIC FORMULATIONS AND NEW PRODUCT IDEAS, OFFERING TESTED FINISHED PRODUCTS. HE HAS MORE THAN 30 YEARS OF EXPERIENCE AND HAS BEEN DIRECTOR OF RESEARCH AT BONAT, NESTLE LEMUR AND TURNER HALL. HE WELCOMES DESCRIPTIVE LITERATURE FROM SUPPLIERS AND BENCH CHEMISTS AND OTHERS IN THE FIELD.