THE MEASUREMENT OF WEIGHT

I BASIC PRINCIPLES Weight is the force of gravity on an object. Balances measure this force.

WEIGHT versus MASS Mass: amount of matter in an object; units are kilograms. Mass doesn’t change when object is moved to new location.  Astronaut is “weightless” in space but mass is the same. In the lab, we weigh objects.

WEIGHING AN OBJECT We compare pull of gravity on sample with pull of gravity on standard(s) of established mass.

BASIC BALANCE When the beam is exactly balanced, gravity is pulling equally on sample and standard they are the same weight. Hence, “balances”.

OLDEST SCIENTIFIC INSTRUMENTS Mechanical balances have one or more beams; objects are placed on a pan attached to a beam. Have been used for hundreds of years.

MECHANICAL BALANCES Still used for some purposes – balancing centrifuge tubes.

ELECTRONIC BALANCES Still measure pull of gravity on objects but do not have beams. Use an electromagnetic force rather than weights to counterbalance the sample.

ELECTRONIC BALANCES Produce an electrical signal when a sample is placed on the weighing pan, the magnitude of which is related to the sample’s weight. To convert electrical signal to a weight value, balance compares the electrical signal from the sample to the signal from a standard(s) of known weight.

ELECTRONIC BALANCES Electronic balances make the comparison between sample and standard sequentially:  Calibrate with a standard at one time  Later the sample is weighed

II CHARACTERISTICS AND TYPES OF BALANCES Range is the span from the lightest to the heaviest weight the balance can measure. Capacity is the heaviest sample balance can weigh.

SENSITIVITY AND READABILITY Sensitivity: smallest weight that will cause a change in the response of the balance. Sensitivity determines the number of places to the right of the decimal point that the balance can read accurately and reproducibly.

Extremely sensitive balances weigh accurately to the nearest 0.1 microgram (or g). Less sensitive balance might read to the nearest 0.1 gram. Manufacturers express the sensitivity of their balances by their readability.

ANALYTICAL BALANCES Analytical balances optimize sensitivity and can weigh samples to at least the nearest tenth of a milligram ( g). Are both mechanical and electronic balances of all types.

RANGE, CAPACITY, SENSITIVITY Range, capacity and sensitivity are interrelated. Don’t use analytical balance to weigh samples in the kilogram range and vice versa. Choose best balance – not simplest to operate.

III PROPER OPERATION; AVOIDING ERRORS Accuracy and precision of modern balances is primarily affected by:  User technique and lab conditions  Maintenance  Design and construction of balance Accuracy and precision of instruments is excellent

OPERATING AN ELECTRONIC ANALYTICAL BALANCE 1. Level balance. 2. Adjust the balance to zero with pan clean and empty and chamber doors closed. 3. Tare the weighing container or weigh the empty vessel. 4. Place sample on weighing pan; read the value for the measurement. 5. Remove sample, clean.

WAYS TO GET SYSTEMATIC ERROR: Don’t level the balance Don’t adjust to zero Allow vibration, drafts, jostling Don’t close balance doors Touch samples and their containers Allow temperature to fluctuate

Ignore static charge Ignore loss or gain of moisture Place overload on weighing pan Select wrong weighing vessel Make a mess and don’t clean up

QUESTION A technician weighs a cell preparation on an analytical balance and observes that initially the weight is g. A few minutes later the weight is g. What might be happening here?

ANSWER Assuming the technician avoided temperature effects and drafts, sample probably lost moisture or other volatile component. Analytical balances are very sensitive, so see these effects.

MAINTENANCE AND CALIBRATION Calibration brings balance readings into accordance with internationally accepted standards. Calibration must be periodically checked in the laboratory of the user. Practice in lab today.

CALIBRATION Level balance Set to zero Place standard on balance, often 100 g Adjust to upper weight

HOW OFTEN TO CALIBRATE? It depends Time, use and abuse affect response Check whenever balance is moved In microgram range, check when weather changes Mechanical balances – when service person comes

STANDARDS Calibrate with standards; metal objects whose masses are known (within limits of uncertainty). Traceability comes from standards. Accuracy of any weight determination is limited by the accuracy of the standards used for comparison.

STANDARDS Handle standard weights with tongs because they are damaged by skin oils and cleaners. Standard weights need to be periodically recertified since change over time due to scratches, wear, and corrosion.

QUALITY PROGRAM Calibrate periodically. Check precision and linearity periodically. Consistently check and record weights of standards – tests accuracy. Follow SOPS.

LINEARITY Linearity error occurs when a balance is properly calibrated at zero and full-scale (the top of its range) but the values obtained for weights in the middle of the scale are not exactly correct. If a balance has linearity error, have it repaired professionally. Practice in lab today.

QUESTION A technician is verifying the performance of a laboratory balance and weighs a 10 g standard six times. a. What feature of the balance is she checking? b. What is the standard deviation for the balance? (Remember the units.)

c. The balance specifications require that its precision be better than or equal to g. Does it meet these specifications? If not, what should be done? WEIGHTS g g g g g g

ANSWER The SD is g, which does not meet the specification. There may be a problem with the balance although it is possible that the operator is not using good weighing practices. The balance should be taken out of use and the cause of the imprecision should be investigated.

IV MASS versus WEIGHT Value read from a balance is the weight of an object, not its mass. May seem surprising. After all, the object is directly compared to a standard whose mass is known.

Standard has a mass of 1 gram and the sample exactly balances the standard. Therefore, it seems logical that the sample has a mass of 1 gram. Not so.

BOUYANCY EFFECT Major force measured in weighing is force of gravity  But there is slight buoyant force by air that makes any object appear “lighter” than it really is. If the same object is weighed in air and in a vacuum, the object will be slightly heavier in the vacuum.

BUOYANCY EFFECT Principle of buoyancy: any object will experience a loss in weight equal to the weight of the medium it displaces.  ships float  helium balloons rise

Balances are calibrated with metal mass standards. Metal has a relatively high density compared to aqueous solutions and other materials.

A 1 g metal mass standard displaces less air than 1 g mass of water  The metal standard is buoyed less by the air. Difference in buoyancy of mass standard and sample explains why the standard and sample have the same weight but different masses.

If a balance were calibrated with a mass standard whose density were identical to that of the sample, then the weight of the sample would equal its mass. Or, if the standard and sample were weighed in a vacuum, then weight and mass would be equal. But, neither is the case.

Call this buoyancy error.  Weight readings for aqueous solutions have a buoyancy error of roughly 1 part in  This buoyancy error is small enough as to be of little concern in most applications. Distinction between mass and weight is generally ignored except when very high accuracy measurements must be made.

If necessary, there are equations to correct for the buoyancy effect that take into consideration the air density at particular atmospheric conditions and the density of the object being weighed.

QUESTION 1. Suppose you need to prepare a solution with a concentration of 15 mg/mL of a particular enzyme. You try to weigh out 15 mg of the enzyme on the analytical balance, but find that it is extremely difficult to get exactly 15 mg. Suggest a strategy to get the correct concentration even if you cannot weigh exactly 15 mg.

ANSWER Weigh out as close to 15 mg as possible and then calculate the amount of buffer required to dilute the enzyme to a concentration of 15 mg/mL.

QUESTION Suppose a 100 g standard mass is accidentally dropped on the floor. As a result, its mass is slightly less than 100 g. If the standard is used to calibrate a balance, what will happen to subsequent readings from that balance? What type of error is this?

ANSWER All subsequent readings with that balance will be a little too high. Using this standard causes a systematic error. Dropping it was a gross error.