Determination of Mass in gravimetric analysis

The weight of an object on the top of a mountain will be less than the weight of the same object at the bottom due to gravity variations. A unit of measurement for weight is the newton. A newton takes into account the mass of an object and the relative gravity and gives the total force, which is weight.

Although mass and weight are two different entities, the process of determining both weight and mass is called weighing.

Balance and Scale Terms

Accuracy The ability of a scale to provide a result that is as close as possible to the actual value. The best modern balances have an accuracy of better than one part in 100 million when one-kilogram masses are compared.

Calibration The comparison between the output of a scale or balance against a standard value. Usually done with a standard known weight and adjusted so the instrument gives a reading in agreement.

Capacity The heaviest load that can be measured on the instrument.

Precision Amount of agreement between repeated measurements of the same quantity; also known as repeatability. Note: A scale can be extremely precise but not necessarily be accurate.

Readability This is the smallest division at which the scale or balance can be read. It can vary as much as 0.1g to 0.0000001g. Readability designates the number of places after the decimal point that the scale can be read.

Tare The act of removing a known weight of an object, usually the weighing container, to zero a scale. This means that the final reading will be of the material to be weighed and will not reflect the weight of the container. Most balances allow taring to 100% of capacity.

Types of Balances and Scales

Analytical Balance These are most often found in a laboratory or places where extreme sensitivity is needed for the weighing of items. Analytical balances measure mass. Chemical analysis is always based upon mass so the results are not based on gravity at a specific location, which would affect the weight.

Generally capacity for an analytical balance ranges from 1 g to a few kilograms with precision and accuracy often exceeding one part in 106 at full capacity. There are several important parts to an analytical balance.

A beam arrest is a mechanical device that prevents damage to the delicate internal devices when objects are being placed or removed from the pan

The pan is the area on a balance where an object is placed to be weighed. Leveling feet are adjustable legs that allow the balance to be brought to the reference position.

The reference position is determined by the spirit level, leveling bubble, or plumb bob that is an integral part of the balance.

Analytical balances are so sensitive that even air currents can affect the measurement. To protect against this they must be covered by a draft shield. This is a plastic or glass enclosure with doors that allows access to the pan

Triple-Beam Balance

This type of laboratory balance is less sensitive than a top-loading balance. They are often used in a classroom situation because of ease of use, durability and cost. They are called triple-beam balances because they have three decades of weights that slide along individually calibrated scales. The three decades are usually in graduations of 100g, 10g and 1g. These scales offer much less readability but are adequate for many weighing applications.

Balance and Scale Care and Use

A balance has special use and care procedures just like other measuring equipment. Items to be measured should be at room temperature before weighing. A hot item will give a reading less than the actual weight due to convection currents that make the item more buoyant. And, if your balance is enclosed, warm air in the case weighs less than air of the same volume at room temperature.

Another important part of using a balance is cleaning. Laboratory scales are exposed to many chemicals that can react with the metal in the pan and corrode the surface. This will affect the accuracy of the scale.

Also, keep in mind that a potentially dangerous situation could occur if a dusting of chemicals is left on the lab balance pan. In many lab and classroom situations, more than one person uses a single scale for weighing.

It would be impossible for each person to know what everyone else has been weighing. There is a chance that incompatible chemicals could be brought into contact if left standing or that someone could be exposed to a dangerous chemical that has not been cleaned from the balance. To avoid damaging the scale or putting others in danger, the balance should be kept extremely clean.

A camel’s hair brush can be used to remove any dust that can spill over during weighing.

Calibration is another care issue when it comes to scales. A scale cannot be accurate indefinitely; they must be rechecked for accuracy. There are weight sets available that allow users to calibrate the scale themselves or the scales can be calibrated by hiring a professional to calibrate them on site.

It is important for you to realize with what sort of accuracy these weighings should be made. Depending on the desired accuracy you should use the proper balance to make your weighings. Also located in the room adjacent to the laboratory.

These will weigh to an accuracy of ± 0.1 mg and must be used whenever you desire four or more significant figure accuracy. This will be the case when you weigh out samples of an unknown, primary standards or when taking crucibles to constant weight. Directions for the use of these balances are posted in the balance room.

Various Types Of Weighing

When mass amounts are specified in chemical procedures the following terms are commonly used:

“Weigh out about 2 g of …… ” This statement means that you are required to weigh an amount of approximately two grams. The accuracy to which this mass amount needs to known is not high and the top-loading balance will suffice.
“Accurately weigh out about 0.2 g of ….” This statement means that you should, with the aid of the analytical balance, weigh out an amount that is close to 0.2 g, but you must know the exact amount to an accuracy of ± 0.1 mg. Note that this does not mean that you must weigh out exactly 0.2000 g. An amount between 0.1900 g and 0.2100 g is perfectly acceptable.

However, you must know the exact amount to the nearest tenth of a milligram. When weighing out triplicate samples it is not necessary that all three weights be exactly the same, indeed, it is poor procedure to attempt to do so.

Rules for Analytical Balances

The following rules summarize those procedures which must be followed in order to obtain accurate and reliable mass measurements with a single-pan analytical balance. Adherence to these rules will, at the same time, prevent damage to the balance.

Close the balance door, while weighing an object, in order to prevent air currents from disturbing the reading. When finished, the operator should close the balance door to prevent dust and dirt from entering the balance.
Only glass, ceramic, metal or plastic objects and containers should be placed in direct contact with the balance pan.
Do not handle objects to be weighed with bare hands. Moisture, grease and dirt on you fingers will affect the weight of the objects.
To be weighed accurately, all objects must be at room temperature. A warm object sets up convection currents inside the balance enclosure, which will make an object appear lighter than it really is. Also, warm air inside the enclosure is less dense than the air that it displaces and this also leads to a negative determinate error.
Never weigh chemicals directly in contact with the balance pan. Use containers such as beakers, flasks and weighing bottles.
All objects and materials that have recently been removed from a desiccator will absorb moisture and thereby gain weight. It is therefore good practice to record weights after identical time intervals. For example if you are taking crucibles to constant weight. Always record the weight of the crucible exactly 5 seconds after having placed the crucible on the balance pan. Using this technique it is possible to minimize the effect of moisture absorption.
The use of weighing paper must be strictly avoided when using an analytical balance.
Do not spill chemicals inside the balance enclosure. If a spill occurs, clean it up immediately.

Calibration of weighing balance

Before weighing anything on this analytical balance, it needs to be “tared,” or recalibrated to read 0.0000 g.

When first turned on, or when left by the previous user, the balance may indicate something other than 0.0000 g.

The Tare button needs to be pressed and released to effect this recalibration.

An analytical balance is so sensitive that it can detect the mass of a single grain of a chemical substance. Thus, if a method of direct weighing is used, the substance ought to be added to the tared container which will hold it, NEVER directly to the pan or even to weighing paper placed on the pan.

The container used should be completely dry and at room temperature, never at an elevated or reduced temperature. Even slight temperature differences can produce APPARENT changes in mass of the container.

Finally, the container ought to be completely dry, inside and out. All that having been said, here are some images showing various correct ways of carrying out weighings using an analytical balance.

Regardless of which method illustrated below is used accurately to weigh a sample, the sample, placed in a weighing bottle set in the upturned cap in a beaker with a watch glass placed on top, must be first dried in the oven. You may identify your sample by marking the beaker but DO NOT mark the weighing bottle.

The tried-and-true method of transferring a precisely weighed sample uses “weighing by difference,”. The empty balance is tared, then the weighing bottle with cap is placed on the pan and weighed to ±0.0001 g.

The weighing bottle is removed in a manner which avoids the transference of oil or other matter from one’s fingers.

The cap is likewise removed from the weighing bottle. The weighing bottle is tipped above the container to receive the sample and a small amount is allowed to fall out of the weighing bottle. The weighing bottle is tipped back up and tapped gently to make sure all of the substance falls back in the bottle and doesn’t remain on the bottle rim.

The cap is replaced and the bottle weighed once again. The difference between the first and second weighings represents the amount transferred.

If your sample has a tendency to absorb water and thus to gain weight when exposed to the moisture of the air, this method MUST be used to minimize exposure to the atmosphere. Still, the method is not foolproof and has its own perils:

several transfers may be necessary until an amount close to that needed is added to the receiving container,
too much may be transferred the first time, forcing one to discard the entire sample (“Drat and blast, I have to start over,” is a common epithet heard in weighing rooms),
sample which remains on the weighing bottle rim may be lost and produce a weighing error. This peril is repeated each time a transfer is attempted.

The direct transfer of samples to receiving containers is possible using modern analytical balances which allow one to tare a receiving container before the transfer is begun. Note the Erlenmeyer flask on the pan. The Erlenmeyer flask needs to be completely dry so that there will be no systematic error due to the evaporation of water.

A spatula can be used to remove a sample from the weighing bottle and the sample placed directly into the flask.

This method has some disadvantages associated with it:

one risks losing the sample on the outside of the Erlenmeyer flask due to the small diameter of the flask neck and
the point of entry is rather high; one must have good coordination to orient the spatula above the mouth of the flask. Advantage:

The sample is placed directly into the flask and the mass read is that of the sample added. Eternal vigilance being the price of good results, note in the image at the right the particles of sodium carbonate which didn’t quite make it to the bottom of the flask.

The titration yielding the molarity of the hydrochloric acid standard for the percent sodium carbonate in soda ash experiment showed a precision within one part per thousand with the other titrated samples but only because both the stopper, which was removed gently before the titration, and the inside of the flask neck were rinsed carefully to dissolve the particles of carbonate and to allow them to drain into the body of the flask.

The mass limit of our analytical balances beyond which one receives an error message is 200.0000 grams. Even this 400 mL beaker can be placed on the pan. Note that its mass is just above 149 g. Once tared, the sample may be placed directly into the beaker.

The perils using this method are similar to those encountered during a direct transfer to an Erlenmeyer flask.

The beaker must be clean, dry and at the same temperature as the room and the balance. The mouth of the beaker is rather high, so one must have good coordination to orient the spatula over the mouth without knocking it against the beaker or the windows of the balance, thus losing the sample.

You can still weigh your sample directly but do it in a manner which doesn’t require as much care and coordination as that illustrated above by removing the beaker from the pan, adding your sample and replacing the beaker on the pan.

The advantage of this method is that you have more room for sample addition and the placement of your hands, arms and the transfer spatula is less awkward, but the disadvantage is that you must check the weight of your beaker and sample after each addition; it is not good practice to remove excess sample from the beaker if you add too much.

You must be sure that the removed beaker is placed on a clean and dry part of the bench so to avoid picking up any particles which will change your weight and decrease your accuracy. The removal and replacement of the beaker must also be done with a small folded piece of paper or paper towel so as to avoid adding weight to the beaker in the form of fingerprints

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