Page 104 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

3 COMMON APPARATUS AN0 BASIC TECHNIQUES

1. Never exceed the stated maximum load of the balance.
2. The balance must be kept clean. Remove dust from the pan and from the
floor of the pan compartment with a camel hair brush.
3. Objects to be weighed should never be handled with the fingers; always use
tongs or a loop of clean paper.
4. Objects to be weighed should be allowed to attain the temperature of the
balance before weighing, and if the object has been heated, sufficient time
must be allowed for cooling. The time required to attain the temperature of
the balance varies with the size, etc., of the object, but as a rule 30-40 minutes
is sufficient.
5. No chemicals or objects which might injure the balance pan should ever be
placed directly on it. Substances must be weighed in suitable containers, such
as small beakers, weighing bottles or crucibles, or upon watch glasses. Liquids
and volatile or hygroscopic solids must be weighed in tightly closed vessels,
such as stoppered weighing bottles.
The addition of chemicals to the receptacle must be done outside the
balance case. It is good practice to weigh the chosen receptacle on the
analytical balance, to transfer it to a rough balance, to add approximately
the required amount of the necessary chemical, and then to return the
receptacle to the analytical balance for re-weighing, thus giving the exact
weight of substance taken.
6. Nothing must be left on the pan when the weighing has been completed. If
any substance is spilled accidentally upon the pan or upon the floor of the
balance compartment, it must be removed at once.
7. Exposure of the balance to corrosive atmospheres must be avoided.
The actual weighing process will include the following steps.
1. Brush the balance pan lightly with a camel hair brush to remove any dust.
2. With the balance at rest, place the object to be weighed, which must be at or
close to room temperature, on the pan, and close the pan compartment case.
3. Set the on/off control of the balance to the 'on' position and record the
weight shown on the digital display: if the balance is linked to a printer,
confirm that the printed result agrees with the digital display. Return the
control to the 'off' position.
4. When al1 weighings have been completed, remove the object which has
been weighed, clear up any accidental spillages, and close the pan compartment.
The above remarks apply particularly to analytical balances of the macrobalance
range; microbalances and ultramicro balances must be handled with special
care, particularly with regard to the temperature of objects to be weighed.
3.6 ERRORS IN WEIGHING

The chief sources of error are the following:
1. Change in the condition of the containing vessel or of the substance between
successive weighings.
2. Effect of the buoyancy of the air upon the object and the weights.
3. Errors in recording the weights.
1. The first source of error is occasioned by change in weight of the containing
vessel: (a) by absorption or loss of moisture, (b) by electrification of the