Introduction:
Analytical chemistry or the art of recognizing
different substance and determining their constituents ,takes a prominent
position among the applications of science, since the questions which it
enables us to answer arise wherever chemical processes are employed for
scientific or technical purposes.
Analytical chemistry clearly encompasses other areas
of chemistry : physical chemistry , for example , is especially relevant to the
way in which many measurements are made , while aspects of the subject directly
relate to the analysis of inorganic or organic compounds . by its very nature ,
analytical chemistry is a generic discipline , and overlaps the realms of
biotechnology , forensic science , physics , materials science and many other
subject area.
Balance and weighing
The balance is the most delicate and
expensive instrument used by the
beginner in a laboratory and it must always be treated with care .
(a) Always weight materials by difference if that is
possible e. g weigh a crucible empty and then containing the substance and so
obtain the mass of the substance by subtraction. This approach eliminates any
minor in accuracy of the balance and prevents materials coming into contact
with the pan of the balance.
(b) Hot objects should not be put on the pan of the
balance .
(c) Do not put objects or weight on, or take them off the
pans while the pans are released from their rest position this prohibition does
not apply to top pan balance.
(d) If there are weights to be put on and off the right
hand pan as in the case of a two pan balance use tweezers 9 forceps) not
fingers.
(e) Record the masses involved immediately.
(f) Respect the mass limit of a balance.
Types of balance used in the lab
1- Two-pan balance.
The object to be
weight is placed on the left pan of the balance and objects of known mass are
added to the other until the pointer comes to center of the scale. At this
points the contents of both pans weigh the same and , since they both
experience the same gravitation acceleration, both pans contain equal masses ,
in chemistry , we generally , measure mass in grams.
2- Modern single pan (analytical balance ) capable of
measurement to the nearest 0.0001 g and produce very accurate mass weight. The
object to be weighted is placed on the pan and the weight are adjusted
internally by turning the knobs on the face until balance is
achieved , these
two kinds have exactly the same principle that is a balance been adjusted to
have the same weight on either side
of the
knife-edge pivot. We used in our lab (mettle balance) of one pan.
Preparation of different types of solutions
Percentage, composition :-
To express the concentration by a percentage of
solution there are three types :-
w/w % :- is the number of grams of solute dissolved in
a certain weight of solvent.
w/v % :- is the number of grams of solute dissolved in
certain volume of solvent.
Prepare 50% -NaOH solution in distilled water.
Weight 50 mg of NaOH solid material then dissolve it
in 100 ml water , caution “ the reaction is very exothermic, so NaOH must be
added slowly with shaking and cooling .
v/v % :- is the number of ml .of solute dissolved in a
certain volume of solvent.
Prepare 30% HCl solution in H2O.
In cylinder prepare 70 ml H2O , then complete the
volume up to 100 ml with hydrochloric acid.
Concentration units
1-Normality of
solution:-
Is defined as the number of
grams-equivalent weight of solute per one liter of solution .
Molarities of solution :-
A molar solution contains one
gram molecular weights of solute dissolved in sufficient amount of solvent to
give 1 liter of solution .
Formality :- is
the number of grams formula weight of solute per liter of solution. used for
partial dissolved solutes (such as acetic acid , oxalic acid etc…)