A
standard solution is a solution whose concentration is known. Standard
solutions are prepared in advance or at demand and kept on the bench in the
laboratory for use. A standard solution of a substance can be prepared, if the
substance can be obtained in pure form state, by weighing accurately a definite
mass of the substance, dissolving it in a suitable solvent, usually water, and
making up the solution to a known volume in a volumetric flask.
When
a substance is not available in pure form, a solution of approximate
concentration required is first prepared, and then standardized (that is, its
concentration determined) by titrating against a standard solution of a pure
substance.
Standard
solutions of common acids and alkalis cannot be prepared by direct weighing.
Reasons: some of them are volatile and corrosive liquids, for example HCl, HNO3
and ammonia; while H2SO4 is hygroscopic. Others are
solids that are deliquescent and hence absorbs moisture and carbon(IV)oxide,
from the atmosphere, for example KOH and NaOH pellets.
Types of standard solutions include;
1.
Primary standard: a substance which can be obtained in
a high state of purity, anhydrous, non-deliquescent, non-hydroscopic, readily
soluble in water or in the required solvent, and hence suitable for the
preparation of standard solution. Examples are anhydrous sodium
trioxocarbonate(IV), NaCO3 and benzoic acid C6H5COOH.
2.
A substance which is hydrated, but does not
efflorescent can be used in the preparation of a standard solution. An example
is the dibasic organic acid called ethanedioic acid. (H2C2O4)
(Oxalic acid).
Example of preparation of some solutions
To prepare 2 Molar HCl
From calculation
89ml is dissolved in 1liter of distilled water to get one molar solution of
HCl.
NB: HCl is a
monobasic acid, therefore its molarity and normality is the same?
89mls of HCl
1Molar
1Litre
2×89mls of HCl
2molar
1Litre
That is, 178mls
of HCl
2molar
1Litre.
For tetraoxosulphate (VI) H2SO4
Molecular weight
Mw =98
(Mw
%purity) × (100÷specific gravity)
Therefore (98.07
)
54.4ml of H2SO4
1000mls of H2O
1M
To prepare 0.1NaOH (sodium hydroxide)
Molecular weight
of NaOH =40g
To get the mass
in o.1m =40×0.1=4g
Therefore 4g in
1dm3 of distilled water
4grams of NaOH
pellets/crystals will be measured and dissolved and be made up to 1liter of
distilled water to get 0.1m concentration.
Stock solution
This is to prepare
different concentration from already prepared solution.
Use the
following equation V1= (M2V2÷M1)
Were V1=Initial
volume; V2=final volume; M1=initial mole; M2=final mole
which is the new desired mole.
QUANTITATIVE DETERMINATION OF CAFFEINE IN BEVERAGES AND SOFT DRINKS USING UV. SPECTROSCOPY
·
Determination of caffeine in Igbo kola nut,
Hausa kolanut and coca cola.
Caffeine is a naturally occurring alkaloid which is found in the leaves,
seeds or fruits of over 63 plant species world wide.
The most common
sources of caffeine are
1.
Coffee
2.
Cocoa beans
3.
Colanut
4.
Tea leaves
Method
Reagent used
1.
Caffeine
2.Dichloromethane
3.Purified
H20
Aim: To extract caffeine from the
sample by the use of chloroform.
Procedure
· Hausa and Igbo kolanut was grated. Then
allowed to dry after drying it was grinded with an electric grinder into powder
form.
· 2g was measure and poured into a conical flask.
· Water
was boiled and then 200ml of the boiled water was measured and then added to the
sample in the beaker and placed in a magnetic stirrer to stir for 30mins.
· After
adding boiled water to the sample and stir for 30min. It was allowed to cool @
room temperature.
· 200ml of coca cola was measured and heated.
· Allowed to cool.
· The sample was washed with 25ml of chloroform
to extract the caffeine.
Note: Wash for 3 times
After
extracting the caffeine from the sample it was taken to uv spectroscopy and
then read @ spec. 450nm.
The
concentration of caffeine was taken in ppm (part per million) which is as
follows 55.358 x ABS.
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