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1. PURPOSE
The purposes of this experiment are to show the importance of standardization of
solutions to be used in environmental chemical work, to demonstrate the use of burettes
and calibration and to display the effects of personnel errors in the use of standardsolutions.
2. PROCEDURE
1. We dilute about 1.1 mL of concentrated H2SO4 to 1 liter with distilled water. (This will
give 0.04 N H2SO4)
2. We weigh about 0.12 g (to the nearest 0.1 mg) of HgO and add 25 mL of distilled water
to it.
3. Together with warming and stirring add solid KI, a little at a time, until the oxide has
completely dissolved.
4. We dilute the mixture to 100 mL very precisely.
5. We fill a calibrated burette with H2SO4.
6. We titrate the standard solution with the acid in the presence of methyl-red indicator (it
will turn from yellow to red).
3. THEORY
A standard solution (or standard titrant) is a reagent of known concentration that is used to
carry out a volumetric analysis. Standards solutions play a central role in all volumetricmethods of analysis. The ideal standard solution for a volumetric method will
1. be sufficiently stable so that it is necessary to determine its concentration only once.
2. react rapidly with the analyte so that the time required between additions of titrant is
minimized.
3. react more or less completely with the analyte so that satisfactory end points are
realized.
4. undergo a selective reaction with the analyte that can be described by a simple balanced
equation.
A titration is performed by slowly adding a standard solution from a buret or other
volumetric measuring device to a solution of analyte until the reaction between the two is
complete. The volume needed to complete the titration is determined from the difference
between the initial and final buret readings.
The equivalence point in a titration is reached when the amount of added titrant is
chemically equivalent to the amount of analyte in the sample. The equivalence point of a
titration is theoretical point that cannot be determined experimentally. Instead, we can only
estimate it by observing some physical change associated with the condition of
equivalence. This change is called the end point for the titration. The difference in volume
between the equivalence point and the end point is the titration error.
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With the exception of absolute methods of analysis that involve chemical reactions of
known stoichiometry (e.g. Gravimetric and titrimetric determinations), a calibration or
standardization procedure is required to establish the relation between a measured physico-
chemical response to an analyte producing the response. In other words, standardization is
the name given to this process of accurately determining the concentration of a standardsolution.
A primary standard is a highly purified compound that serves as a reference material in all
volumetric titrimetric methods. Important requirements for a primary standard are:
1. High purity. Established methods for confirming purity should be available.
2. Stability in air.
3. Absence of hydrate water so that the composition does not change with variations in
relative humidity.
4. Ready availability at modest cost.
5. Reasonable solubility in the titration medium.
6. Reasonably large formula weight so that the relative error associated with weighing is
minimized.
HgO is a primary standard for H2SO4.A weighed sample is dissolved in a solution of KI as
given by below formula.
HgO + 4I- + H2O HgI42- + 2OH-
The resulting alkaline solution is titrated with the acid. Methly-red (or any other indicator
changing within a pH range 4.5-9.5) may be used as the indicator.
4. DATA ANALYSIS AND CALCULATIONS
1. The exact concentration of acid solution:
Weight of HgO=0.114 g
H2SO4 used in titration (mL) =30.8 mL
Normality=Accurate wt. of HgO g /meq.wt. of HgO*volume of H2SO4 mL
Normality=0.114g / 0.108305*30.8 mL
Normality of H2SO4=0.034 N
Where,
Miliequivalent weight of HgO=molecular weight of HgO /2*1000
Miliequivalent weight of HgO=216.61 / 2000 =0.108305
2. Sulfuric acid 98% density=1.84, M=18.4, Normality=36.8to make 1000 mL solution
Normality=1.1 mL *36.8 / 1000 mL =0.040 N (from
www.erowid.org/archive/rhodium/chemistry /equipment/molarity.html)
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Error 1: (0.04-0.034) /0.04*100 = 15% error
Error 2: (0.04-0.040) /0.04*100 = 0% error
5. DISCUSSION AND CONCULUSIONS
1. Standard solutions are also commonly used to determine the concentration of an analyte
species. By comparing the absorbance of the sample solution at a specific wavelength to a
series of standard solutions at differing known concentrations of the analyte species, the
concentration of the sample solution can be found via Beers Law. Any form of
spectroscopy can be used in this way so long as the analyte species has substantial
absorbance in the spectra. The standard solution is a reference guide to discover the
molarity of unknown species.
2. A solution of acid can be standardized by titrating it against a solution of alkali (NaOH)of known concentration. Once this has been calculated, it can in turn be used as a standard
solution to find the concentration of a solution of alkali.
3. Probably, we used more than 25 mL of distilled water.
Maybe, we weighed less than 0.12 g HgO.
Perhaps, we did not stir the mixture.
6. REFERENCES
1. Skoog, D.A., West, D.M., Fundamentals of Analytical Chemistry 6
th
edition, Holt,Rinehart and Winston, Inc.,2004 pp. 94-97
2. Rodojevic, M., Bashcin, V.N., The Royal Society of Chemistry, Cambridge, 1999 p.21
3. Kealey, D., Haires, P.J., Instant Notes Analytical Chemistry, Bios Scientific Publishers
Limited, Oxford, 2002 p.15
4. Freiser, H., Nancollas, G., Compendium of Analytical Nomenclature: Definitive Rules,
Blackwell Scientific Publications, Oxford, 1987 p.48
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