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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