KWAME NKRUMAH UNIVERSITY OF SCIENCE AND
DEPARTMENT OF CHEMISTRY
YEAR TWO (CHEM 270)
TITLE: DETERMINATION OF SULPHATE AS
BARIUM SULPHATE USING GRAVIMERTY WITH
DRYING OF RESIDUE
NAME: OPOKU ERNEST
DATE: 17TH FEBRUARY, 2014
DETERMINATION OF SULPHATE AS BARIUMSULPHATE USING
GRAVIMETRY WITH DRYING OF RESIDUE
AIMS AND OBJECTIVES:
To determine the amount of sulphate in an unknown sulfate sample
To perform and develop skills in precipitation Gravimetric method of qualitative
The term gravimetric pertains to a Weight Measurement. Gravimetric method is one in which
the analysis is completed by a weighing operation. Gravimetric Analysis is a group of
analytical methods in which the amount of analyte is determined by the measurement of the
mass of a pure substance containing the analyte.
Gravimetric Methods can also be defined as quantitative methods based on the determining the
mass of a pure compound to which the analyte is chemically related.
There are four methods of gravimetry, namely, precipitation gravimetry, electrogravimetry,
volatilization gravimetry and particulate gravimetry. In this experiment precipitation
gravimetry, in which the signal is the mass of the precipitate, is used. Precipitation gravimetry
is based on the formation of an insoluble compound following the addition of a precipitating
reagent, or precipitant to a solution of the analyte. The steps involved in gravimetry subsume:
1. Preparation of the solution 2. Precipitation
3. Digestion 4. Filtration
5. Washing 6. Drying or ignition
7. Weighing 8. Calculation
A precipitate for precipitation gravimetric analysis must be of low solubility, high purity and
of known composition if its mass is to accurately reflect the mass of the analyte. Also, the
precipitate must be in a form that is easy to separate from the reaction mixture. The steps
involved in precipitation gravimetry are precipitation, digestion, filtration, washing, drying and
weighing. The sulphate content of an unknown sample is to be determined in this experiment.
It is precipitated in dilute HCl with barium chloride as barium sulphate (BaSO4). The formed
BaSO4 precipitate is allowed to stand in solution with the analyte solution. It is then washed
and separated from solution by filtration and dried. Then the precipitate is weighed and its
Sulphate is widely distributed in nature and may be present in natural water in concentrations
ranging from a few to several thousand milligrams/litre. Sulphates are of considerable concern
because they are indirectly responsible for two serious problems often associated with the
handling and treatment of wastewater.
Sulphate is precipitated in hydrochloric acid medium as barium sulphates by the addition of
barium chloride. The precipitation is carried out near the boiling temperature and after a period
of digestion the precipitate is filtered; washed with water until free of chlorides, ignited and
weighed as barium sulphates.
If organic matter is not present in the sample, first method can be done without igniting and
instead drying the residue and weighing.When a solution containing sulfate ion is mixed with
one containing barium ion, sparingly soluble barium sulfate precipitates. The reaction may be
This reaction can be used for the gravimetric determination of sulfate or, less commonly, for
barium. Barium sulfate usually precipitates as very fine particles. The high surface area of the
particles facilitates contamination by adsorption.
Larger particles can be obtained by heating the precipitate in the presence of its mother liquor.
During this digestion process, recrystallization takes place resulting in a precipitate of larger
particle size. Since barium sulfate is stable in air and is nonhygroscopic, the weighing can be
performed in an open crucible.Recrystallized BaSO4 is collected on filter paper.
The precipitate and filter paper are dried in a crucible. The filter paper is charred and ashed,
leaving the dried sample in the crucible for ease in weighing. To reduce error, the paper ashing
operation must be carried out with care.
The paper should be charred carefully at low temperature with the crucible lid in place so that
escaping gases do not burst into flame. When smoke formation has ceased, the temperature is
slowly increased and air is allowed ample access to the interior of the crucible.
If this practice is not followed, carbon will reduce sulfate ion to sulfide ion, forming carbon
BaSO4 + 4C → BaS+ 4CO.
Low mass determinations will result.
SOLUTIONS AND CHEMICALS
1. 1+1 HCl
2. 0.05M BaCL2
3. 0.1M HNO3
5. Unknown sample
6. Warm water
1. 400ml beaker
2. 10ml and 200ml measuring cylinder
3. Stirring rod
4. Heating plate
6. Analytical balance
7. Filter paper
8. Vacuum pump compressor
250ml of the unknown sample was
measured into a 500ml beaker and then
3ml of 1:1HCl solution added.
A colourless solution was formed.
The solution was heated to near boiling
point and an amount of 80ml of 0.05M
BaCl2 was added slowly to it with
Cloudiness was formed and later a white
precipitate of BaSO4 was formed beneath
The precipitate was digested for about
30mins. Test for completeness was done
by adding two drops of BaCl2 to the
solution, when cloudiness was noticed;
40ml of BaCl2 was added to the solution
and allowed to digest for about 20mins.
There was cloudiness when two drops of
BaCl2 was added to the solution.
Test for completeness was performed
continuously until no cloudiness formed.
A filter paper of constant weight and
cooled in a desicator was weighed and its
The recorded mass was 1.048g.
The precipitated BaSO4 was filtered
using the weighed filter paper and
drained in a suction pump.
A white crystalline precipitate of BaSO4
was left on the filter paper and the filtrate
obtained contained white gelatinous
precipitate suspected to be BaCl2 and lost
The filter paper and the residue (BaSO4)
were dried for about 6mins and weighed.
The recorded weight was 3.5363g.
CALCULATIONS AND EVALUATION OF DATA
Mass of precipitate= mass of (dry filter paper + BaSO4)- mass of filter paper
Mass of precipitate (BaSO4)= 3.5363g-1.048g= 2.4883g
Weight of SO2-
4= weight of BaSO4 × gravimetric factor
= weight of BaSO4 × a( gram formula weight of SO2-
b( gram formula weight of BaSO4)
(aq) + SO2-
thus a=1 and b=1
hence weight of SO2-
4= 2.4883 × [32.066+4(16)]/[137.33+32.066+4(16)]
weight of SO2-
Hence weight of SO2-
Mg/L SO4= mgBaSO4 x 411.6
250 x 10-3
The solution was digested to break the bonds in the unknown sample in order to make the
sulphate in the solution free to precipitate with the BaCl2 to form BaSO4. The completeness
of the precipitate is check by adding BaCl2 to the clear solution on the surface of the
precipitate if no cloudiness is not form then it indicate that there is no sulphate left out in the
solution. Either wise excess BaCl2 must be added to precipitate the sulphate.
The Barium sulphate precipitate was washed with warm distilled water to free the Cl-
the precipitate. The precipitate freeness of Cl-
ion is tested by performing qualitative test of
Cl. This is done by 1ml of 0.1M HNO3 into the filtrate collected during the washing and
drops of Ag NO3 is added in drop wise, if AgCl precipitate is formed then it indicates that the
Barium sulphate precipitate needs further washing . if not the then precipitate and the filter
paper is dried and weighed it mass noted as 3.685g
HCl was added to the sample solution to prevent the precipitation of barium salts of weak
1. HCl was added to the sample solution to prevent the precipitation of barium salts of
weak acids such as CO2-
4 etc., which will volatilize during ignition.
2. The precipitate was digested to increase the average particle size and reduce co-
3. BaCl2 was added slowly amidst effective stirring to reduce relative supersaturation.
4. Completeness of the precipitation was tested for after each digestion by adding drops of
5. The container that contained BaSO4 was washed with warm water to remove the Cl-
6. The filter paper and the precipitate were well dried to ensure accurate weight
7. The experiment was carried out at a near boiling point temperature to increase the
solubility of the precipitate at equilibrium and reduce relative supersaturation.
The anticipated aims of the experiment were explicitly achieved. Thus we have successfully
determined the amount and concentration of the sulphate through the adopted method:
gravimetry. The mass of the sulphate was found as 1.024184g and hence its mass
concentration was deduced through the formula given Mg/L SO4= mgBaSO4 x 411.6
as 4.09760 mg/l.
1. Gary D. Christian, Analytical Chemistry, sixth edition, University of Washington, 2004,
chapter ten, pages 313 to 338.
2. Analytical Chemistry for Technicians, John Kenkel, 3rd
edition, page 130, 142.
3. KNUST- Kumasi, Department of Chemistry, Laboratory Manual, CHEM 269 & 270,
Page 29 & 30.
4. Analytical Chemistry Handbook, Pradyot Patnaik, 2nd
edition, page 297.