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Experiment on the Determination of sulphate as barium sulphate
Experiment on the Determination of sulphate as barium sulphate
Experiment on the Determination of sulphate as barium sulphate
Experiment on the Determination of sulphate as barium sulphate
Experiment on the Determination of sulphate as barium sulphate
Experiment on the Determination of sulphate as barium sulphate
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Experiment on the Determination of sulphate as barium sulphate


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  • 2. 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 analysis. INTRODUCTION 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 mass recorded.
  • 3. 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 written as: SO4 2- +Ba2+ → BaSO4 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 monoxide: BaSO4 + 4C → BaS+ 4CO. Low mass determinations will result. SOLUTIONS AND CHEMICALS 1. 1+1 HCl 2. 0.05M BaCL2 3. 0.1M HNO3 4. AgNO3
  • 4. 5. Unknown sample 6. Warm water APPARATUS 1. 400ml beaker 2. 10ml and 200ml measuring cylinder 3. Stirring rod 4. Heating plate 5. Desicator 6. Analytical balance 7. Filter paper 8. Vacuum pump compressor PROCEDURE Experiment Observation 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 continuous stirring. Cloudiness was formed and later a white precipitate of BaSO4 was formed beneath the solution. 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 mass noted. 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 BaSO4 precipitate. The filter paper and the residue (BaSO4) were dried for about 6mins and weighed. The recorded weight was 3.5363g.
  • 5. 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- 4) b( gram formula weight of BaSO4) BaSO4 Ba2+ (aq) + SO2- 4(aq) 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- 4= 2.4883×96.066/233.396 = 2.4883×0.4116 Hence weight of SO2- 4= 1.024184g Mg/L SO4= mgBaSO4 x 411.6 ml sample = 2.48883x10-3 x 411.6 250 x 10-3 =4.09760 mg/l DISCUSSION 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- ion in 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
  • 6. PRECAUTION 1. HCl was added to the sample solution to prevent the precipitation of barium salts of weak acids such as CO2- 3, PO3- 4 etc., which will volatilize during ignition. 2. The precipitate was digested to increase the average particle size and reduce co- precipitation. 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 BaCl2. 5. The container that contained BaSO4 was washed with warm water to remove the Cl- salt present. 6. The filter paper and the precipitate were well dried to ensure accurate weight measurement. 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. CONCLUSION 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 ml sample as 4.09760 mg/l. REFERENCE 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.