Published on March 9, 2014
KWAME NKRUMAH UNIVERSITY OF SCIENCE AND TECHNOLOGY, KUMASI COLLEGE OF SCIENCE DEPARTMENT OF CHEMISTRY PRATICAL CHEMISTRY IV ANALYTICAL PRACTIAL 2.2.2 NAME: ADU AUGUSTINE DATE: 3RD 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 analysis. INTRODUCTION Gravimetric includes all analytical methods in which the analytical signal is a measurement of mass or change in mass. There are four types of gravimetric method: o o o o Precipitation gravimetry Electrogravimetry Volatilization gravimetry Particular gravimetry In precipitation gravimetry an insoluble compound forms when we add a precipitating reagent or precipitant to a solution containing the analyte. In most methods the precipitate is the product of a simple metathesis reaction between the analyte and precipitant, however, any reaction generating a precipitate can potentially serve as gravimetric method. All precipitation gravimetric analysis share two attributes. Firstly, the precipitate must be of low solubility, of high purity and of known composition if its mass is to accurately reflect the analyte`s mass. Secondly, the precipitate must be separate from the reaction mixture. To provide accurate result, solubility must minimal. Sulphate is widely distributed in nature and may be present in natural water in concentration ranging from a few to several thousand milligram/liter. Sulphates are of considerable concern because they are indirectly responsible for two serious problems often associated with the handling and treatment of water. Odour and sewer corrosion problem result from the reduction of sulphate hydrogen sulphide under anaerobic conditions. Sulphate can be determined by Gravimetric method of with ignition of residue Gravimetric method with drying of residue Turbidimetric method Gravimetric method with ignition of residue Sulphate is precipitated in hydrochloric acid medium as barium sulphate by the addition of barium chloride. The precipitation is carried out near the boiling point temperature and after a period of digestion, the precipitate is filtered, washed with water until free of chloride, ignited weighed barium sulphate.
Gravimetric method with drying of residue If organic matter is not present in the sample first method can be done without igniting and instead drying the residue and weighing. Turbidimetric method Turbidimetric method is method of measuring sulphate is based upon the fact that barium sulphate tends to precipitate in a colloidal form and that this tendency is enchance in presence of sodium chloride – hydrochloric acid solution containing glycerol and other organic compounds. The absorbance of barium sulphate solution is measured by nephelometer or turbidmeter and the sulphate ion concentration determined by the comparison of the reading with a standard curve. SOLUTION AND CHEMICAL 1+1 HCl 0.05M BaCL2 0.1M HNO3 AgNO3 Unknown sample Warm water APPARATUS 400ml beaker 10ml and 200ml measuring cylinder Stirring rod Heating plate Desicator Analytical balance Filter paper Vacuum pump compressor PROCEDURE 250ml of the unknown sample is measured and put into 400ml beaker. Addition of 3ml 1+1HCl is poured into the 400ml beaker containing the unknown sample. The solution is heated to near boiling point and slowly 80ml of 0.05M BaCl2 is added with efficient stirring. The mixture solution is left to digest for about 30minutes near boiling point. Afterward the solution is allowed to settle for the precipitate to form at the basement. A few drops of BaCl2 is added to the clear solution at the top of the precipitate check all the sulphate in the solution is precipitated out. If not additional Bacl2 is added in excess to precipitate the sulphate out.
A filter paper of constant weight and cooled in a desicator was weighed and its mass noted. The precipitated BaSO4 was filtered using the weighed filter paper and drained in a suction pump. The filter paper and the residue (BaSO4) were dried and weighed. CALCULATION Mass of filter paper=1.048g Mass of precipitate and fitter paper=3.685g Mass of precipitate BaSO4 is =3.685-1.048=2.637g 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= 2.637 × [32.066+4(16)]/[137.33+32.066+4(16)] weight of SO2-4= 2.637×96.066/233.396 = 2.637×0.4116 2weight of SO 4= 1.0853892g = 1085.3892mg Mg/L SO4= mgBaSO4 x 411.6 ml sample = 2.637x10-3 x 411.6 250 x 10-3 =4.342 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 PRECAUTION 1. The precipitate was digested to increase the average particle size and reduce coprecipitation. 2. BaCl2 was added slowly amidst effective stirring to reduce relative supersaturation. 3. Completeness of the precipitation was tested for after each digestion by adding drops of BaCl2. 4. The container that contained BaSO4 was washed with warm water to remove the Cl- salt present. 5. The filter paper and the precipitate were well dried to ensure accurate weight measurement. 6. 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 It can be deduced that the aims of the experiment were met out as expected. The mass of the sulphate was 4.342mg/l. REFERENCE Analytical Chemistry for Technicians, John Kenkel, 3rd edition, pg 130, 142 Analytical Chemistry Handbook, Pradyot Patnaik, 2nd edition, pg 297 Modern Analytical Chemistry- Harvey, Pg248, 250, 251. KNUST- Kumasi, Department of Chemistry, Laboratory Manual, Pg 29 & 30.