SynopsisThe objective is to determine the amount of Sulphate in anhydrous Barium Sulphate precipitate by gravimetric method. Gravimetric method is by the quantitative determination of the mass of anhydrous Barium Sulphate precipitate. Barium sulphate precipitate is form when Barium Chloride is added excessively to a hot given sulphate solution slightly acidified with concentrated Hydrochloride acid. The white precipitate of hydrate Barium Sulphate formed is than digest, filtered out, washed and dried than cool down in a desiccator. Anhydrous Barium Sulphate is formed and weight using analytical balance. We will find that the mass of Barium Sulphate precipitate form is 0.1990g and with the knowledge of its chemical composition, we can calculate the amount of Sulphate. From these, we found that amount of Sulphate is 0.0820g. And the major experimental finding is that gravimetric analysis is one of the most accurate and precise methods of macro-quantitative analysis as we can calculate till high figure accuracy.
ObjectiveThe purpose of this experiment by gravimetric method is to determine the amount of Sulphate in a solution. In addition, to learn to carry out vacuum filtration of Barium Sulphate precipitate and use a desiccator correctly.
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TheoryThe amount of Sulphate is determined quantitatively from Barium Sulphate by gravimetric method.
This determination consists of slowly adding, with vigorous stirring a dilute solution of barium chloride to a hot solution of the Sodium Sulphate which is slightly acidified with concentrated hydrochloride acid for the removal of impurities and preventing carbon dioxide in the air from reacting to barium to form barium carbonate precipitate. Dilute and hot solutions with slowly adding and effective stirring are done to decrease the degree of contamination of impurities.
Barium Sulphate precipitate form is allowed to digest which can help to reduce the amount of co precipitation. Co precipitation may result in excess mass hence, there may have inaccurate result. Than a slight excess of Barium Chloride is added when precipitate has settled, this is to decrease the solubility by mass action and to assure precipitation is complete. But a large excess of Barium Chloride should be avoided as this increases chances of absorption on the surface of precipitate.
Ionic Equation: Ba2+(aq) + SO42-(aq) BaSO4(s)The white precipitate Barium Sulphate form is filtered off using vacuum filtration, washed with warm deionised water, died in oven and weight in analytical balance. Washing helps to remove co precipitated impurities, especially those on the surface. However, washing may lead to redissolving of a slightly soluble salt; hence there will be lost of precipitate.
Some of instrument that is used in this experiment are vacuum pump filter, analytic balance and desiccator.
Procedure1.25.0mL of the given sulphate solution is pipette into a 250mL breaker. The solution is treated with concentrated Hydrochloride acid and an addition of 50mL of water so that the proper precipitate is formed and to suppress the formation of other precipitates.
2.10mL of 10% barium chloride is added drop by drop to the heated sulphate solution and stirred vigorously.
3.After the precipitate has formed and been allowed to digest, a few drops of Barium Chloride is added to ensure that analyte has been completely precipitate; if a precipitate is observed, the precipitation is incomplete.
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4.The solution is carefully filtered at the vacuum pump, using a crucible with a piece of filter paper and washed the precipitate twice with warm deionised water.
5.After filtration, the precipitate(including the filter paper and crucible) is heated. This is to remove the remaining moisture and convert the precipitate to a more chemically stable form.
6.After the precipitate is allowed to cool in a desiccator to keep it from absorbing moisture, it is weight (in the crucible) with analytical balance. The mass of the crucible is subtracted from the combined mass, giving the mass of the precipitated Barium Sulphate.
Results and CalculationsWeight of crucible with the precipitate:21.9494gWeight of crucible (with a piece of filter paper): 21.7504gWeight of BaSO4 precipitate: 0.1990gBa2+(aq) + SO42-(aq) BaSO4(s)Moles of SO42- 1Moles of BaSO4 1Molarity of SO42- x Volume of SO42- 1Weight of BaSO4 1Mr of BaSO4Molarity of SO42- = 0.03411 MConcentration of SO42- in g/L = Molarity of SO42- x Mr of SO42-= 3.278 g/LAmount (g) of SO42-= No. of mol of SO42- x Molecular mass of SO42-% of SO42-= x 100%= x 100%= 41.19%DiscussionThe result of weight of Barium Sulphate precipitate may be wrong if co precipitation or re-dissolving of precipitate have occurs. Result in lesser or more precipitate form. To have a totally quantitative gravimetric analysis, several requirements must be met.
First, the precipitate to be separated must be obtainable in a pure compound with a known stoichiometry.
Secondly, there must be complete precipitation of desired substance and the solubility product for the solid must be negligible.
Lastly, the precipitate itself should be free of impurities and not much surface area for adsorption of impurities but large enough to filter.
With these entire requirements met and methods followed carefully, provides for exceedingly precise analysis, gravimetric analysis is typically the most accurate analytical method that can be done in a normal chemistry laboratory as gravimetry provides very little room for instrumental error and does not require a series of standards for calculation of an unknown. For most laboratories, the measurement of mass on an analytical balance is the most precise and accurate measurement that is done. In fact, gravimetric analysis was used to determine the atomic masses of many elements to six figure accuracy. Moreover, methods often do not require expensive equipment. Hence, gravimetric analysis is an effective way of analysing an element.
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Questions1.Why do you add HCl to the mixture of BaCl2 and Na2SO4?Adding HCl is to remove impurities that might be present in BaCl2 and Na2SO4 and prevent formation of other precipitates like Barium Carbonate, which form from carbon dioxide in air reacting with barium. Moreover, pH is important because often it influences the solubility of the analytical precipitation and the possibility of the interferences from other substances, like formation of barium carbonate.
2.What are the factors that favour the formation of BaSO4 crystals?Factors that must be considered that favours the formation of BaSO4 include the volume of the solution during precipitation, the concentration range of the test substance, the presence and concentrations of other constituents, the temperature, and the pH. When precipitate at high temperature, solubility is lower hence precipitate will be quantitative when cooled. Precipitate at a pH near acidic end to maintain quantitative precipitation as many precipitate are soluble in acid medium hence slow down the rate of precipitation and lead to growth of larger crystal. Slow addition of dilute Barium Chloride with effective stirring allows nuclei site to come together to form large crystal too.
3.How do you check the purity of BaSO4 crystals formed?We can check purity of BaSO4 crystals formed by checking the melting point. A pure substance has a fixed melting point while an impure substance melts over a range of temperature and at a lower temperature than the pure substance. The greater the range of melting temperature the more impurities present. A range of less than 1°C indicates that the purity is high. Melting point of BaSO4 crystals can be checked by using a narrow capillary tube in an oil bath and by heating an aluminium block which has a thermometer inserted and with crystal on the block. Another method to check the purity is by calculating the percentage of purity.
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4.How do you improve the purity of BaSO4?By making BaSO4 crystals larger, as the larger crystals have a smaller specific surface area and so have less chance of absorbing impurities. Hence concentration of impurities is lower. BaSO4 crystals can be made larger by digestion and slower rate of precipitation. To improve the purity, washing, dissolving and reprecipitating maybe help.
5.ConclusionFrom this experiment, we found that the molarity, concentration in g/L and percentage of SO42- is 0.03411M, 3.278g/L and 41.19% respectively. We can conclude that gravimetric analysis was indeed an effective but tedious way to analysis an element and gravimetric analysis usually only provides for the analysis of a single element, or a limited group of elements, at a time. In addition, a slight mis-step in a procedure can often mean disaster for the analysis. Despite these disadvantages of gravimetric analysis, it is still an effective way in all mean as gravimetry provides very little room for instrumental error and does not require a series of standards for calculation of an unknown in addition; this experiment is an inexpensive way to analysis an element. Moreover, gravimetry is among the most accurate analytical techniques.
References
1.http://chemistry.olivet.edu/classes/chem301/pdf/Gravimetric%20Precipitation.PDF2.http://en.wikipedia.org/wiki/Gravimetric_analysis3.http://kinardf.people.cofc.edu/154L_HONS_HonorsChemistryLaboratory/154_Exp5_GravimetricSulfateAnalysis.pdf4.Analytical Chemistry(5th edition) Gary D. Christian5.http://www.sciencetechnologyaction.com/lessons2.php?studyid=26
