Quantitative Determination of Sulphate by Gravimetric Analysis

ANALYTICAL & PHYSICAL CHEMISTRY

Formal Report of the Quantitative Determination of Sulphate by Gravimetric Analysis (Experiment 4)

Maryjo Lee Wei Min

(A) Synopsis

The amount of sulphate is determined quantitatively as barium sulphate by gravimetric analysis.

This determination consists of slowly adding a dilute solution of barium chloride to a hot, unknown sulphate solution which is slightly acidified with concentrated hydrochloric acid. A white precipitate will formed and is then filtered off, washed with deionised water, dried in the oven and weighed as barium sulphate. The percentage of sulphate is calculated from the weight of barium sulphate.

(B) Objective

The purpose of the experiment is to quantitatively determine the amount of sulphate, in barium sulphate, by the gravimetric method.

(C) Theory

Gravimetric analysis gets its name from the process of isolating the desired constituent in weighable form. In summary, it involves changing one compound containing the constituent into another compound containing that constituent and measuring the percentage of sulphate in the new compound to determine the percentage of sulphate in the previous compound.

This experiment also demonstrates the concept of stoichiometry. Stoichiometry is the determination of the proportions in which chemical elements combine and the mass relations in any chemical reaction. Also known as mass analysis, gravimetric analysis is used to determine an analyte by selective precipitation of the substance from an aqueous solution. The analyte, in this case, is sulphate.

For a successful gravimetric analysis, barium sulphate must be completely precipitated so as to avoid contamination. The number of moles of sulphate can be found out from the mass of the barium sulphate.Since barium chloride is added in excess, and since the precipitation reaction is assumed to proceed to completion, we can assume that the number of moles of sulphate in the precipitate to be equal to the number of moles of sulphate in the original sample. Hence, we can calculate the percentage by mass of sulphate in the original sample of the given sulphate solution. As gravimetric analysis involves precise weighing, the instruments used during the experiment, such as the analytical balance, should also be accurate enough for the gravimetric analysis technique to yield useful results.Hence, when done cautiously, gravimetric analysis provides for an exceptionally accurate analysis.

Veritably, gravimetric analysis was used to conclude the atomic masses of many elements to a six-figure accuracy. Another advantage is that this method also provides for very little room of instrumental error and does not need a series of standards for the calculation of an unknown substance. Usually, the method does not involve the usage of expensive equipment. Because of its high degree of accuracy, when gravimetric analysis is performed correctly, it can also be used to calibrate other instruments instead of reference standards.However, gravimetric analysis generally caters for the analysis of either a single element, or a limited group of elements, at a time.

Thus, it is incapable of analysing multiple elements concurrently. The methods are usually quite complicated as well, and even the most trival mistake in a procedure can lead to disaster in the analysis. For example, if a colloid was to be formed during precipitation gravimetry, the results would be deeply affected, calling for inaccuracy. Therefore, by comparison, hardy methods such as spectrophotometry provide for a much more efficient analyses.

D) Procedures

1) Precipitation of Barium Sulphate

1. Using a pipette, add 25 mL of the given sulphate solution into a 250 mL beaker.
2. Add 50 mL of water into the beaker, and then add 5 drops of concentrated HCL.
3. Heat to boiling.
4. Add, drop-wise with vigorous stirring, 10 mL of 10% barium chloride solution into the heated solution.
5. Cover the beaker with a watch glass and digest for 20 minutes.
6. Test for complete precipitation by adding a few drops of barium chloride to clear the supernatant liquid.

2) Washing and Filtration of Barium Sulphate Precipitation

1. Pre-weigh a crucible.

   Ensure that the crucible has a filter paper that covers the base of the crucible completely.

2. Decant the clear supernatant liquid by filtration at the vacuum pump into the crucible.
3. Use a ‘rubber-policeman’ to dislodge any particles on the beaker, and then rinse the beaker with warm deionised water. Empty the contents into the crucible while the vacuum pump is at work.
4. Further wash the precipitate with warm, deionised water at the vacuum pump twice more.
5. Discard the filtrate.

3) Drying and Weighing of Barium Sulphate Precipitate

1. Dry the crucible with the barium sulphate precipitate in the oven at 150°C for about 30 minutes.
2. Cool the crucible in a desiccator for about 10 minutes.
3. Weigh the crucible when it has cooled down.
4. The difference between this weight and the empty crucible is the weight of the barium sulphate precipitate.
5. Results and Calculations

E) Weight

Weight of crucible with the precipitate: 14.

0996 g Weight of crucible (with piece of filter paper): 13. 9030 gWeight of barium sulphate precipitate: 0. 1966 g Ba2+ + SO42- ————-> BaSO4 (Moles of SO42-)/(Moles of BaSO4 ) = 1/1 (Molarity of SO42- x Volume of SO42-)/[(Weight of BaSO4)/(Mol Weight of BaSO4)] = 1/1 Mol Weight of BaSO4 = 233. 33 g/mol ? Molarity of SO42 = 0. 1966 g x 0.

025 L = 0. 004915 M ?Concentration of SO42- in g/L = Molarity of SO42- x Mol Weight of SO42- = 0. 004915 x [32 + 4(16)] = 0. 47184 g/L

(F) Discussion

Experimental errors might have occurred during the experiment, causing inaccuracy of results. For example, poor techniques might have been used when weighing the sample and handling the equipment. Although the scales used in the experiment were accurate to 0.

001 grams, small amounts of errors to the masses obtained might affect the weight of the precipitate. For instance, during the handling of the glassware, if we were not cautious enough and touched them with our bare hands, there might be a change in the composition of the weighing form as sweat on the hands could contaminate reagents.Another error that could have occurred was the parallax error. This could have caused the inaccuracy in the weighing of the limiting reactant and the precipitate formed would also be inaccurate. Failure to collect all the precipitate was also a possible source of error.

During handling, any substance that were dropped or had an atmospheric loss would result in a lower yield of product. Having to transfer all the precipitate to the glassware was no easy task either, even with the help of the rubber policeman which aided in the scrubbing off of traces of precipitate on the wall of the beaker.Thus, any precipitate that was not transferred would result in a low final mass of barium sulphate measured. Another factor that might have contributed to the experimental errors was the imperfection of the filtering process. When carrying out filtration, the position of the filter paper was not sitting flat on the crucible and as a result, a small amount of precipitate would have failed to be filtered off and passed through the filter paper, causing a lower amount of solid precipitate at the end of the experiment.Also, due to time constraints, we could not carry out the vacuum filtration for a third time to ensure that the solution was completely filtrated.

Hence, some of the precipitate that might be in the flask was thrown away. An important assumption made was that the inevitable loss of material did not affect the overall accuracy of the technique. So as to obtain better results, these errors can be avoided by repeating the vacuum filtration for at least more than two times, so that the maximum amount of precipitate can be collected.Extending the time for drying and cooling should lead to the removal of moisture content before weighing. The proper way of handling glassware as the experiment is to the highest degree of accuracy in gravimetric analysis.

One should also aware and alert when handling concentrated hydrochloric in the fumehood. This is because the acid is extremely corrosive and wearing gloves upon handling the hot equipment, such as the oven, is necessary to avoid accidents from occurring.

(G) Conclusion

As there were experimental errors made during the gravimetric analysis, the purity of the sulphate is affected. As the compounds that were precipitated were insoluble, insignificant errors might have resulted from the incompleteness of the precipitation. However, to get an extensively pure precipitate that is the precise composition of its chemical formula is rather challenging and this subjects the methods of gravimetric analysis to some degree of error. Hence, the results of the experiment were not satisfactory due to the mentioned experimental errors.