Quantitative Determination of Sulphate by Gravimetric Analysis

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Formal Report of the Quantitative Determination of Sulphate by Gravimetric Analysis (Experiment 4)| Maryjo Lee Wei Min 1111107 DBS/FT/1A/01|

(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
A) Using a pipette, add 25 mL of the given sulphate solution into a 250 mL beaker. B) Add 50 mL of water into the beaker, and then add 5 drops of concentrated HCL. C) Heat to boiling.
D) Add, drop-wise with vigorous stirring, 10 mL of 10% barium chloride...
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