Quantitative Determination of Sulphate By Gravimetric Analysis

The objective of this experiment is to determine the amount of sulphate by gravimetric analysis. The amount of sulphate is determined quantitatively as barium sulphate, BaSO4, by gravimetric analysis. For the experiment, a dilute solution of barium chloride was slowly added to a hot unknown sulphate solution slightly acidified with concentrated HCl. The precipitate is then filtered off by suction filtration, washed with water and dried in the oven at 150 degree Celsius for about half an hour and cooled in the desiccator, and weighed as barium sulphate and the percentage of sulphate was calculated from the weight of barium sulphate. The Molarity of sulphate= 0.004269 M and the concentration of sulphate in g/L= 0.4098g/L. The weight of barium sulphate was calculated to be 0.0249g. There were also no hiccups during the experiment.

(133 words)

OBJECTIVE

To determine the amount of sulphate by gravimetric analysis.

Students can also be familiar to the ways to carry out Gravimetric Analysis through this experiment.

THEORY

Gravimetric analysis has become one of the most accurate classes of analytical methods available. Commonly, gravimetric analysis is being carried out by the following steps: (1) preparing a solution containing a known weight, (2) isolation of the desired elements, (3) weighing the separated elements and (4) computation of the amount of constituent in the sample from the observed weight of the isolated substance. Precipitation and filtration are the most commonly used methods to isolate the desired substances from a solution. The reagents used in the reactions will form an insoluble compound with the desired constituents but not precipitate of other elements.

Firstly, exactly 25mL of given sulphate solution was used so that it would be a fair experiment in relation to how much sulphate will be obtained at the end of the experiment with regards to the amount of sulphate solution used.

Secondly, concentrated HCL was added to water in the fume hood. This is because water should not be added to concentrated acid as the heat of the solution will cause water to boil and the acid to splatter. Also, the process is done in the fume hood as concentrated HCL gives out vapour that is toxic and it should not be inhaled.

Thirdly, the beaker was covered with a watch glass while heat and the use of the watch glass is to let condensation occur to prevent the lost of solution through evaporation. After that, a few drops of barium chloride was added to see if everything has mixed together. If the few drops of barium chloride added do not dissolve in the clear solution, it means that there is complete precipitation.

Next, the filtration of barium sulphate precipitate is done by the use of suction filtration. For this process, the use of a filter flask is that it can withstand pressure without imploding, and also helps speed up the process of filtration. Also, the filter paper has to cover the entire base of the crucible so that now solvent will actually leak through when the suction starts up, resulting in the loss of solvent and also in severe cases, causing the flask to break due to pressure inside it. Also, the use of a rubber policeman is to help dislodge any particles on the beaker into the crucible and the use of the rubber policeman instead of just a glass rod is that the surface area that the rubber policeman can cover is more and thus speeding up the dislodging of particles. Also, plucking out the tube away from the filter flask before switching off the pump is important as it is to ensure that there isn’t a sudden drop in pressure that could cause the filter paper to be sucked in and tearing in the process.

For the drying and weighing of barium sulphate, the crucible with the barium sulphate precipitate was dried in the oven to let the remaining moisture be evaporated so that only the dried sample is left behind. Cooling the precipitate in the desiccator is to prevent the capture of moisture in the outside air as the desiccator controls the level of humidity in it while the precipitate cools down.

For the weighing of the crucible, it is done in a closed balance, so that it can measure a very accurate weight of the collected sulphate.

PROCEDURES

PRECIPITATION OF BARIUM SULPHATE

25mL of the given sulphate solution was pipette into a 250mL beaker.

50mL of water and 5 drops of concentrated HCl was added.

The solution was heated till boiling and vigorously stirred. 10mL of 10% barium chloride solution was added drop wise from a measuring cylinder.

The beaker was covered with a watch glass and digested for 20 minutes.

A few drops of barium chloride was add to the clear supernatant liquid to test for complete precipitation.

WASHING AND FILTRATION OF BARIUM SULPHATE PRECIPITATE

The filter paper was positioned to cover the entire base of the crucible completely. The clear supernatant solution was decanted by filtration into the pre-weighed crucible with filter paper.

A “rubber-policeman” was used to dislodge any particles on the beaker and the beaker was rinsed with warm deionised water. The contents were emptied into the crucible while the vacuum pump was still at work.

The precipitate was further washed with warm deionised water at the vacuum pump two more times.

The filtrate was discarded.

WASHING AND FILTRATION OF BARIUM SULPHATE PRECIPITATE

The crucible with barium sulphate was dried in the oven at 150 degree Celsius for about half an hour.

The crucible was cooled in the desiccator for about 10 minutes.

The crucible was weighed when it has cooled down.

The weight of the barium sulphated was calculated.

RESULTS & CALCULATIONS

Tabulated results for experiment 4:

A: Weight of crucible with precipitate

17.3337g

B: Weight of crucible (with a piece of filter paper)

17.3088g

Weight of barium sulphate= A-B

17.3337g-17.3088g= 0.0249g

Molarity of SO42-

0.004268632M

Concentration of SO42- in g/L= Molarity of SO42- x Molar weight of SO42-

0.4098g/L

DISCUSSION

From the results, it is shown that the weight of barium sulphate is 0.0249g and the concentration is shown to be 0.4098g/L. The barium sulphate precipitate appeared to be white and powdery, as shown in internet sources. The results was compared and it was quite high compared to others results and it has got to do with Thoroughly rinsing of beaker and vacuum pump during filtration as it ensures that none of the precipitate is lost and all the precipitate are collected during suction filtration. According to Texas A&M University, sufficient washing helps to reduce the level of impurity. Lesser impurities will give higher amount of pure precipitate.

However, another reason why the precipitate would weigh much more may be also because there is moisture present in the precipitate due to improper drying or the time taken to dry the precipitate was not enough. As my group have only done the process of drying the precipitate in the oven and cooling in the desiccators for only once, there is a possibility that there is still moisture in the precipitate and one way that this can be overcome is to repeat the drying process for a few times until a constant weight have been recorded and this would make the result more accurate.

Moreover, due to human error and environmental conditions, it could cause slight inaccuracy to the measurements such as while taking the measurement for the barium chloride solution and measurement for the sulphate solution, parallax error may occur, causing us to read off the value at a wrong angle and thus resulting in either more or less concentration of the solution that we used. Also, as mentioned in theory, the part where the beaker was covered with a watch glass and throughout the digestion process, if the end results obtained was lesser than expected, it would probably be because not all the barium chloride has dissolved in the solution, which results in the loss of some sulphate and contributing the inaccuracy of the end results.

Lastly, the loss of precipitate may be also caused by not washing off all the precipitate into the crucible, as some precipitate may be left behind in the beaker and if they are not washed down, it would result in the loss of precipitate and affect the amount of sulphate obtained. Also, from the process where the solution was heated and stirred vigorously, some of the solution may have accidentally spilled out, and also when the water heats up, some of the solution may have actually evaporated, causing the loss of some of the solution.

CONCLUSION

Through this experiment gravimetric analysis is known as a method for determining the amount of a desired constituent present in a sample and also it helped my group gets familiarized with the process of carrying out gravimetric analysis. The method involves precipitation of the wanted substance and suction filtration to isolate the substance, and finally drying the substance and weighing it to find out the exact amount of the substance retained. The results of the experiment are shown that the weight of barium sulphate is 0.0249g and the concentration is shown to be 0.4098g/L. Also, the precipitate is white and powdery, and it is similar to what internet sources show. However, the weight of the precipitate is higher than a few other results of other groups, and this may be due to moisture being present in the precipitate, because of either improper drying or the time taken to dry the precipitate was not long enough, thus causing inaccuracy in the results tabulated. Overall, the experiment was a success.

REFERENCES

Dr.Nutts (2010) Definition of Gravimetric Analysis [online] Available from : http://www.chm.davidson.edu/ronutt/che115/viii_gravi.pdf – INFORMATION FOR GRAVIMETRIC ANALYSIS (THEORY SECTION)

http://www.dartmouth.edu/~chemlab/techniques/vfiltration.html – INFORMATION ON SUCTION FILTRATION (THEORY SECTION)

http://www.laboratoryequipmentworld.com/filtering-flasks.html- THE USE OF A FILTER FLASK (THEORY SECTION)

http://en.wikipedia.org/wiki/Gravimetric_analysis – COMPARING RESULTS AND PROCEDURES WITH INTERNET SOURCES (DISCUSSION SECTION)