FE 211

ANALYTICAL CHEMISTRY

EXPERIMENT-1

THE GRAVIMETRIC DETERMINATION OF SULFATE IN

A SOLUBLE SAMPLE

Ugur ASİT

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PURPOSE:

In this experiment, we will determinate the value of SO4-2 in the

sample solution. By this way we will become familiar with basic processes of gravimetric analysis.

THEORY:

Gravimetric method is one of the three subgroups of quantitative analytical method. It is based on measuring the weight of a known substance which is related to the analyte and, by using the relation, determining the quantity.

Gravimetric analyses divides into three subgroups; precipitation, volatilization, electro deposition. In this experiment we are going to use precipitation method.

As we all know solubility varies for all compounds. There are such compounds that nearly insoluble. Precipitation method is based on this principle that precipitating a soluble analyte as insoluble compound of it and weighing the precipitation.

In this experiment we are going to determine the SO4- quantity by

precipitating it as BaSO4 for that BaSO4 has a solubility of 0.4 mg/100 g water. We are going to use BaCl2 and as barium source.

Ba+2+SO4-2 BaSO4(s)

We will separate BaSO4 crystalline precipitate by filtering and wash it. After drying and ignition we’ll weigh the pure BaSO4.

Advantages

Gravimetric analysis, if methods are followed carefully, provides for exceedingly precise analysis. In fact, gravimetric analysis was used to determine the atomic masses of many elements to six figure accuracy. Gravimetry provides very little room for instrumental error and does not

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require a series of standards for calculation of an unknown. Also, methods often do not require expensive equipment. Gravimetric analysis, due to its high degree of accuracy, when performed correctly, can also be used to calibrate other instruments in lieu of reference standards.

Disadvantages

Gravimetric analysis usually only provides for the analysis of a single element, or a limited group of elements, at a time. Comparing modern dynamic flash combustion coupled with gas chromatography with traditional combustion analysis will show that the former is both faster and allows for simultaneous determination of multiple elements while traditional determination allowed only for the determination of carbon and hydrogen. Methods are often convoluted and a slight mis-step in a procedure can often mean disaster for the analysis (colloid formation in precipitation gravimetry, for example). Compare this with hardy methods such as spectrophotometry and one will find that analysis by these methods is much more efficient.

MATERIALS:

BaCl2 solution (5 g / L). HCl solution. AgNO3 solution. Ashless filtering paper. Flask. Porcelain crucible. Beaker. Water. Bunsen burner.

PROCEDURE:

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10 ml of sample was taken in a beaker. 50 ml water was added. Add 4 ml 6 M HCL This solution was heated until boiling. And 100 ml of BaCI2.2H2O was heated until boiling an other

beaker. Then BaCI2.2H2O was added quickly into hot sample. Solution was waited and cooled. Until the top of solution became

clean. After that solution was decanted. Solution was filtered and washed with hot water three times. Filteted solution was tested with AgNO3 solution. If color does not

change.solution was waited one week to dry. One week later, filter paper and solid substance were put in

porcelain crucible. and they were chared off about one hour in bunsen burner.

Then porcelain crucible was transfered at oven and ignated at 800 0C .

finally,we had only solid (Ba2SO4 (s) ) and weighted. Results were recorded.

RESULTS:

Weight of sample and porcelain crucible = 20.8282 g

Weight of porcelain crucible = 20.1516 g

Weight of (Ba2SO4 (s) ) = 0.6766

DISCUSSION:

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Gravimetric analysis is one of the oldest analytical techniques and for this reason is referred to as a "classical method." Gravimetricprocedures are usually very accurate, but more tedious than othermethods. The only major equipment needed for gravimetric analysisis an accurate balance.

Sulfur can be determined gravimetrically. Sulfur is found inminerals such as iron pyrite, gypsum and coal. Coal may contain asmuch as 4% sulfur in various forms: pyritic sulfur , sulfur insulfates and in a variety of organic forms, such asthiophenols and thiophenes. When coal with high sulfur content isburned sulfur dioxide is formed which is a major primary airpollutant. Obviously the value of coal as a fuel decreases as thesulfur content increases. For this reason, the analysis of coal forsulfur is extremely important.

In any precipitation gravimetric analysis, the species to be deter-mined is reacted with a reagent to yield a product that is: a) of known composition, b) is relatively pure, and c) of low solubility. Knowing the mass of the dried product and the mass of the original sampleand their formula weights (to create a gravimetric factor), the percentof the species to be determined can be calculated..

In the analysis for sulfate, the sample to be analyzed is brought intosolution and reacted with a solution of BaCl2 , a source of Ba 2+ , toyield BaSO4 as a white precipitate. In the procedure, the solution isfirst acidified with HCl (aq) to: a) prevent the precipitation of BaCO3and Ba(OH)2 , and b) aid the formation of larger precipitate crystals because fewer crystallite nuclei are formed. Also, the BaCl2 must be added quickly to minimize occlusion of foreign ions in the precipi-tate crystals. The crystalline BaSO 4 precipitate is then purified by digestion: a process in which the solution from which the precipi-tate came (the mother liquor) and the precipitate are heated to dissolve smaller precipitate particles which subsequently reprecipi-tate on larger particles. The digestion process improves filterability because of the larger crystalline particles, and the precipitate is isolated by gravity filtration through paper.

If paper is used to isolate the precipitate, it must be removedcompletely without losing any precipitate. This can be done by

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using an filter paper which can be burned away. Care mustbe taken in this process to avoid flaming and to prevent conversion of BaSO4 to BaS, a yellow precipitate of lower mass than BaSO4 . The final ignition removes the carbon residue and any volatile impurities carried down with the barium sulfate precipitate. Ignition is continued until the mass of the crucible plus precipitate is constant.

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