GRAVIMETRIC DETERMINATION OF SULFATE

PurposeYou will be given a solid powder, which has been dried to constant mass. The sample is water soluble.The purpose of this experiment is to determine the concentration of sulfate in your solid sample usinggravimetric analysis.

Background

Gravimetric Analysis

The goal of most quantitative chemical analysis measurements is to estimate the relative abundance ofan analyte in a chemical sample. For solid and liquid samples, a very common expression of analytecontent is based on the mass fraction. This fraction is commonly expressed as a percentage (w/w %),as parts per million (ppm) or as parts per billion (ppb), depending on the concentration level of theanalyte. Classical chemical analysis methods are excellent for the determination of analyteconcentrations in the range of 1−100 w/w %. In order to estimate the analyte mass fraction of any solidsample, we typically need two measurements, one to estimate the sample mass and one to estimate theanalyte mass.

In gravimetric analysis, measurements of mass are used to determine the analyte concentration in asample. In the most common form of gravimetric analysis, the analyte is separated from the rest of thesample in the form of a solid precipitate. In this experiment, the analysis of sulfate is performed usingbarium chloride as a reagent to precipitate sulfate from the dissolved sample. The mass of sulfate in thesample can be calculated by simple stoichiometry from the mass of the weighing form of the sulfateprecipitate.

In order to ensure maximum filterability and minimum contamination, the sulfate is precipitated underconditions that maximize the particle size. These conditions include: (a) avoiding excessively highconcentrations of precipitating reagent; (b) slow addition of reagent to a hot solution, with vigorousstirring; (c) avoiding a great excess of reagent. The digestion of the precipitant after the reactionminimizes errors due to coprecipitation.

For accurate measurements, we must be sure that the composition of the weighing form is knownexactly. You must be very careful in your mass measurements throughout this experiment − forexample, do not handle your weighing bottle with your bare hands, to avoid leaving fingerprints thatwould increase the mass of the bottle. Of course, the recovery of the sulfate precipitate must bequantitative, and the final weighing form must be pure.

GRAVIMETRIC ANALYSIS: Procedure

Chemicals and materials:6 M HCl0.05 M BaCl2 (note: you will be provided with 0.5 M BaCl2)Whatman #42 ashless filter paper

Place the sample in a beaker and cover it with a watch glass to protect your sample from dust in the air.Dry the sample for at least one hour at 110 °C. Allow the sample to cool in a desiccator for at least 30minutes before weighing. Weigh three samples by difference (your instructor will show you how to dothis) into 400 mL beakers. The samples should be approximately 0.4 to 0.5 g in size, and their massmust be known accurately (to the nearest 0.1 mg). Be sure to label the beakers; you do not want to mixup your samples.

While the sample is drying, clean three porcelain crucibles and covers and identify each numericallyaccording to the instructor’s direction. Heat the crucibles without covers to constant weight at thehighest temperature of the burner. To heat the crucibles to constant weight, place the crucibles intriangles, bring the tip of the flame close to the bottom of the crucible, and heat for 15 minutes (seeHarris Fig 29-1). Allow the crucibles to cool in air until only moderate radiated heat is felt when theback of the hand is brought near the crucible (about one minute or so), then transfer the crucibles to thedesiccator. After the crucibles reach room temperature in the desiccator (about 30 minutes), weigh thecrucibles accurately to the nearest 0.1 mg. Repeat the heating, cooling and weighing until successiveweights are within ±0.2 mg.

Dissolve each sample in approximately 200 mL of deionized water and add 4 mL of 6 M HCl to each.Heat nearly to boiling.

You will need to add an excess of precipitating reagent (BaCl2) to the samples in order to precipitatethe sulfate. To calculate how much reagent to use, use the following calculation: assume that thesamples are pure sodium sulfate (they aren’t) and calculate the volume of 0.05 M BaCl2 needed toprecipitate the largest of your samples. Add to the calculated amount a 10% excess of BaCl2 solution.This is the volume of reagent that will be added to each of your samples. You will be provided with 0.5M BaCl2; you must make your precipitating reagent from this solution.

Heat three portions of reagent BaCl2 solution nearly to boiling. Slowly pour the hot BaCl2 solution intothe hot sample solution with vigorous stirring during the addition. Use a separate stirring rod for eachsample and leave it in the solution throughout the determination. Allow the precipitate to settle and testfor complete precipitation by adding a few drops of BaCl2 solution and observing the result.

After the precipitation is complete, cover each beaker with a watch glass and digest just below theboiling point for at least one hour. Longer periods are desirable, if time permits. If digestion iscompleted near the end of the laboratory period, so that there is insufficient time for filtration, set thesamples in the cupboard where they will not be disturbed until the next laboratory period. You willwant to heat the solution nearly to boiling before filtering at the beginning of the next laboratoryperiod.

Carefully decant the supernatant liquid through Whatman #42 ashless filter paper (obtained from yourinstructor). Wash the precipitate three times by stirring with warm distilled water and decanting the

water after the precipitate has settled. Empty the filtrate from the beaker below the funnel unless thefiltrate is cloudy. If the filtrate is cloudy, refilter the filtrate through another piece of ashless filterpaper.

Using a stirring rod and wash bottle, transfer the filtrate to the filter paper. Police the walls of thebeaker to remove the last traces of the precipitate. Wash the precipitate in the filter three times with 3-5mL portions of warm distilled water each time.

Fold the filter paper and place in the previously weighed porcelain crucible (see Harris fig 29-2). Placethe crucible in a triangle and heat very gently to dry the paper. Next char the paper with gentle heat.The heat should be regulated so that the paper does not burst into flame. A clean crucible cover shouldbe located nearby, ready for use if necessary. If the paper does burst into flame, the burner must bequickly removed and the cover used to snuff out the flame.

After the paper has burned away, you will want to burn away deposited carbon particles by applyingincreased heat to the crucible. Do not point your flame directly into the crucible − you will probablyblow your precipitate across the room. You should always position your flame so that the precipitatehas access to atmospheric oxygen; otherwise you might reduce your precipitate from BaSO4 to BaS.

When the carbon is gone from the crucible, place the crucible upright in the triangle and ignite for atleast fifteen minutes at the highest temperature of the burner. You should note that the hottest portionof the flame is just above the top of the inner blue cone. After ignition, allow the crucible to cool untilthe red glow disappears and then place it in the desiccator until it cools to room temperature. Weigheach crucible to the nearest 0.1 mg. Finally, bring each crucible to constant weight, as directedpreviously.

GRAVIMETRIC ANALYSIS: DATA SHEET

Directions: fill in this sheet and turn it in with your abstract. Also turn in a sheet showing yourcalculations (i.e., how you transform this raw data into your final results).

Name: Unknown #:

estimated analyte conc(as a confidence interval)

g g gmass: crucible + ppt(heated to constant mass)

g g gcrucible mass(heated to constant mass)

g

g

g

g

g

g

before:

after:

dry sample mass(mass by

difference)

sample Csample Bsample A