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Chemistry 3000Experiment 3

Redox Titration of Iron in Ore

Reading. Before coming to lab to work on this experiment, read the background material onredox titrations in Chapter 16 of your textbook. You will two lab periods to complete thisexperiment.

Iron ore is analyzed by dissolving the ore in hydrochloric acid solvent (Equation 3-1) togive iron III, followed by pre-reduction (Equation 3-2 and Equation 3-3) to give iron II, andtitration of the iron II with a standard oxidizing agent such as potassium permanganate or

potassium dichromate (Equation 3-5). The simplest determination involves pre-reduction ofiron III with stannous chloride and titration with potassium dichromate. Your potentiometric

titration curve from later in the lab should look similar to Figure 3-1.

Figure 3-1 Theoretical curve for the titration of 100.0 mLof 0.0500 M Fe +2 with 0.100 M Ce +4 in 1 M HClO 4

Although stannous chloride reduction is more convenient than using a column of metallic

reducing agent such as amalgamate zinc, it is subject to serious errors if a large excess ofstannous chloride is added. When mercuric chloride is added to react with the excess stannouschloride, a high concentration of the latter tends to reduce mercury II to finely divided mercurymetal (Equation 3-4). During the titration with dichromate, the mercury metal will interfere withthe determination by being oxidized itself. If only a small excess of stannous chloride is present,mercuric chloride is reduced only to insoluble mercurous chloride.

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Chemistry 3000 Experiment 3, Page 2

Fe2O3 + 6H + = 2Fe 3+ + 3H 2O (3-1)(HCl)

Sn II + 2Fe 3+ = Sn IV + 2Fe 2+ (3-2)(excess) (yellow) (light green)

Sn II + 2Hg 2+ + 2Cl - = Hg 2Cl2(s) + Sn IV (3-3)(HgCl 2)

Sn II + Hg 2+ = Hg 0 + Sn IV (3-4)(excess) (HgCl 2) (black)

Cr 2O72- + 6Fe 2+ + 14H + = 2Cr 3+ + 6Fe 3+ + 7H 2O (3-5)(K 2Cr 2O7)

Laboratory Procedures

A. Preparation of Potassium Dichromate

You will find primary standard grade potassium dichromate in one of the ovens. Transferabout 1.5 grams to a weighing bottle and cool to room temperature in your desiccator. While thedichromate is cooling, transfer your ore sample to a weighing bottle and put it in the oven for atleast one hour to dry (you might want to get your unknown one week ahead of time to do thismost efficiently). When the potassium dichromate is cooled, weigh out exactly about 0.61 gramsand transfer it to your 250 mL volumetric flask and dilute to volume. Calculate the molarconcentration of this solution for analysis of your results.

B. Dissolution of Ore and Pre-reduction of Iron III

Caution: Hot, concentrated acids are used in this experiment. Wear protective clothingand be careful to keep your area clean to avoid acid burns to your skin or clothes.

1. Exactly weigh four iron ore samples of about 0.15 g each into 250 mL Erlenmeyer flasks, add10 to 15 mL of 12 M hydrochloric acid to each. Do this in the fume hood . Solutions should

be swirled instead of using a stir bar to avoid splashing in the next two steps.

2. Heat on a hot plate in a hood below the boiling point. You will have to watch to make sure thesolutions don't boil. Addition of 0.5 M stannous chloride solution will greatly acceleratedissolution of the sample. Add drop-wise (about 10 drops) until it is orange; add a few moredrops which will produce the bright yellow color of Fe III. Your dissolved sample may have acolorless silicate precipitate that will not dissolve; this will not interfere with the analysis. Iftoo much stannous chloride is added, the solution will go colorless or light green. If thisoccurs, then after the sample has completely dissolved, eliminate the excess Sn II by drop-wise addition of KMnO 4 until the bright yellow color of Fe III is observed.

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Chemistry 3000 Experiment 3, Page 3

3. Evaporate the solution carefully to about 5 mL and dilute with distilled water to 15 mL. Sincethe iron II produced in the reduction is easily oxidized by air, treat each sample individually atthis point (if there is not enough time to finish all the samples, they may be safely left in thiscondition until the next period) and titrate the sample before reducing the next sample. Heatthe solution to boiling, remove from the heat, and add 0.5 M stannous chloride dropwise,

until the yellow ironIII

is completely reduced to light green ironII

. Most samples of ironII

areso dilute that they appear virtually colorless. Add exactly three drops of excess stannouschloride but no more. If a large excess of stannous chloride has been added, oxidize theexcess with a few drops of potassium permanganate solution. Heat and a high concentrationof chloride ion favor a rapid rate of reaction.

4. Cool the sample flask to room temperature. Mix 25 mL of water and 5 mL of 0.125 M mercuric chloride in a separate beaker and add the mixture all at once with rapid stirring tothe sample flask. This avoids reduction of the mercuric chloride to mercury metal. Allexcess Sn II is now oxidized to Sn IV.

5. Wait 3 minutes. If a white precipitate of mercurous chloride is observed, proceed with thetitration as directed below. If no precipitate is observed, an excess of stannous chloride wasnot added. Discard the sample. If the precipitate is grey or black, mercury metal is present.Discard the sample and begin again. Note: use the waste container specifically for thisexperiment to dispose of mercury- and chromium-containing solutions and not the sink.

C. Titration of Iron II

Carefully add a stir bar to the sample flask at this point to ensure rapid stirring during thetitration. Add 10 mL of 1:5 sulfuric acid:water, 5 mL of 85% phosphoric acid, and 8 drops ofsodium or barium diphenylamine sulfonate indicator to the flask containing iron II; titrate slowlywith potassium dichromate to the end point. As dichromate is reduced, it produces the blue-green chromium III ion; hence, the color change at the end point proceeds from a blue-green (or amilky-green) through grayish tinge to a purple end point. The titration should be conducteddropwise or with fractional drops when the grey tinge is noted. Because the oxidation of theindicator is somewhat slow, allow adequate time for the reaction to go to completion.

Calculation of Results

Calculate and report the percentage of iron by weight in the ore sample. Obtain at leastthree (preferably four) results on the analysis of the ore before proceeding with the followingstandardization.

Standardization

Since the titrant was prepared from a primary standard grade material, this step is notnecessary in principle. However, check the standardization of the dichromate solution byanalyzing a sample of pure iron wire. You will also use this opportunity to illustrate the principleof a redox titration, by titrating the standard potentiometrically.

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Chemistry 3000 Experiment 3, Page 4

1. Warm about 5 mL of concentrated hydrochloric acid in a 250 mL Erlenmeyer flask in thehood. This expels any oxygen from the flask. Obtain approximately 0.3 m of iron wire.Lightly sand the wire to remove any oxide (rust), and then carefully remove the dust. Thiscan be done by pulling the wire through a folded piece of #600 (fine) sandpaper, and thenthrough a clean cloth. Weigh exactly about 0.1 g of iron wire cut into 2 cm pieces and add it

to the flask. Allow to stand for about 10 minutes at room temperature in the hood. The ironwire dissolves as iron II and releases hydrogen gas. The hydrogen maintains a reducingatmosphere and prevents oxidation of iron II to iron III. Hence, there is no need to reduce theiron with stannous chloride. Heat the solution to boiling on a hot plate in a hood to completedissolution. The solution should be light green due to the presence of iron II. If the solution isyellow (due to iron III), carefully add one to two drops of stannous chloride to reduce anyiron III to iron II followed by addition to mercury II chloride to react with any excess stannousion. Quantitatively transfer the sample into a breaker so that the titration can be monitored

potentiometrically.

2. Work in groups of two for this part. Titrate the standard iron sample according to the

procedure above. Monitor the potential of the solution using a platinum electrode andsaturated calomel reference electrode (SCE). Set the pH meter to read millivolts. See Figure3-2 below and Demonstration 16-1 on page 332.

Figure 3-2 Apparatus for potentiometric titration of Fe +2 with Ce +4 (note that you will use Cr 2O 7-2 instead of Ce +4)

Titrate the entire iron wire sample and record the potential at 1 mL intervals. In addition,make note of any changes in the color of the indicator as the titration proceeds. Plot the titrationcurve data using a spreadsheet program such has Excel. Make sure the axes are labeled similarto Figure 3-1. Save the data and the plot for a problem set on redox titrations which will becoming shortly.