Robert D. Braun Vassar College Poughkeepsie. New York 12601
I
The Simultaneous Potentiometric Titration of CU and Fe in Nonaqueous Media
There are several standard nonaqueous acid-base titrations ( 1 , Z ) which have been used as student experiments to dem-
onstrate the advantages of nonaqueous, acid-base titrimetry; however nonaqueous redox titrimetry has been neglected as a possible area of student experiments. The only often nsed nonaqueous redox titration is the determination of water in methanol hy Karl Fisher titrimetry (3).The endpoint of this particular titration is most often located using an amperometric technique. The purpose of this paper is to report a potentiometric, redox titration in N,N-dimethylformamide (DMF) which has been used in the chemical analysis course a t Vassar College. Hinton and Tomlinson (4) were first to report the potentiometric titration of several inorganic reducible species in DMF with titanium(II1) chloride. Copper(I1) and iron(II1) were among the species which they were able to titrate. The titration of copper(I1) was straightforward and resulted in a relatively large change in potential in the vicinity of the endpoint. The titration of iron(II1) gave a significantly smaller endpoint potential change necessitating the use of a modified second derivative method to locate the endpoint. In a later paper Braun and Stock (5)observed that the endpoint of the titration of iron(II1) with titaninm(II1) chloride could he determined by direct inspection of the titration curve. These workers also found it possible to simultaneously titrate copper(I1) and iron(II1) in DMF solutions of samples derived from various copper-iron alloys. In aqueous solutions a separation of the copper from the iron is normally required prior to titration (6).The first endpoint of the titration with titanium(II1) chloride was found to he the copper(I1)-copper(1) endpoint and the second endpoint was found to he the iron(111)-iron(I1)endpoint. During the course of the titration titanium(II1) was converted to titanium(1V). The student experiment reported here is the titration of mixtures of copper(I1) chloride and iron(II1) chloride with titanium(II1) chloride in DMF. Experimental Apparatus
Potential measurements were made with a Radiometer model PHM62 p H meter (The London Co.) in the millivolt mode. Any comparable p H meter is adequate for this experiment. A platinum wire indicator electrode was nsed with a silver/O.Ol M silver nitrate in DMF reference electrode. T h e 0.01 M silver nitrate reference solution was held in a glass tube with a fine porosity glass frit separating the reference solution from the titrand solution in the titration cell. A silver wire was held in place in the reference solution by a ruhher stopper inserted in the open end of the reference electrode tuhe and through which enough of the silver wire protruded to allow electrical connection to the p H meter. Each student prepared his own silver nitrate reference solution. The titration vessel was a 150.m1, tall-form beaker which was capped with a rubber stopper containing holes for a 50-ml buret tip, a nitrogen inlet line, a constricted nitrogen exhaust line, and the indicator and reference electrodes. A constricted nitrogen exhaust line was used to ensure a slight positive pressure of dry, inert gas over the titrand solution. The top of the buret was capped with a ruhher stopper which contained a single hole for one leg of a glass T-piece and which was inserted in the nitrogen line upstream of the titration vessel. A magnetic stirrer and Teflon-coated stirring bar were used to effect stirring in the titration vessel. A sketch of the titration
Figure 1. Thetitrationassembly.( A ) 50-ml buret. (4nitrogen line. IC) nitrogen exhaust tuba, (4platinum indicator electmde, (0Ag/AgN03 in DMF reference alenrade, (F) 150-mltall-form beaker. (G)magnetic stiner assembly is shown in Figure 1.A disposable 17 X 17 glove hag (Instruments for Research and Industry) was used when required. Chemicals
N,N-dimethylformamide was used as received from Eastman Kodak Company. Iron(II1) chloride hexahydrate (J.T. Baker Chemical Co.) and copper(I1) chloride dihydrate (Merck and Co., Inc.) were used as received and assumed to he pure. Titanium(II1) chloride was ohtained from K and K Laboratories. An individual unknown sample was prepared for each student by accurately weighing varying amounts of copper(I1) chloride and iron(II1) chloride into a test tuhe and numbered for idenwhich was subseanentlv . stoooered .. tification. The total weight of the unknown mixture varied between 0.4 and 0.7 g. Procedure
Accurately weigh the entire iron(II1)-copper(I1)unknown mixture into a 100-mlvolumetric flask. Add about 50 ml of DMF to thevolumetric flask to dissolve the mixture and then dilute to the mark with more DMF. Accurately weigh two, known, approximately 0.18 g copper(I1) chloride samples into separate, labeled, 150-mltall-farm beakers and set aside for later use as standards. Weigh a dry, 100-ml volumetric flask with its glass stopper and insert both in the glove bag containing the sealed titaniumiII1) chloride container and a spatula. After thrice flushing with dry ni. trogen, seal the glove hag, open the titanium(II1)chloride container, and using the spatula add about 1.6 g of the salt to the volumetric flask. Place the glass stopper on the volumetric flask and reseal the titanium(II1) chloride container. Open the glove hag, remove and Volume 53, Number 7, July 1976 / 46%
reweigh the volumetric flask to ensure the correct approximate mass of its contents. Coasiderahle caution should he exercised when working with titanium(II1) chloride in the solid state. The solid reacts vigorously with moist air to yield hydrogen chloride gas and consequently the titanium(II1)chloride container is opened only under an atmosphere of a dry, inert gas. Add about 25 ml of DMF to the volumetric flask containing the titanium(II1)chloride and swirl to cause dissolution. The dissolution process is exothermic and care should be used not to touch the bottom of the flask during this step. The DMF addition may be carried out either in the glove hag or in a hood. After the titanium(II1) salt is completely dissolved, dilute to about 100 ml with DMF. Insert the power cord from the pH meter in an appropriateac outlet and allow the pH meter to equilibrate while preparing the 0.01 M silver nitrate reference electrode solution (0.017 g of silver nitrate diluted with DMF to 10 ml). Place the silver nitrate solution in the reference electrode tube and insert the silver wire. Connect the reference electrode to theglasselectrodesocketof the pH meter and the platinum indicator electrode to the millivolt socket Fill the buret with titanium(II1) titrant solution, insert the rubber stopper containing the nitrogen line in the top of the buret, and start the nitrogen flow. Remove the air in the buret tip by allowing titrant to flaw into the vacant space. Add about 50 ml of DMF to one of the known copper(I1) samples, dissolve the solid, add a magnetic stirrer bar, and stopper with the cell cap-buret-electrodes-nitrogen lines assembly. Adjust the nitrogen regulator valve to give a slight positive pressure af nitrogen in the beaker. Turn on the magneticstirrer and potentiometrically titrate the copper(I1) sample with the titanium(II1)solution. After completion of the first titration, remove the 150-ml beaker, empty its contents, and dry the beaker with a paper towel. Using a pipet add 50 ml of the unknown solution to the beaker, replace the stirring bar, and reinsert the beaker in the titration assembly. Without the addition of mare titrant to the huret, titrate the first 50 ml of the unknown solution. The best results were obtained when the titration was continued until the titrand solution turned the dark violet color ofthe titrant solution. After this titratian, refill the buret, remove the beaker, and repeat the entire titratian procedure with the second known copper(I1)sample and the second 50 ml of the unknown salution. Plot all four titration curves and determine the six endpoint volumes (one from each "known" titration and two from each"unknownn titration). Typical titration curves are shown in Figure 2. The concentration of the titanium(II1)solution was calculated from the mass of the copper(I1)salt in eaeh known solution and from the volumeof
Figure 2. Typical Writ tibation clnves for !he titration with titaniumllll) chlwide of (A) a copper(ll~imn(lll)"unknown" mixture and (6)a copper(l1) chloride "known" sample.
464 / Journal of Chemical Education
titanium(II1)solution required to reach the titration endpoint. The percent ofcopper and iron in the original unknown salt was calculated mine the concentrationof the titanium(II1) . . solution and the volume required to reach eaeh endpoint in the unknown titrations, 0
Discussion
The entire experiment required about three hours for completion. Initially an attempt was made to have the students analvze mixtures of Copper and iron metal. That at.. tempt was soon ahandoned as a consequence ot'the lung times reauired to dissolve the metal samples in hydrochloric acid and hydrogen peroxide. The only student to try the experiment with an unknown sample from the metals re. .prepared . quired a full day for diisolutiot~. DMF solutions of titaniumrII1) chloride rapidly decrease in strength when exposed to air (5)as a resultof oxygen oxidation. The decrease in strength is considerably slowed when the solutions are stored under nitrogen. As a result both the titrand and titrant solutions were stored under a nitrogen atmosphere throughout each titration and the buret containing the titanium(II1) chloride solution was not opened to the air during " each set of one known and one unknown titration. Since it was necessary to refill the huret with air-exposed titrant after the first two titrations were comdeted. the calculations were done in such a manner that the titrant concentration calculated from the first known sample was used to find the copper and iron masses in the first 50 ml of the unknown solution, and the titrant concentration calculated from the second known sample was used to calculate the copper and iron masses in the second 50 ml of the unknown solution. A eood portion of the air oxidation of the titrant could be eliminated by designing an air-tight apparatus which connects the titanium(II1) chloride reagent reservoir to the buret. The reagent could he transferred to the buret through an appropriate glass tuhe which connects through a stopcock to a tuhe entering the bottom of the buret. I n t h i s way the reagent reservoir could be kept under a positive pressure of nitrogen and there would be no possibility of contact with air. In designing such an apparatus care should be taken to ensure that all connections are of glass or Teflon since Tygon tubing is readily dissolved by DMF. Ruhber tubing may be used; however it has the disadvantage of being opaque and of becoming brittle and cracking when in constant contact with DMF. For those laboratories which have the necessary equipment to allow all of the students to simultaneously oerform this exoeriment. the procedure could he simplified by enlarging t i e titani"rn(1f1) chloride reservoir to hold enough reagent for all of the students and by having the instructor prepare the reagent. It is difficult to keep the reagent near its prepared strength for more than one or two days and consequently this procedure should not be attempted if the groups of students using the reagent are in laboratory periods separated by more than one day. The 0.01 M silver nitrate reference solution may also be prepared ior rhe srudenb by the instnlrtor. If this is done, the solut~onshould he stored in a dark hortle out of the light since silver nitrate photodecomposes, to yield metallic silver 17). If the leads to the p H meter are reversed, the titration curves would rise as the titration proceeds rather than fall (Fip. 2). The reference electrode was connected to the glass electrode terminal of the p H meter since in this configuration, a positive potential applied to the indicator electrode results i i a positive meter reading. As may he seen in Figure 2 the change in potential near the copper endpoint was about four times as large as the corresponding potential change near the iron endpoint. This perhaps in part accounts for the students' results which showed an average relative error of +0.90 f 4.26% for the first endnoint and a lareer relative error of +2.83 f 11.1%for the second endpoint. The uncertainties are expressed as standard deviations for the 13students who completed the experiment. ~
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The unknown samples varied in copper content between 10 and 26% and in iron content between 6and 15%.The student results appear to he fairly good when the relatively small endpoint volumes and the students' lack of experience are considered. There were two major sources of student error in the exoeriment. A few students did not titrate sufficientlv. hevond . ;he second unknown endpoint to he able toaccurately locate it. This orohlem is ensilv eliminated by carrying out the titrations-until the titrand solution becomes the-dark violet color of the titrant solution. Some students did not completely dissolve the titanium(II1) chloride while preparing the titrant solution. This led to a relatively~. large increase in titrant strength between the first and second set of titrations. Since the titanium(II1) solution is opaque, this error can he easily
overlooked unless the students are cautioned to check the bottom of the volumetric flask for undissolved solid prior to starting the titrations. Literature Cited (1) Reilly. Charles N.. and Sawyei, DonaldT.,"Experiments for instrumental Mathads: McGrsw-HillBook Co., N.Y., 1961,pp. 26'29. (2) Guilbault. George G.. and Harpis. Larry G.. "Lnsfmmental Analysis Manual," Marcel Dekker,N.Y., 1910,pp. 208-207. (3) Stock, John T., "Ampmmotric Titration%," Inferscience Publishers. N.Y., 1965, pp. 627.664. ( I ) Hinton, James F., and Tomlin~on,Hazel M.. Awl. Chem., 33.1502 (1961). (5) Brsun,Roberf D.,andSfoek,John T.,Anol Chim. Aelo, 60,167 (1972). Fd.,D. Van (6) Furman. N. H m l l (Editor)"StandsrdMethodsofChemicalAnalysi~~5th Natrand, N.Y.,vol. 1,pp. 351-355. (7) Skng, Douglas A..and West, Donald M.."Fundamonfslr ofhalytieal Chemistry,"Holf, Rineharf and Winston. N.Y.. 1 9 6 3 , ~251. .
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