Potentiometric and Photometric Methods for Determining the Solubility of Lead Iodide Gary W. Rice College of William and Mary, Williamsburg, VA 23185 syringe down to approximately the l-mL mark. Cap, and reweigh the bottle. The actual delivery rate in mL/min can be calculated from the density of water at the temperature measured, the weight of water delivered. and the time interval. Set the recorder chart speed to 5 cmlmin, and allow the recorder to run for 5 min, marking the beginning and ending times. Measure the length of the [raring and determine the actual chart speed. fie-paroldon of rhe lead aodlde solut~onand ekctrode assembly. Transfer a 50.00-ml.al~quotof the saturated lead iod~desolut~onto a 250-mL plastic beaker;taking care not to get any solid precipitate from the bottom of the bottle into the pipet. Add a stir bar and 50 1/21, Ce4+ 2C1ICI; Ce3+ (2) mL of concentratedHCI using agraduated cylinder.Immerse the Pt indicator and calomel reference electrodes into the solution to the Accordingly, the overall titration reaction can be written as extent that they are not in contact witb the stir bar, and begin stirring the solution at a rate that produces a moderate whirlpool. Attach the Pt and calomel electrode leads to the and - terminals of the recorder, respectively. Set the recorder on 1V full scale with a We have used a potentiometric titration to determine the chart speed of 5 cm/min. two endpoints for a number of years in our instrumental Potentiometric titrationprocedure. Rinse the syringe and tubing analysis course with a simple, student-built semiautomatic titrator. The exoeriment has recentlv been s u ~ ~ l e m e n t e d with small portions of the Ce(W solution to be used for the titration..and then fill the swinee described.After insurine" .. as oreviouslv . with a photomekc titration to monitor the i o d i k produced that the syringe has delivered several drops from the tubing, wipe and consumed as the result of eqs 1and 2, respectively. Such the tip of the tubing, and insert it about 2-3 cm intn the lead iodide practical exercises have allowed the students to observe solution. Simultaneouslyturn on the syringe pump and the recordfirst-hand that ohvsical constants can be determined bv er, noting the starting point on the chart paper, and continue the very different analytical approaches and concepts. addition of titrant until-0.5 mL is left in the syringe. Determine the two inflection points on the titration curve by the method of bisection. The distance between the starting point and inflection points can be converted to minutes from the recorder chart speed. The volume of titrant used can then be determined from the delivery Equipment rate of the syringe. From these volumes and the concentration of the The electrochemical cell was constructed using a Pt wire indicatCe(IV)solution, the solubility of lead iodide can be calculated. ing electrode and a saturated calomel reference electrode. A caliPhotometric titrotionprocedure. Add 15-20 mL of the Ce titrant brated syringe pump (Sage Instruments, Model 341A) was used for to a 50-mL buret, and record the initial volume reading. Prepare the continuous addition of titrant to the cell. The potential was recordlead iodide solution as previously described, taking care to measure ed by attaching the electrode leads to a standard strip chart recorder the 50 mL of HCI as accurately as possible to obtain a total solution (Fisher Recordall Series 5000) with a l-V full-scale range. volume of 100 mL. Carefully transfer -5 mL of the solution with a Chanees in absorbance for the ohotometric titration were monipipet from the beaker into a cuvette, and adjust the 100 %Ton the tored ona Hausrh and Lomb ~pe&onir 20 set at a waeelengfh of spectrophotometer. Carefully pour the cuvette solution back into 435 nm. Titrant volumes were added with a rtnndard 50-ml. buret. the beaker, returning as much of the solution as possible without All titrations were stirred witb magnetic stirrers. splashing. Add 0.50 mL of titrant to the beaker, stir for 10-15 s, transfer -5 mL back to the cuvette,and measure the %T.Return the Reagents cuvette solution back to the beaker, and continue adding 0.5 mL A saturated solution of lead iodide was prepared as described by increments of titrant and measuring the %T after each addition Meites and Thomas (I)and washed free of other electrolytesused in until a total volume of 10 mL of titrant has been added. Convert the the preparation of the solution to insure that the ionic strength of %T readings to absorbance, and correct the absorbance values to the solution was due only to lead and iodide ions. A 0.100 M sulfatoaccount for dilution effects from the titrant (4). d o t of absorbance . . A. ceric acid solution was prepared and standardizedwith ferrous ethyvs. millilrtera of titrant should generate three linear segments from lenediammonium sulfate (3)and diluted to 0.0500 M with 2 M HCI which the intersects yield the two endpoint volumes. The srduhilrty immediately before lab far student use. of lead iodide can then be calculated and compared to the potentiometric results. Procedure T h e solubility of lead iodide in water can readily be determined by measuring the iodide concentration in a saturated solution of the salt (1).One such method utilizes a redox titration with c ~ ( I v ) ' in 6 M HCI (2). T h e mechanism through which the redox reaction occurs is rather complex but can be approximately described by a sequential two-step reaction,
+
+
-
+
+
~
~
.
-
..
- .. .
Calibration of the svrin~e and recorder. Althourha swinee . oumo gives nomlnal flow rates for a particular syringe wlume, exact flow rates must be established prior ru the titration. Fill a 10-mLdisposshle syrrnge and deliver) tube with deionized water of known temperature, making sure that no air bubbles are present in the syringe and tubing. Securely fasten the syringe in the pump, and set the pump to give a nominal delivery flow rate of 1.5 mL/min. Turn the pump on until 5-10 drops have been displaced from the delivery tubineof the svrinee. Record the weieht of a caooed. .. . 30-mL bottle to the niarest 0.601,; and place the hottle in such a position as to rereive the water to be delivered fnm the syringe. Turn on the pump and record the time required u, displace the water from the 430
Journal of Chemical Education
~
~
-
~~
~
~
Dlscusslon Typical potentiometric and photometric titration curves are shown in Fieures 1and 2. resoeetivelv. from one set of student data. A compilation of 28 k b m i t t e d reports yielded averaee ~otentiometricendooint volumes of 3.33 ( f 0.12) and 6:73 (f 0.17) mL, and'average photometric endpoint volumes of 3.29 ( f 0.15) and 6.61 ( f 0.25) mL. The two methods of endpoint determination were very comparable as indicated by the results and precision of the measurements. T h e slightl~lowervalues obtained by the photometric meth~
~~~
~~
Chorf
Length (cm)
Flgve 1. PomntlomehiCtitrationm e Dbtainsd imm a recorder tracing of ihe potential change from wlume ot titrani added as determined by ihe chart length. Points A and B mark the endpointsof me tltration.
od- are nrobablv due to losses of iodine from the method of sample transfer necessary to measure the transmittance. The averaee solubility for lead iodide as determined from the above eApoint volumes and a 0.0486 M Ce titrant solution was 0.746 g/L. This value agrees well with values compiled by Seidell and Liuke (0.762 and 0.756 @ 25 "C) (5). The lower experimental values can probably be attributed to the lead iodide solutions being equilibrated a t room temperature (21-23 OC). The solubility product constant can also be determined by calculating the activities of the lead and iodide ions from the molar concentrations present and nssumine no other ions are oresent in the solution. In this .- case the?& was found to b e i . 1 X lo-", which is well within given in and 1.4 X the ranee of two values (8.5 X CRC ~ i n d b o o k s(6). The onlv unusual equipment required for this experiment - is the sy&e pump; however, a potentiometric curve can easily be produced by monitoring the potential with a pH meter after incremental additions of titrant with a buret. A
I 00
2.0
4.0 6.0 8.0 Volume Ce (IV) (mL)
10.0
~ l p u r e2. Phoiornenlc tltratlon m e gsnareted from monkoring changes in iodlne concenirat;on amn svccsaslve addltlons of Ce (IV) Points Aand B marK me endpoints of me tltration
smaller buret (25 mL) could improve the precision in the volumes addedfor both methods of titration. The addition of HCI to the samoles should be performed in a ventilated hood, and titrated samples shouldbe neutralized and poured down a drain with ample amounts of water. Literature Clted 1. Mcites. L.:Thomas. H. C. Aduonced Analylicol Chemistry; Mffiraw-Hill: New York, 1958: p p 4 1 M 2 4 . 2. Will8rd.H. H.;Young,P. J. J.Am. Ckam.Soc. 1928.50,1368-1372. 3. Diehl. H.Quanfifofim Analysis; Oakland Street Science: Am*, IA, 1970: pp 235-238. 4. Goddu, R. F.: Hume, D. N. Anal. Chsm. 1954,26,174&1748. 5. ~ i ~ kW. e .F.; Seidell,A. Solubilities olhorgonic and Mefoi-Ownic Compounds, 4th ed.; American Chemical Society: Washingtan. DC, 1965; Vol. 2. p 1301. 6. CRCHondbook olchemistry and Physics, 62nd and 65th ids.; CRC: B- Ratan, FL.
Volume 67 Number 5 May 1990
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