Analytical Methods Development Chris L. Basel Rockhurst College, Kansas City, MO 64110 Great emnhasis is currentlv heine-. olaced on the develonment of reasoning power in students. This emphasis is especiallv in analvtical chemistrv classes as manv in. anoarent .. structors expand upon the traditional curriculuml~"that covered only measurement methods, calculations, and sometimes interpretation of results. A more complete approach emphasizes all aspects of analytical chemistry from sampling to interpretation of results, often including methods development, better preparing the student to solve realworld prohlems. Analytical methods development is often too time-consuming to he done during a teaching lab, especially for freshmen and sophomores. As an alternative, test questions can he used to helo students develoo an understandine of how methods are developed without great expenditure-of time. Althoueh " manv students will not choose methods develooment as a career, these questions are still beneficial. Students freauentlv eain a s e a t e r understandine of the use of chemistry for anzysis wien they must formurate the methdeveloped od themselves rather than discuss in^- a oreviouslv . method. The following are questions I have used in the sophomorelevel introductory analytical chemistry course. Answers that show logical reasoning hut are probably not feasible are given partial credit. The goal is to support the reasoning process as well as an understanding of the chemistry involved. Additional questions and a discussion of the problems associated with administering these questions can he ohtained by writing to the author.
a
Queotions (1) A solutiou is known to contain hoth Pb2+ and Ba2+. Suggest a gravimetric method you could use to determine their concentrations given that hoth Ph2+ and Ba2+ form insoluble sulfates and chromates. Clearly outline your proce-
528
Journal of Chemical Education
dure. Show the reactions involved and.how to set UD the calculations. (2) . . Sueeest a method for the determination of lanthanum in water using a potentiometric titration. Be specific about what electrodes and titrant you will use. Show the reaction(~)involved and how youwill calculate the lanthanum concentration. (Note: The student has studied the fluorideselective electrode before taking the exam containing this question.) uu
Acceptable Solutions (1) Add excess sulfate (a solution of sodium sulfate might be used) to an aliquot of the sample to precipitate lead sulfate and barium sulfate. Filter, dry, and weigh the precipitate (mass = A). Take another aliquot, and add excess chromate to it to precipitate lead chromate and barium chromate.
Filter, dry, and weigh the precipitate (mass = B ) . Mass relationships can then he set up.
+
A = mass of PbSOl mass of Bas06 B = mass of PhCr04 +mass of BaCr01
Using appropriate gravimetrie factors, the two equations can he solved simultaneously for the moles of both Baz+and PbZt present in the aliquots taken,
' Chem. Eng. News 1983,61(37),37.
Neman, R. L. J. Coll. Sci. Teach. 1983, 13, 16. PIeva, M. A,; Settle, F. A. J. Chem. Educ. 1985, 62, A 8 5
where, X equals moles of Pb2+present and Y equals moles of Ba2+ present. The concentrations are then found hy dividing X and Y hy the aliquot volume (V).
(2) Add a buffer to the sample to prevent formation of fluoride complexes, to convert any HF to F-, and to make the ionic strength constant for all samples. Place a fluoride-selective electrode and a reference electrode (a saturated calomel will work) connected to a pH meter into an aliquot of the sample. While recording the potential, titrate the sample with s dilute solution of fluoride.
+
La3+(aq) 3F-(aq)
+
LaF3(s)
As the fluoride concentration in the solution increases the potential goes down
E =k
- 0.059 log F-]
When the equivalence point is reached a rapid decrease in potential will he seen. The volume of titrant required to reach the endpoint can he ohtained from the titration curve (potential versus titrant volume). The lanthanum concentration can then he calculated from the endpoint volume and the titrant concentration,
where n is the numher of moles.
Volume 64
Number 6
June 1967
529
