Recycling Analytical Samples Joyce A. Sweeney, Dennis W. Mahoney, and Derek A. Davenport Purdue University, West Lafayette, IN 47907
There are a number of analytical reagents which can be quantitatively determined in more than one way. Thus oxalic acid may he &,rated as an acid with sodium hydroxide and as a reducing agent with potassium permanganate in hot acidic solution. Iodic acid can be titrated as an acid and by sodium thiosulfate in the presence of acid and excess potassium iodide. Similarly ascorbic acid (Vitamin C) can he titrated as an acid and by potassium iodate in the presence of excess potassium iodide and acid (1): HC6H70et NaOH
- NaCeHvOs + Hz0
carried out. It is advisable to keep the amount of phenolwhthalein (or other) indicator to a minimum and to dissolve it in a relatively non-reducing solvent such as acetone. Since local lahoratory circumstances vary and the basic procedures have been run for several years a t Purdue and mimeographed copies of our laboratory instructions are available by writing to the authors. Each of the sequential procedures can of course be applied separately to different samples or aliquots. Indeed in the case of sulfamic acid this opens up the possibility of producing harium sulfate by homogeneous precipitation ( 4 ) .However, students learn much about stoichiometry. by recognizing . .that certain reagents can he added in non-measured excess and that the exploitation of one kind of quantitative functionality does not necessarily preclude subsequent exploitation of another. Furthermore, even if the initial sampling or weighing in in error, ratio comparisons enahles an internal check to he made of experimental precision. Various other m u l t i ~ l edeterminations sueeest themselves but have not yet been subjected to the ordeal of mass student trial. Obviously hydrochloric acid could he determined by titration with base and then as silver chloride. Nitric acid by titration with base, reduction with Devarda's allov and distillation of the ammonia into an aliquot of standard acid or a saturated solution of boric acid, followed bv back-titration. ~
Thanks to an observant if inattentive student we have found that the excess potassium iodide is unnecessary; the same end-point is obtained in its absence as in its presence! Apparently direct oxidation of ascorbic acid by iodic acid is rapid and further oxidation of dehydro-ascorbic acid, C~H606,is sufficiently slow. Ascorbic acid may also he titrated directly with potassium permanganate, hut in this case a small crystal of potassium iodide must be added to determine the end-point by excess permanganate since further oxidation of CBHFOG takes place fairly rapidly (2).Sulfamic acid can he titrated as an acid or be estimated gasometrically ( 3 )hy treatment with excess sodium nitrite in acid solution: HOS02NH2 + NaOH
-
NaOS02NH2 + Hz0
After the nitrogen gas has been collected and measured the resulting solution may be analyzed by gravimetric determination as barium sulfate: NaHSOn
+ RaCIP
--
BaSOi
+ NaCI + HCI
Thus, sulfamic acid can be estimated titrimetrically, gasometrically, and gravimetrically. All of these processes are familiar enough and in normal practice each would he carried out on separate weighed samples or on an aliquot. fil~wruer, double (and i n one case triple) determinations can readily be performed on the same sample. All that need be done is that after the acidibase titration is complete the resulting solutions be re-acidified with the appropriate acid and the suhsequent redox titrations then
742
Journal of Chemical Education
--
are more elusive but even a quadruple-play is not ruled out. Students could well be asked to come up with new systems and then set to demonstrating whether they can make them work. Literature Cited
la) rker8. n, r... H=Y
