netic importance of these three species in the oxidation of hydrazine in weakly basic media. Experimental Hydrszinium bisulfate may he used as the source of hydraeine and may be standardized with iodate to an iodine monochloride endpoint (4). Depending on the sophistication of the students and the time d o t t e d for the experiment this may be done by the st,ndent, or for him. Alternativelv the bisulfate mav he dried a t in the table.
Reagents Needed Approximate Molarity
Reagent
0.025 Standadized 0.055 in b 0.15 0.10 Standardized 0.02 Standard
-- --
0 5
13-:
NLHPRatio
-
-
To a suitable conical flask, add the following in order: 10.00 ml 0.025 M N1H,.H2S04; 25.00 ml 0.055 M KII; 10 ml water; 10.OOmlN 0.15 M Na.B,O, buffersolution. In our experience it has not been necessary to take precautions against the volatilization of Iaor the photo-sensitized oxidation of iodide, though it is perhaps instructive to mention these possibilities in class discussion. The reaction mixture is swirled and allowed to stand for about five minut,es: ten ml 0.5 M HCI are added to quench the reaotion and the'mixture is allowed to stand for about three minutes before hack-titrating the excess iodine with standardized thiosulfate to a starch endpoint. For s. blank determinrttion, prepare a mixture as above, adding no hydrasine and a convenient volume of the same 0.055 M XIa. Typical results are Is-/N1H4 = 1.99 0.01. It should he emphasized to the student a t this point that the data obtained here are not sufficient to confirm the stoiehiometry of the reaction, even if it is assumed that the sole iodine-containing product is iodide ion. For example, at pH 10, hoth of the following reactions are consistent with the data obtained and are thermodynamically possible.
-
-
*
-
+
+
-
+
+
NzH4 21s40HNz 61- 4Hp0 (2) and NzHl 2L- 40H- -,l/lN~O NHzOH 5/2Hs0 61- (3) Additional work is necessary to determine which among these and perhaps other stoichiometries obtain. Depending on the time allotted the experiment, it may he advisable to present this point as s. dilemma and then describe, demonstrate, or assign the followine ..section to illustrate the im~ortrtneeof a full stoichiometric detemmntim. iu :r < hemml prohlrnm. Pmtidarly for t h dv ~ n r r dstudrnt, we tl.ink ir I. rmportant ta vnrourage him to struggle w t h thcdilrnm,~u* thmgh i t rrre his own, w t h n minimum of professorial coaching.
+
+
+
+
+
OH-:N2H4 Ratio To determine the OH- to N,HI ratio, it is convenient to initiate the reaction with a measured excess of standardized Na$B,O, and hack-titrate the excess with standard dilute HCI to a potenpH 5. The endpoint is defined in the tiometric endpoint usual way by the maximum in slope of a plot of pH versus volume of titrant. At pH 5 the excess borate is neutralized and any hypoiadite and iodide are reconverted to iodine. A potentiometric endpoint is chosen hoth because of the difficulty of detecing a. colored indicator change in the presence of excess iodine and because of the relative shallowness of the vH-break in the borate neutralization. To a suitable reaction flask, add the following in order: 10.00 ml 0.025 M NZH,.H2S04; 10.W m l 0.055 M KIa; 20 ml water; 10.00ml- 0.15 MNa2B,0, buffer solution. Swirl vigorously and allow to stand for 5 min. Transfer the product mixture quantitatively to a suitable beaker and titrate
-
-
-
-
+
-
+
+
-
~
0 :i Standardized
HCI
with -0.10 M standardized HCI to a potentiometric endpoint, pH-5. For a blank determination, mix 10.00 ml 0.15 M Na*B,O, in 40 ml water and titrate. Typical results are OH-/N2H4 = 5.9.5 i- 0.5, which corresponds to the neutralization of the hydrazinium sulfate N2H,.H$04 20HN,H, H70 S 0 F (4) followed by reaction (2). We note here that although reactions (2) and (3) exhibit the same OH-/N&, the choice of a pH .5 endpoint serves as a quantitative demonstration of the ahsmce of hydroxylamine in the product mixture. At pH 5 hydroxylamine is present aNHIOH+: if stoichiometrv" (3) . . were obtained,. the .roto on at ion of hydroxylamine H + + NH20H NHaOH+ (5) would contribute to the apparent mole rat,io giving an OH-/ NIHI = 5.00. On the basis of these results one may conclude that, of the stoichiometries considered above, eqn. (2) is consistent with the data and eqn. (3) can at most contribute only of the order of five percent. A ~t,oiohiometrvbased onlv on the ratios of the reactants consumed is necessarily incomplete. In this case the assumed products, I- and excess b- in dilute aqueous acid and NZ are relatively stable; the assumption they are the products is not unreasonable but should he confirmed in a careful, complete study. We find it convenient here to paint this out to the class and ask for suggestions, how the form of the products containing I and N might be determined. A class that has had experience with the iodate-iodine monochloride determination of hydrazine will probably recommend extracting iodine qusntitat,ively from samples of products and reactant KIa with successive portions of CCl, or CHCls and then determining the remaining total iodide with an iodate titration ( 6 ) similar to the one mentioned ahove. The quantitative separation and determination of the gaseous produet is rather difficult without a. preparative vacuum line, largely because of the supersaturation of the gases in the product mixture. It seems unlikely that most analytical teaching labs are equipped with a sufficient number of vseuum lines to recommend that the entire class perform this determination. With the stoichiometric data gathered above, a qualitative demonstration of the absence of N20 in the dried product gas would he strong supporting evidence for stoichiomet,ry although strictly speaking the stoichiometry is established only when mass balance has been achieved. Two obvious and fairly straightforward ways of detecting the presence of NZO, if my, t,aken from a dried gas product sample are measuring the gas phase ir spectrum or chromatographing the gas using a moleculxr sieve column ( 6 ) ; the latter can also be used to confirm the identity of the principal product as N.. Once the gas composition is determined, the product ratios of N1O and N%to hydrszine can be determined with conventional vacuum techniques, trapping the condensible N.0, or with gas chromatography, calibrahing the detector response with known mixtures of N1O and Nz.
We believe that precise stoichiometric determinations are of such importance in the quantitative study of chemistry that at least one such experiment should be attempted during each student's career. The problem introduces a wide range of analytical procedures and emphasizes the importance and application of precise analytical techniques in research chemistry. Finally, the class results can be used with a realistic analysis of probable errors to discuss and show the significance, if any, of deviations from integral whole number stoichiometries. Literature Cited (1) CHILD,W., AND RAMETTE, R., J. &EM. EDUC.,44, 109 (1967). (2) CHIA,Y., U.S. AtomicEnergy Comm. UCRL.-8311, (1958). (3) . . BERTHOUD. . A,.. AND PORRRT. . D... Helu. Chim. Aeta... 17.. 32 (1934). (4) VOQEL,A. I., "Quantitative Inorganic Analysis," Longmans, Green and Co., Ltd., London, 1961, p. 380. (5) Reference (4) p. 375. (6) COOPER,J. N., Ph.D. Thesis, University of California, Berkeley, 1964, p. 60. Volume 46, Number 12, December 1969
/
873
