ACKNOWLEDGMENT
The authors thank the directors of the Mathematical Centre, Amsterdam, for placing the electroriic computer X1 a t their disposal. They also express their appreciation to J. Groot for the careful construction of the cell holders. LITERATURE CITED
(1) Arends, J. M., Cerfontain, H., Prinsen, A. J., to be published.
(2) Cerfontain, H., Rec. Trav. Chim. 80, 296 (1961). (3)TCerfontain, H., Duin, H. G. J., kollbracht, L., ANAL. CHEM.35, 1005 (1963). (4) Cerfontain, H., Kaandorp, A. W., Sixma, F. L. J., Rec. Trav. Chim. 82, 565 (1963). (5) Cerfontain, H., Sixma, F. L. J., Vollbracht, L., Ibid., 82, 659 (1963). (6) Herschberg, I. S., Sixma, F. L. J., Koninkl. Akad. Wetenschap. Proc. B 65,244, 256 (1962). ( 7 ) Mandel, J., Linnig, F. J., ANAL.CHEM. 29, 743 (1957).
(8) N. V. Electrologica, Willem Fene-
gastr. 31, The Hague, The Netherlands, description of X1 computer. (9) Sternberg, J. C., Stillo, H. S., Schwendeman, R. H., ANAL.CHEM.32. 84 (1960). (10) Streitwieser, A,, Jr., Fahey, R. C., J. Org. Chem. 27, 2352 (1962). (11) Vollbracht, L., Cerfontain, H., Sixma, F. L. J., Rec. Trav. Chim. 80, 11 (1961).
RECEIVED for review February 24, 1964. Accepted March 31, 1964.
A Reversible Internal Indicator for Sodium Nitrite Titrations ALFRED A. SCHILT and JERRY W. SUTHERLAND' Departmenf o f Chemistry, Northern lllinois Universify, DeKalb, 111.
b Dicyanobis( 1,lO-phenanthro1ine)iron (11) (ferrocyphen) reacts reversibly with nitrous acid to give the corresponding iron(l1l) complex (ferricyphen) and nitric oxide. Theoretical verification of its applicability 12s an internal, reversible indicator for diazotization titrations is presented. Ferrocyphen is a reliable indicator for titrating sulfarnates and azides, as well as certain aromatic amine derivatives, with sodium nitrite.
The present paper reports some experimental results that explain the unexpected reversible behavior of ferrocyphen in diazotization titrations. The results also demonstrate that ferrocyphen can be employed to good advantage in a number of other titrations that use sodium nitrite as the titrant. EXPERIMENTAL
Present address, Analytical Research Dept., Abbott Laboratories, North Chi-
Reagents. Dicyanobis(1,lO-phenanthroline)iron(II) dihydrate (ferrocyphen) was available from earlier studies; its preparation has already been described (8). The indicator solution was prepared by dissolving 1 gram of ferrocyphen in 100 ml. of concentrated sulfuric acid. A4nilinehydrochloride (technical grade) was purified by decolorization with charcoal (Norite), followed by threefold recrystallization from water. The purified product was dried a t 100" C. in vacuo over 1fg(c104)2. Sodium azide (technical grade) was purified by three successive recrystallizations from water and dried a t room temperature in vacuo over Mg(C104)*. The sulfanilamide (L.S.P. reference standard), sulfamic acid (G. F. Smith Chemical Co., reference grade) , hydroxylamine hydrochloride (Eastman Kodak Co., White Label Grade), and sulfanilic acid (J. T . Baker, analyzed reagent Grade) were used without preliminary treatment. Titrant solutions of reagent quality sodium nitrite in distilled mater were prepared in approximate 0.1M concentrations and standardized us. aniline hydrochloride or sulfanilamide. Nitric oxide was generated as needed by addition of 1 ml. of HNO, (32%) to 100 ml. of 1.5.11 FeS04 in dilute sulfuric acid; the nitric oxide was liberated by heating and bubbled through a solution of FeS04 (0.05M) in dilute &SO4 to remove any trace of
cago, Ill.
K-02.
S
can be employed as a titrant for the determination of sulfamates, azides, and aromatic amines, including various sulfa drugs (8). A serious deterrent to its more frequent use has been the lack of any simple, convenient end point detection method. Potentiometric methocls are extremely time-consuming because of sluggish electrode response (5:i and, in most cases, more than an hour is required to complete the titration. Moreover, the technique may not always be successful (3). Visual methods also have proved cumbersome. Starch iodide solution, the most commonly used indicator, behaves irreversibly and must be employed externally ( 2 ) . Recently it was found that dicyanobis(1,IO-phenanthroline)iron(II), which was given the trivial name ferrocyphen, serves as a reversible indicator in the diazotization titration of aromatic amines with sodium nitrite ( 7 ) . Banick and Valentine confirmed this result by successfully titrating sulfanilamide and a number of its derivatives using ferrocyphen indicator ( I ) . ODIUM NITRITE
Titration Procedures. Titrations of weighed samples of ferrocyphen (-50 mg.) in 6 M hydrochloric acid (200 ml.) with standard sodium nitrite were performed potentiometrically using a Beckman Model G p H meter equipped with platinum and saturated calomel electrodes. Dissolved oxygen was removed from the ferrocyphen solutions prior to titration by purging with pure nitrogen for 15 minutes. Atmospheric oxygen was excluded during titration by passing nitrogen over the titrate solution. Formal potent'ials for the titrate and titrant redox couples were deduced in the usual way from the E.M.F. data at the 50 and 2000/, titration points, respectively. The equivalence points in these same titrations also could be estimated by visual means, because any significant amount of unoxidized ferrocyphen could be readily detected by its orange-yellow color, even in the presence of relatively greater concentrations of the pale violet ferricyphen. Titrations of various other substances with sodium nitrite, either to standardize the titrant or to assay the purity of the t.itrate, involved the following procedure. An accurately weighed sample was dissolved in 50 ml. of 6 J 1 hydrochloric acid, 14 drops of ferrocyphen indicator solution were added, and the resultant solution was titrated with 0 . 1 X sodium nitrite to a pale violet end point' that persisted for at, least 3 minutes. A titration blank was measured and applied to the results to correct for the amount of titrant, consumed by t'he indicator. For titration of sodium azide a t 0" C., to minimize loss of hydrazoic acid by its volatilization, the procedure was modified. The indicator solution and the 6 M hydrochloric acid were combined and cooled to 0" C. in an ice bath prior to mixing with the weighed sample of NaN,; the titration was begun immediately after adding the cold solution to the sample. Ai reminder for caution is appropriate: VOL. 36,
NO. 9 , A U G U S T 1964
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Hydrozoic acid is poisonous, attacking the mucous membrane; also some of its heavy metal salts in the dry state may explode violently (10). RESULTS A N D DISCUSSION
Ferrocyphen and nitrous acid undergo the following reversible reaction: [Fe(phen)z(CN)~] .H+ (orange-yellow)
+ +
HOKO= [ F e ( ~ h e n ) z ( C N ) ~ ] + (violet)
Hz0
+ NO
(1)
The formula for ferrocyphen is written as a protonated species, in keeping with results of a n earlier study (9). The stoichiometry of reaction 1 was established by titration in an oxygenfree system. Pertinent data are given in Table I. Dissolved oxygen caused low results. The explanation is found in the following sequence of reactions: (2)
2h-0+02=2~02
KO2
+ 2 [Fe(phen)z(CX)Z].H+= 2 [Fe(phen)dCN)~l++ H20 + S O
(3)
Reaction 2 is well known and proceeds very favorably. A simple, direct test verified that reaction 3 is also rapid and complete. Thus it is evident that the oxidation of ferrocyphen to ferricyphen by oxygen (an energetically favor-
Table I.
Titration of Ferrocyphen in 6M HCI (Oxygen-Free) with 0.0885M NaNO2
Ferrocyphen, gram taken 0.0581 0.0553 0,0452 0.0736 b
able process) is induced or catalyzed by nitric oxide. The identities of the products in reactions 1, 2 , and 3, with the exception of nitric oxide, were readily determined from their characteristic absorption spectra in the visible region. The presence of NO was confirmed by the action of oxygen upon it to produce the brown-colored gas NOs. Confirmatory evidence for the validity of reaction 1 is the close agreement of the formal potentials in Table I, obtained from the potentiometric titrations, with those reported for ferrocyphen (the titrate) ('7) and nitrous acid (the titrant) (4). The equilibrium constant for reaction 1, calculated in the usual manner from the formal potentials, is approximately 1 X lo3. Accordingly, if HONO were to be chemically removed from the reaction system by the presence of some reactive substance, one would expect reaction 1 to proceed in reverse of the order written. Our experiments confirmed the predicted result. The violet ferricyphen is rapidly reduced to the yellow-orange ferrocyphen when nitric oxide is bubbled into acidified solutions containing either sulfamic acid, sodium azide, or aniline. Furthermore, reduction would not occur unless both nitric oxide and one of the three nitrous acid reactive substances were present. The reversible behavior of ferrocyphen as an indicator in diazotization titrations is based entirely upon reaction 1. Xeither aromatic amines nor their diazotized products show reactivity
End points, ml. of titrant Potent. Visual Theoryb 1.2 1.2 1.0 I .8
1.2 1.1 1.0 1.7
1.30 1.24 1.01 1.65
Formal potentials," volts
Titrate
Titrant
0.82 0.84 0.82 0.82
0.99 1.01 1.00 1.01
Vs. the standard hydrogen electrode. Assumes that titration reaction involves a molar ratio of 1 to 1 for the reactants.
Table II.
Titrations Utilizing Ferrocyphen as Indicator
STANDARDIZATION O F SODIUM NITRATE TITRANT Molarit of No. of ubstance titrated NaN& detn. Aniline hydrochloride 0 09673 5 Sulfanilamide 0 09670 5 TITRIMETRIC ASSAYS No. of detn. Substance titrated Purity, % Sulfamic acid 99.64 7 Sulfanilic acid 99.52 5 Hydroxylamine hydrochloride 97.6 5 Sodium azide 99.945 8 98,56b a h
Solutions titrated at 0" C. (conc. = O . O 2 M ) . Solutions titrated a t 25' C. (conc. =O.lM).
180 6
ANALYTICAL CHEMISTRY
9
Rel. std. dev., % 0 12 0.30
Rel. std. dev. % 0.33 0.45 1.4 0.15 0.81
with either oxidation states (I1 or 111) of the indicator. The unique features of reaction 1 are reversibility, speed, and the optimum value of its equilibrium constant. If K,, were too large, a momentary excess of nitrous acid prior to the equivalence point would give rise to a premature, permanent end point in those cases where the indicator reacts more rapidly and more completely with the titrant than does the titrate species. If K,, were too small, reaction 1 would not be sufficiently sensitive to indicate the presence of free nitrous acid. Clearly, some optimum value for K,, is necessary. The indicator reaction of ferrocyphen with nitrous acid will serve for end point detection in certain titrations only-those in which the titration reaction has a K,, significantly greater than that of the indicator reaction. Other metal complexes, closely related to ferrocyphen, were tested for possible utility as indicators in titrations involving sodium nitrite. They included [Fe(phen)31SOd, IFe(bipy)31SO~, [Rubipy) 3 ]SO,, [Os(bipy)3lSO4, [Ru(bipyb(CN)*], and (Os(bipy)z(CY),]. The formal potentials in various acid concentrations have been measured previously for all of these complexes (6, 7 ) . Yeither of the two ruthenium complexes could be oxidized on treatment with sodium nitrite, regardless of the acid concentration used. Because both complexes exhibit formal potentials that are greater than that for nitrous acid, this result is expected. The [Os(bipy)s]SOc complex proved to be readily oxidized by nitrous acid; however, addition of aniline or other substances that are reactive with nitrous acid failed to produce the original color of the osmium(I1) complex. In this case, also, the results are consistent with the low formal potentials of the osmium complex as a function of acid concentration (0.80 - 0.44 volt in 1 to 10M acid). Neither of the two iron(I1) complexes could be oxidized by nitrous acid unless the hydrochloric acid concentration was approximately 6-11 or greater. Rapid loss of color by dissociation of the complexes (especially when oxidized) occurs in such high acid concentrations; hence further consideration was not given to their possible utility as indicators. The last mentioned complex, [Os(bipy) (CN) ], appeared to behave reversibly in simulated diazotization titrations, provided that a hydrochloric acid concentration between 2 and 4 M was employed. Further study of this complex as an indicator, however, was considered impractical, since the color change on oxidation is not very distinctive (the golden yellow color in hydrochloric acid is converted to a pale yellow when excess sodium nitrite is added). These results, considered with those observed for ferrocyphen and its
2,2’-bipyridine analog iferrocypyr) , suggest that a formal potential between 0.8 and 0.9 volt is an important requisite for the type of indicator characterized by reaction 1. As an adjunct to elucidating the nature of the indicator response of ferrocyphen, it was pertinent to examine the behavior of the complex in reactions between nitrous acid and substances other than aromatic amines. Thus it was discovered that ferrocyphen will function as an internal indicator for the titrimetric determination of sulfamic acid, sodium azide, and hydroxylamine, as well as various aromatic amine derivatives. Titration data for these substances are compiled in Table IT. A number of other substances, known to be reactive with nitrous acid to various degrees, were also examined but could not be titreked successfully. These included urea, thiourea, phenol, n-butylamine, nitroethane, acetamide, benzenesulfonamide, diphenylamine, N ethylaniline, and hr,h’-dimethylaniline. The time required to complete a given titration varied somewhat with the
nature of the substance titrated. Sulfanilamide, sulfanilic acid, and sodium azide react reasonably fast with the titrant; their titrations could be completed within a period of 8 to 10 minutes. For the somewhat slower reacting aniline and sulfamic acid, titration times of 10 to 15 minutes were necessary. Titration of hydroxylamine proved very slow, requiring 40 to 50 minutes for completion. The time period could not be appreciably shortened by use of higher temperatures. It is pertinent to note that use of the backtitration technique is precluded by the instability and volatility of nitrous acid. Titration precision and accuracy are poorest for hydroxylamine, as evidenced by the results in Table 11. The cause no doubt stems from the very slow reaction rate. I n the case of sodium azide, precision and accuracy are low under conditions that favor loss of hydrazoic acid by volatilization-Le., when relatively high temperature and high concentrations are employed. Loss of H S 3 is of course aggravated by the
evolution of nitrogen (one of the reaction products) during the course of the titration. Loss of HY3 is not significant when cold. dilute solutions are titrated. LITERATURE CITED
(1) Banick, W. M., Jr., Valentine, J. R.,
Preprint, 1964 Metropolitan Region Meeting, Yew York, N. Y. (2) Kolthoff. I. M.. Belcher. R.. “T‘olumetric Analysis,”’ Vol. 111; pp’. 660-2, Interscience, Sew York, 1957. (3) Kolthoff, I. M., Furman, X. H., “Potentiometric Titrations,” 2nd ed., p. 409, Wiley, New York, 1931. (4) Latimer, W., “Oxidation Potentials,” 2nd ed., Prentice-Hall, New York, 1952. (5) Muller, E., Dachselt, E., 2. Elektrochem. 31,662 ( 1925). (6) Schilt, A. A,, ANAL.CHEM.35, 1599 (1963). (7) Schilt, A. A,, Anal. Chim. Acta 26, 134 (1962). (8) Schilt, A. A,, J . Am. Chem. SOC.82, 3000 (1960). (9) Ibid., p. 5779. (10) Sidgwick, N . V., “The Chemical Elements and Their Compounds,” pp. 713-18, Oxford Universitv Press. London, 1950. RECEIVED for review March 16, 1964. Accepted April 24, 1964. \
,
Tabulation for Deriving Procedural Rate Constants from Dynamic I hermograms J. A. MAGNUSON Silicone Products Department, General Electric Co., Waterford,
b A
scheme is presented for the tabulation of kinetic data from a dynamic thermogram. The numerous measurements, derivations, calculations, and appropriate cor rextions necessary to derive the standard graphical expression for log ,k vs. I / T are broken down into a step-by-step outline form. Errors are thus less likely to occur and computations are completed with less fatigue. The pyrolysis of a high niolecular weight dimethylsiloxane polymer is used as an example.
T
analysis records (thermograms) may be viewed and treated in many ways to extract useful conclusions. One treatment, employed to a great extent in this laboratory, is the calculation of a n Arrhenius plot from dynamic thermograms. On the assumption that t.he first 25% of weight loss for a particular substance fits tjhe equation for a first-order reaction, a typical and useful Arrhenius plot may be obtained. The plot, particularly a t high heating rates, probably will not match one derived from isothermal data ( 7 ) . For purposes of comparison HERMOGRAVIMETRIC
N. Y.
of the thermostability of related polymers, however, procedural (3, 4)kinetic data are as useful as data isothermally derived and are certainly more quickly obtained. A simple tabulation has been constructed to reduce fatigue and the chance of errors in the many operations involved in deriving kinetic data from dynamic thermograms. The details of measurement and calculation are broken into a step-by-step form. This is very helpful since these thermograms, being procedural in nature, complicate somewhat the usual straightforward kinetic calculations because an apparent weight gain and also the resultant apparent rate of weight gain for the inert sample crucible are included in the overall weight change data. Corrections, therefore, must be properly evaluated and applied. hlore complex attempts to derive kinetic parameters having some fundamental significance have been included in a recent review of thermogravimetric analyses by Coats and Redfern ( 2 ) . Work too recent for this review has been presented by Anderson ( 1 ) and by Friedman ( 5 ) .
EXPERIMENTAL
The thermogram in Figure 1 was recorded by a Chevenard recording thermobalance (6) TH-59, Model 1A, S o . 40. The sensitivity of an earlier model Chevenard thermobalance has been thoroughly examined (8). Estimates of weight change with the present thermobalance are accurate to within 1% when compared with a standard analytical balance. Temperature was measured by means of a Model 8690 Leeds & Northrup potentiometer and a platinum, platinum-rhodium thermocouple embedded in the alumina support rod directly beneath the sample crucible. Unless the thermocouple is inserted directly into the sample there must be some difference between the measured and true sample temperature ( 7 ) . This difference should be very small, however, at the slow 20’ C. per hour (more correctly, 20” to 22’ C. per hour) heating rate used. The sample was General Electric Co. SE-30, a dimethylsiloxane polymer, from which the low molecular weight cyclic siloxanes were previously removed by heating the sample in nitrogen at 275” C. for 4 hours. The sample weight was 218.8 mg. The sample container was a Coors high-form porcelain crucible No. 000 VOL. 36, NO. 9, A U G U S T 1964
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