Figure 5 B shows that the P P D . + spectrum was completely absent n-hen the same electrolysis was performed on a mixture of 1 X l O - 3 M P P D and 1 x 10-3LVM P D in the same buffer. The settings of the EPR spectrometer were identical for both runs. Even if a faint signal exists above the noise level in Figure 5 B, the P P D . + concentration admittedly has been greatly diminished. T h a t this is a chemical interaction and not a part of the electrode process is proved b y Figure 5 C where OPD was used as the interacting species. Here the electrolysis was carried out as in 5 A , on P P D only. Because P P D . + is relatively stable, a n E P R signal level corresponding to that of Figure 5 9 was obtained. Then the polarographic leads were removed and 4 drops of 1 x 10-3.11 OPD was added carefully t o the top of the electrolysis cell. After a short time the OPD diffused down the tube to the region of the cell in the microwave cavity and the PPD.+ signal diminished in magnitude as indicated. (Again the same spectrometer settings were em-
ployed so the signal heights are approximately representative of relative amounts of P P D . +.) The interaction between OPD and M P D cannot be shown by this technique because, of the three isomeric phenylenedianiines, only P P D gives an E P R signal on electrolysis in aqueous solutions (8). The data at present do not reveal if LIPD (or OPD) reacts chemically with the diimine or the radical ion of PPD (or both). Attempts t o distinguish between the two, by generating only P P D . + at very low applied potentials, were not successful. Only a very small E P R signal is obtained in this fashion and any diminution in intensity in the presence of M P D is not conclusive. Figures 2 and 3 both show that the i, for P P D is relatively uninfluenced b y the presence of OPD or M P D above about p H 8. I n this p H range PPD. + is quite unstable, undoubtedly because the diimine undergoes rapid deamination to form quinone-imine type products (IO) and no EPR spectrum is obtained from PPD in this pH range.
REFERENCES
( 1 ) Geske, D. H., Maki, A. H., J . Am. Chem. SOC.82, 2671 (1960). ( 2 ) Rfaki, A. H., Geske, D. H., Ibid., 83, 1852 (1961). ( 3 ) Maki, A. H., Geske, D. H., J . Chem. Phys. 3 3 , 8 2 5 (1960). ( 4 ) hlatsuda, H., Ayabe, T.,Z. Electrochew 59, 494 (1955). ( 5 ) Michaelis, L., Shubert, RI. P., Granick. S.. J . d i n . Chem. Soc. 61. 1981 119391.' ( 6 ) Olson, C.; ildams, R. S., Anal. Chirn. Acta 22, 582 (1960). ( 7 ) Parker, R . E., Adams, R. S . , AXAL. CHEJI.28, 828, (1956). 18) Piette. L. H.. Ludwig. P.. Adams. R. N., ANAL.CHEM.34, g16 (1962). ( 9 ) Piette, L. H., Ludwig, P., Adams, R. S . , J . Am. Chem. SOC.83, 3909 (1961). (10) Tong, L. K. J., Glesmann, M. C , J . Am. Chem. Soc. 78, 5827 (1956). (11) Vandenbelt, J. XI., Henrich. C., Vanden Berg, Y. G., AKAL.CHEW26, 726 (1954). ~
RECEIVEDfor review May 18, 1962. Accepted August 29, 1962. Work supported in part by the Atomic Energy Commission through contract AT( 11-1 )686 and in part by the General Research Fund of the University of Kansas.
Nonaqueous Potentiometric Titration of Acylamidines and Related Compounds BEVERLY H. BEGGS and R. DONALD SPENCER Mellon Institofe, Pitfsburgh 7 3, Pa.
b A simple method of analysis was sought to permit rough kinetic measurements in reactions involving acylamidines and other closely related classes of very weak acids and bases. Quantitative determination of these compounds, alone and in mixtures, was achieved by means of successive titrations with hydrochloric acid and with tetrabutylammonium hydroxide in nonaqueous systems. In certain solvents, solutions containing two or more of the compounds changed in titer as a result of interaction among the dissolved components, or in some cases, of a reaction in which the solvent participated. Preparation of analytically pure benzamidine is reported here for the first time.
I
of the reactions of acylamidines which lead to s-triazine formation (II), a n analytical procedure was needed for the determination of reaction products individually and in mixture. The reaction studies were primarily investigations into the thermal breakdom-n of acylaniidines in the molten state and in a variety of refluxing solvents. N A STUDY
1590
ANALYTICAL CHEMISTRY
Classes of compounds used or expected in such reactions are: acylamidines, thio acylamidines, amidines, and amidine salts of carboxylic acids, diamides or diacylamines, imido esters. amides, esters, and nitriles. Because compounds in the latter three categories cannot be titrated by any method studied, this investigation was limited to the six former groups. The compounds specifically studied were : N-benzoylbenzamidine (SBB) and its analog Y-thiobenzoylbenzamidine (XTBB) , dibenzamide, benzamidine, methyl Cellosolve benzimidate. and benzamidine benzoate. Structural formulas are given in Figure 1. I n the latter part of the original research (11), mixtures of acylamidines and amidines Tvere determined by nonaqueous potentiometric titration with standard hydrochloric acid in 1 to 1 ethylene glycol-isopropyl alcohol ( I S ) . However, in the presence of other weak or very weak bases such as the imido esters, this system was inadequate. in that differentiation was not possible. Titration of the individual bases with perchloric acid in glacial acetic acid was used to some extent (9). Although
the inflection points obtained were much stronger than with the HCl system, there was no differentiation in miutures among compounds of different basic strength, because of the leveling effect of acetic acid. Because S B B and benzamidine benzoate are amphoteric, the problem of differentiation was reduced. N-Thiobenzoylbenzamidine is too weakly basic to be titrated b y HCl, but it also is amphoteric with respect to the stronger perchloric-acetic acid system (Table I). Table 1.
Titratability of Some Related W e a k Acids and Bases
HC1 HC104 TBAH Dibenzamide @a 0 X N-Benzoylbenzamidine Xb x X LY-Thiobenzoylbenzamidine 0 X X Methyl Cellosolve benzimidate x x 0 Benzamidine X X 0 Benzamidine benzoate X X X 0. Will not titrate b X. Will titrate Q
N-Benzoylbenzamidine INBB) OH
SH
N -Thiobenzoylbenzamidine (NTBB)
SH
NH
0$--&-fa H
Dibenzamide
0
Benzamidine
0
e c c x H
YH2
,-"
7
1
-
XIethyl Cellosolve benzimidate
O
C
,SH , OCHSCH~OCH~
Benzamidine benzoate Figure 1.
Compounds studied
Sodium methoxide, with dimethyl iormamide as solvent, was tried for determining the acidic compounds (8). K i t h one exception, the compounds were too weakly acidic to be titrated b y this 111ethod . Recently, a substantial amount of m r k has been devoted to the use of tetraalkylammoniuni hydroxides as titrants for strong t o very weak acids and acid mixtures (2-5, 12, I S ) . Fritz and Tamamura (10) gave considerable attention to systematically studying differentiating titrations of acid mixtures in a nonaqueous system, utilizing differences in acidity as a basis for analysis. Cundiff and liarkunas ( 7 ) further demonstrated the versatility of tetrabutylammonium hydroxide. For most weak acids, the titrant used \\-as tetrabutylammonium hydroxide (TBAH) in benzene-methanol with pyridine as a solvent for the samples, Other titrants and solvents were also tried. Because S B B , the most weakly acidic compound studied, \vas also the major component in most of the reaction systems, the analytical procedure had to be tailored around it. Therefore, some solvents usable for a few of the more acidic compounds were unsuitable for general use, because no inflection was obtained with XBB. Although the concentration of all constituents in a reaction mixture could riot be determined b y a single titration, by a combination of both acid and base tit'rations. differentiation among all the coinpounds was possible (Table I). In this scherne perc,hloric acid in acetic acid is needed only to differentiate between S-thiobenzoylbenzamidine and dibenzamide.
Various combinations of these compounds, however, reacted with each other in solution so that the amounts introduced could not be accounted for b y titration of a mixture. This effect mas not studied in detail, except for one combination which dl be described below. One of the largest problems encountered was titrant stability. Upon standing, the T B A H developed a n increasing amount of a weaker base which caused a substantial error in the calculation of very weak acids. EXPERIMENTAL
Reagents. HYDROCHLORIC ACID. A 0 . 2 5 solution of HC1 was prepared by diluting concentrated H C l with a 1 to 1 mixture of ethylene glycol and isopropyl alcohol (18). The 1 to 1 solvent mixture was also used to dissolve the samples. Potassium acid phthalate is commonly recommended as a primary standard, but requires heating to dissolve. Tris(hydroxymethyl) - aminomethane (Fisher primary standard grade) has been found to be more satisfactory. PERCHLORIC ACID. ,4 0.1L17solution of perchloric acid was made in glacial acetic acid (9), which was also used as the sample solvent. Potassium acid phthalate can be used as a primary standard, but it is not recommended because of the precipitation of potassium perchlorate. 1,3-Diphenylguanidine is preferred, although a primary standard grade does not seem to be commercially arallable. Technical grade material n as purified by extraction from methylene chloride solution with hydrochloric acid and reprecipitation with excess ammonium hydroxide. The resulting material v a s recrj stallized
successively from ethanol and toluene to constant melting point. TETRARUTTLAYhlOSIUh% HYDROXIDE. I n most cases 0.05~47T B A H in 20 to 1 benzene-methanol was used. This was prepared b y diluting with benzene 8 1.ON methanol solution of the base procured from Southwestern Analytical Chemicals. TBAH, 0.05N in isopropyl alcohol, was used for a few titrations. This solution was prepared by the modified method described by Cundiff and Markunas (6). When not in use, the TBAH was stored a t 5" C. SOL\ENTS. C.P. commercial solvents were used without further purification, n i t h the exception of diethylene glycol dimethyl ether, methyl ethyl ketone, and methyl isobutyl ketone nhich were redistilled. The methyl isobutyl ketone was also passed through activated alumina. Preparation of Research Samples. BESZAVIDIKE.Free benzamidine was obtained b y gentlj shaking benzamidine hydrochloride monohydrate with eycess cold K O H in the presence of niethjlene chloride. T h e KC1 formed stayed mostly in t h e aqueous layer, or adhered to the sides of t h e separatory funnel. The combined solvent extractions were dried over crushed K O H pellets at 0" C., decanted, and evaporated under d r y nitrogen to give white crystals; m.p. 69.0" to 70.6" C. (corr.), assay 100.6%. Two sublimations (50' to 55' C./0.5-1 micron) of benzamidine prepared in this way yielded a product with a higher melting point, 70.8' to 71.6" C. (corr.), assay 100.6%. Calculated for CiHsXs: C, 69.97; H, 6.71; X, 23.32. Found: C, 70.04; H, 6.68; ?;, 23.64. The only other melting point listed in the literature for free benzamidine which is supported by analytical data is that of Pinner (16), ahose analyses, however, were at considerable variance a i t h calculated values. H e reported a melting point of 75' to 80" C. His product was probably contaminated b y small amounts of the carbonate. At a later time, a batch of benzamidine was prepared b y the usual procedure, and a product was formed which melted sharply (57.5' to 57.8" C. corr.) but considerably below the previously observed melting points. The possibility that the two forms are polymorphic is strongly supported b y the infrared absorption spectra of the Nujol mulls and supercooled melts of both forms. Whereas the melts showed identical absorption, a number of differences TI ere apparent in the spectra of the S u j o l mulls. All four spectra had in common r&tively st>rong absorption a t 3.0, 6.1, 6.4, 7.0, and 14.4 microns. The S u j o l mull of the loner melting form shoned a poor13 resolved band a t 2.9 microns, apparently not found in the higher melting form, and also absorbed relatively more strongly a t 6.1 microns. The higher melting form on the other hand, absorbed more sharply and strongly at 6.3, 6.7, 7.7, 8.45, 8.65, 10.0, 10.8, and 11.7 microns. Benzamidine is hygroscopic and on standing has a tendency to decompose VOL. 34, NO. 12, NOVEMBER 1962
1591
with the evolution of ammonia. Vials were therefore stored at about 5" C. over desiccant in a tightly closed polyethylene container, and were warmed t o room temperature before they were opened. BENZAMIDINE BENZOATE. Prepared by the addition of a concentrated solution of sodium benzoate to a saturated solution of benzamidine hydrochloride and recrystallized from 1 to 1 methanolether. Assay 100.8% with base and 100.7% with acid. DIBENZAMIDE. Prepared by the hydrolysis of KBB with aqueous HCI and recrystallized twice from benzene. The assay of 99.301, did not appreciably change over a period of 4 months. METHYLCELLOSOLVE BENZIMIDATE. Obtained from its hydrochloride (11) by neutralization with cold sodium bicarbonate and extraction with methylene chloride. Evaporation of the dried solution a t room temperature under reduced pressure left the free imido ester as a pale yallow liquid, assay 98.0%. An attempt t o purify further by distillation a t 75" C./0.2 mm. reduced the assay to 71.5y0 as a result of thermal decomposition.
Table II.
Differentiation via Titrations of Mixtures
Perchloric HCl" TB.4Ha acid"
Mixtures NBB 1 1 Methyl Cellosolve benzimidate 1 2 1 NBB 1 1 NTBB Benzami dine benzoate 1 Methyl Cellosolve benzimidate 1 Benzamidine 1 7 NBB 2 1 I I Benxamidine Methyl Cellosolve benzimidate 2 2 2 NBB Benzamidine 1 1 benzoate 1 Benzamidine 1 Dibenzamide 2 NBB 1 Dibenzamide 1 Dibenzamide Benzamidine 1 benzoate Benzamidine 1 benzoate 2 NBB Methyl Cellosolve 2 benzimidate 1 Bemamidine Benzamidine 2 1 benzoate 3 1 KRB 1 Dibenzamide a Inflection points in titration curves are denoted by the numbers shown on the table. Compounds numbered identically titrate together. In titrations containing more than one inflection, the numbers increase as acid or base strength decreases. - indicates compound does not titrate.
1592
ANALYTICAL CHEMISTRY
NBB. Prepared as previously described (11) and further purified b y dissolving in cold dilute HC1, precipitating with ice cold 5y0 sodium bicarbonate, and recrystallizing from 1 to 1 methylene chloridepetroleum ether, assay 100.8% with acid. NTBB. Prepared from thiobenzamide and benzonitrile by the method described by Peak (15). All of the research samples were stored at 5" C., although dibenzamide and benzamidine benzoate should be perfectly stable a t room temperature. Apparatus. All titrations reported were r u n on the Metrohm Potentiograph E336 as supplied b y Brinkniann Instruments Inc., Great S e c k , N. Y. This is a n automatic titrating instrument which plots the whole course of the titration as a function of the titrant added. T h e addition of the reagent from the synchronized motorburet is continuous and uniform. The speed of delivery can be varied. T h e 5-ml. piston buret was used. The electrodes used were the Beckman general purpose glass electrode, No. 41252, and the Beckman sleeve-type calomel electrode] KO.40250, modified by replacing the aqueous potassium chloride solution by a saturated solution of potassium chloride in methanol. A magnetic stirrer was used for mixing. Procedure. An illuqtration of a typical titration is a < follows: SAhfPLE. The sample was weighed directly into a 30-ml. beaker by means of a semimicro balance with an accuracy of 1 0 . 0 2 mg. I n cases where synthetic solutions had been prepared, aliquots were taken using a syringe as a weighing buret. The sample size was adjusted to utilize 2 to 5 ml. of titrant, depending upon whether a single compound or a mixture was being analyzed. TITRATION. The instrument was set with 0 at the extreme right of the scale b p means of the zero potentiometer. After the piston buret was filled with titrant and the electrodes were filled and adjusted] a 10-ml. portion of solvent was pipetted into the sample beaker. I n titrations where pyridine or dimethylformamide was used as the solvent, the solution was protected from atmospheric CO, by a fitted rubber dam and nitrogen blanket. A suitable millivolt range and speed (usually 0.5 ml. per minute) of titrant delivery were set, and from this point the titration proceeded automatically. BLANK DETERMINATIO?;. Determination of the solvent blank by direct titration of a n aliquot was not used, because results were not reproducible within reasonable limits and were not felt to be true values. The use of regression analysis as illustrated by Bauer ( I ) was considered to be a superior method. Of the two solvents used routinely, no blank was found for 1 to 1 ethylene glycol-isopropyl alcohol. Blanks were found to vary from batch to batch of reagent grade pyridine, from 0 to 0.12 ml. of 0.05N base for a 10-ml. sample, but did not change nithin a given batch for periods up to a month. However.
care was taken to prevent undue exposure to atmospheric COz. CALCULATIOSS.End points were determined from the plot of potential us. volume of titrant. When two or more inflections were present, the difference in successive end points was used to calculate the volume of titrant for the compound represented, and solvent blank was subtracted from the final segment. The decision to subtract the solvent blank in this manner mas arbitrary and was based on the observation that this system gave the most reasonable titers for known samples. I n perchloric acid titrations, the titrant volume was corrected to a n arbitrary standard temperature by a factor of 0.11% per " C., to allow for the coefficient of expansion of acetic acid (17 ) . RESULTS AND DISCUSSION
Table I gives a n example in each of the categories of compounds studied and indicates how they can be titrated. Table I1 charts the mixtures which have been analyzed, the number of inflections present, and the order in which the compounds titrate. By using successive acid and base titrations, the concentration of any of the constituents in a reaction mixture can be calculated. Figure 2 is a graphic representation of the relative mid-point potentials expected with each method. Generally, two compounds in a mixture having a difference of 100 mv. in mid-point potential will give two inflection points. Titration of Bases and Base Mixtures. Hydrochloric acid, 0.2N in 1 to 1 ethylene glycol-isopropyl alcohol, was used almost exclusively for base mixtures with very good results. Although perchloric acid in glacial acetic acid (9) gives much stronger inflections for individual components, the leveling effect of acetic acid prevents resolution of mixtures. Titration of Acids and Acid Mixtures. According t o previous methods of classification, (3, 6) acids similar in strength to the unsubstituted monocarboxylic acids are designated weak acids, and those similar in strength to phenol are designated as being very weak. Benzoic acid and phenol have been added in the table of midpoint potentials (Figure 2) for comparison. All the acids studied are weaker than benzoic, and S B B is sufficiently weaker than phenol to give a separate inflection when a mixture is titrated. The use of 0.05N TBAH in benzenemethanol, with the samples dissolved in pyridine, was the most satisfactory titrant system. While methyl isobutyl ketone would be amore desirable solvent, owing to the preclusion of the need for a nitrogen blanket and the more agreeable odor, it gave no inflection for NBB. The only other solvent tried was methyl ethyl ketone. The resulting inflections
were smaller and less well defined than in pyridine or methyl isobutyl ketone, and it also gave no inflection for XBB. TBAH in isopropyl alcohol was tried in several cases. Although the curves obtained were weaker than those in benzenemethanol and in some cases gave a strange upward tailing effect after the endpoint had been reached, they nere u-able. This particular tit,rant, holyever, decomposed quite rapidly on st,aiiding. Precision. Table 111 indicates the precision t h a t can be expected in titration of individual compounds. This talde rrpresents samples run in duplicate or triplicate with t h e automatic titrator. As many of t h e compounds decompose slorvly upon standing, :incl assays were run over a period of several mont’hs, absolute values are not given. For the sake of consistency, titrations run with a conventional pH meter and plotted b y hand, and those not run in duplicate were not included. In calculating precision, the only samples deleted Tvere those in which the deviation !\-as more than four times the average deviation excluding the sample in question. Of the 79 samples assayed in the precision st’udy, seven were deleted. Benzoic acid is listed for comparison. Solvent Effect. T o simulate actual reaction mistures and t’o simplify sampling procedures, some synthetic solutions of various compound combinations were made in several solvents, and weighed aliquots were titrated IT-ith both acid and base. I n a few other cases, solvents were added to previously weighed samples. I n all instances, 10 ml. of either pyridine or glycol-isopropyl alcohol, depending upon t h e system, was also added immediately before t’itration. Some solvents markedly reduced the size and sharpness of the inflection point when titrated with TBAH. Among the solvents used, this effect was greatest with ethanol. Curves lvere not noticeably affected in the HCl system. In certain solvents, solutions containing two or more of these compounds changed in titer as a result of interaction among compounds, or in certain cases,
Table 111.
(strong base)
--+ Benzamidine -THAM
---t
100 mv.
__
samples A-Benzoylbenzamidine Methyl Cellosolve benzimidate
, . .
Benzamidine benzoate Dibenzamide Benzamidine Benzoic acid
3 4 ... 28
A--ThiobenzoyIbenzamidine
13 4
1-1-
100 mv.
Benzamidine benzoate
c
+
i
c
--c Pyridine -
Benzainidine benzoate c NTBB c Dibenzamide t
-
-
Methyl Cellosolve
SBB benzimidate
(strong acid)
Benzoir acid
-1 I
Titration with 0.05A’ TBAH Figure 2.
Relative midpoint potentials
Glass and modified calomel electrodes used
of a reaction in which the solvent participated. The longer the compound remained in contact with the solvent, the greater the error. This effect was noted with ethanol and diglyme and to a lesser estent with isopropyl alcohol. The following represents one such case. When dibenzamide was titrated alone in the presence of ethanol, it titrated normally and recovery was complete, both immediately and after 4 hours’ standing. However, when a mixture of dibenzamide and benzaniidine was allowed to stand for 4 and 22 hours in the presence of ethanol before titrating, the titer became progressirely lower. After 22 hours, only 5y0 of the original material could be accounted for. When this 17-as noted, a similar misture mas prepared and titrated without delay. Although the result was slightly over 1 0 0 ~ oit, was more in line with the espected recovery. Evidence that ethanol was responsible for the lon-ering in titer Ivas provided by the fact that there was no drop when titrating the same mixture after 4 hours’ standing, when ethanol was not included. Tetrabutylammonium Hydroxide Purity. T h e presence of basic impurities in some T B A H solutions has been recognized for several years.
Titration Precision
TBAH No. of
+ I
(weak acid)
HC1
Av . precision, To 0.30 ... 0.19
T o . of samples
Av. precision, yG
4 6
0.11
0.07
... 6 ...
0.26
...
0.25
...
4
0.22 ... 0.44 ... 0.37
...
Recent reference5 ( 6 , 24) d i s c u s how two of these. tributylamine and tetrabutylammonium carbonate, may be recognized, and how titrant, free from these impurities, may be prepared and kept pure. I n the present study, the posbible presence of tributylamine iyas of no great concern, because this compound is not a strong enough base to titrate any of the acids under study (Figure 2 ) . including the primary standard, benzoic acid. Hence, tributylamine arts as a neutral diluent. The carbonate, on the other hand, was found to titrate benzoic acid, but not S B B . Thus, vhen the TRAH titrant, containing small amounts of the carbonate and standardized with benzoic acid, !vas used with the acid. of appro.;imately the same strength a. benzoic acid, no discrepancies were noticed. However, the very weak acid S B B showed a high assay under these conditions, because it was titrated only b y the hydroside portion of the titrant. To utilize some of the early data run with titrant which was not cttrbonatefree, it \vas necessary to employ tP;o normalities in making the calculations. The normality derived by using benzoic acid as a primary standard was used for all calculations escept the very weak acid XBB. These were calculated with a normality obtained by using pure S B B as a primary standard. LITERATURE CITED
(1) Bauer, E. L., “A Statistical Manual for Chemists,” p. 85, Academic Press,
Inc., New York and London, 1960.
( 2 ) Bruss, D. B., Wyld, G. E. A., ANAL. CHEM.29, 232 (1957). (3) Cundiff, R. H., Markunas, P. C., Ibzd., 28,792 (1956). 1 4 ) Zbid.. 30. 1447 (19%). ( 5 ) Ibid.: p.’1450. ‘ (6) Ibid., 34, 584 (1962). VOL. 34, NO. 12, NOVEMBER 1962
1593
(7) Cundiff, R. H., Markunas, P. C., Anal. Chirn. ilcta 20,506 (1959). (81 Fritz. J. S..“Acid-Base Titrations in r\Tonaqueous Solvents,” p. 28, G. Frederick Smith Chemical Co., Columbus, Ohio, 1952. (9) Fritz, J. S.,ANAL. CHEx 22, 1028 (1950). (10) Fritz, J. S., Yamamura, S. S., Zbid., 29, 1079 (1957). (11) Gormley, W. T., Spencer, R. D., Tech. Rept. No. 5 , Section VI, Office
of Naval Research, Contract Sonr2693(00), Task No. SR356-407. (12) . , Harlow. G. A.. Brues. D. B.. A N A L . CHEM. 30,‘1833 (i958:. ’ (13) Harlow, G. A., Noble, C. li., Wyld, G. E. A., Zbid., 28,787 (1956). (14) kfarple, L. W., Fritz, J. S.,Ibid., 34, 796 (1962). (15) Peak, D. A., J . C h m . SO?., 1952, 215. (16) Pinner, A., “Die Imidoather und ihre Derivate,” p. 154, Robert Oppen-
heim (Gustav Schmidt), Berlin, Ger many, 1S92. 117) Seaman. IT., Allen, E., ANAL.CHEM. 23, 592, (1951). (18) S.J “Quantitative Organic Analysis via Functional Groups,” p. 104, M-ilel-, Yew York, 1954. RECEIVEDfor review May 14, 1962. rlccepted September 11, 1962. Presented at the Pit,tvburgh Conference on Analytical Chemistry and -4pplied Spectroscopy, March 5-9, 1962.
Instrumentation for the Automatic Simultaneous Measurement of m, t, w, and Drop Count of a Dropping Electrode HELEN
P.
RAAEN and H. C. JONES
Analytical Chemistry Division, Oak Ridge National laboratory, Oak Ridge, Tenn.
b Instrumentation i s described that will measure automatically and simultaneously the m, t, w, and drop count (c) of a dropping electrode at any h within the geometric limitations of the standtube-dropping electrode assembly rather than measuring only m at a single fixed h or recording only electrocapillary curves. The significant features are a precision-bore uniform-diameter standtube, a photoelectric mercury-level detecting unit t,iat can be positioned anywhere along the standtube, and the complete separation of the circuitry of the measuring system from the circuitry of the polarographic system. The accuracy and precision of the measurements are shown to be entirely satisfactory for polarographic work.
(Figure 1) are discussed. It consists of a precision-bore constant-diameter standtube, a device for measuring the time of f l o of ~ mercury, ti, from a known segment of the standtube a t any level of mercury, and a drop-counting device. The circuitry of the measuring system has no part in common with the polarographic system and no shock hazard exists. Measurement of t r is the one critical measurement made in the calibration and use of the apparatus. The accuracy and precision of this nieasurement are entirely satisfactory for polaroymphic work. DESIGN
Principle. T h e design of the instrunientation is based on t h e relationships m = k/tt
T
o evaluate dropping-electrode capillaries or to apply the IlkoviE rquation, it is necessary to measure the characteristics of the dropping electrode. The automatic recording of electrocapillary curves was accomplished by k i h a (9). The measurement of t was made automatically and accurately by Corbusier and Gierst ( 1 ) and by Gierst, Bermane, and Corbusier ( 3 ) . Both Lingane ( 5 ) and T s q i (10) have described apparatus for the automatic deteriinnation of only the flon- of mercury, m, a t a single filed height of mercury, I t . The instrunientation described herein has the advantage of meaAuring autoinntically and simul, drop taneously in addition to ~ n the time, f, the drop mass, to, and the drop count, c, a t any level of mercury in the standtube rather than a t a single fixed level. The design, calibration, evaluation, and use of this instrumentation 1594
ANALYTICAL CHEMISTRY
t = t,/c
w
=
k/c
where
m
flow of mercury from the dropping electrode, mg. per second k = standtube constant = niass of niercury contained in the known segment of standtube. mg. t t = time of f l o ~ of mercury from the known segment of standtube, seconds t = drop time, seconds c = drop count = number of drops that form and fall during tt w = drop mass, mg. Lingane’s (5) device for the automatic measurement of m is based on the first relationship. Standtube. The standtube is shown as a part of Figure 1. I t is constructed from precision-bore uniform-diameter =
tubing of 0.07Stj =k 0.0002-inch i.d. and 10-mni. 0x1. The tubing is available on special order from Fischer & Porter Co., Warminster, Pa. The use of precision-bore uniform-diameter tubing makes unnecmsary the determination of the k value at more than one position of the mercury-level detecting unit. To eliniiiiate the cushioning effects and air pockets that are sometimes associated ivit’li flexible tubing connections, the standtube is connected to the mercury reservoir via a Teflon-plugged stopcock and to the dropping electrode via both a Teflon-plugged stopcock and a standard-taper joint. A platinum wire contact is sealed into the standtube to permit grounding of the dropping electrode. The condition of the surface of the inner x i 1 1 of the glass standtube is a significant factor in the precision of the measurement of tt. If the wall of the standtube is wet to any extent by mercury, the mercury tends to cling to the wall and to form an irregular surface rather than a smooth meniscus. hft,er the standtube rvas fabricated, it was rinsed thoroughly with concentrated HSOs, clistilled water, and ethyl alcohol in that order and was dried in air. It Iyas cleaned subsequently in the same way. This treatment leaves t’he glass surface in such condition that it is not wet significantly by mercury. The treatment of the standtube a t any tinie with any cleaning agent that might attack glass, especially a caustic solution, m s avoided. Timing Device. The timing device f Figure 2 ) consists of a mercury-level detecting unit (Figure 3), a timer, and associated circuhy. The mercur?--level detecting unit contains tn-o light source-photocell sets, ~nounted in an aluminum block, that {Till detect the passage of the mercury surface past two points, fixed with respect to each other, on the standtube.
