642
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 15, No. 10
opening of the tapered tip, thereupon the StoPcock ofthe reservoir is closed. The lower trap then cuts off suction of gas through the right-hand tube. If the system were erfectly gas-tight at constant temperature, the pressure woulBremain constant and the pressure regulator would be unaffected. If air does leak in, mercury rises in the manometer and the two other tubes to de ress the level in the lower trap, so that the higher pressure 0% the system causes gas to rise through the right-hand tube, carrying the mercury with it. The mercury returns to the lower trap by way of the left-hand tube, Gas is withdrawn by the evacuator until the pressure and mercury drop and the level is restored in the lower trap to its original position, at which further evacuation is prevented by the seal.
ing atmospheric. By using vertical tubes of smaller bore, the regulator can be made more sensitive. The manometer can be read to approximately 0.1 mm.9 SO that in distillations the Corresponding temperature should be read simultaneously with the pressure, when the pressure drifts to the desired value. Since vapor-fiquid equilibria are insensitive to changes in as large rts 10 mm., the control of the regulator is sufficiently precise for this work as well as for most vacuum distillations.
The regulator maintains constant pressure within about 2 mm. a t lower pressures and within about 5 mm. a t pressures approach-
(1) Gilmont, R,, and Othmer, D, F.,to be published, (2) O t h e r , D. F.,IND.ENQ.CHEM.,35, 614 (1943).
Literature Cited
Apparatus for Photoelectric Titrations Application to Dark-Colored Resins ROBERT H. OSBORN, JOHN H. ELLIOTT, AND ARTHUR F. MARTIN Hercules Experiment Station, Hercules Powder Company, Wilmington, Del.
A photoelectric instrument, generally applicable to acid-base and oxidation-reduction titrations employing colored or fluorescent indicators, is described. It is especially useful for titrations which cannot be carried out satisfactorily by the usual visual or electrometric means. The application of this instrument to the determination of acid and
T
H E first objective measurement of the color change of an indicator as a means of determining the end point of a titration was made in 1926 by Field and Baas-Becking (7), who applied it to the starch-iodine reaction. These workers used a radiomicrometer with a receiver consisting of a silver-bismuth thermocouple. In 1928, Miiller and Partridge (33) described a photoelectric apparatus for automatic titrations employing a single vacuum phototube, an amplifier, a relay, and an electromagnetic buret release. This equipment wrts used for acidimetry and alkalimetry, permanganate, dichromate, and iodometric titrations. Furthermore, it was found that certain precipitation reactions, such as the determination of chloride ion by silver ion in the presence of chromate ion, could be followed. The precision in every case was greater than that of visual estimation. During the past decade, several papers have been published by Hirano (f3-24), Somiya (44-47), Kasai (96), and their coworkers in Japan describing the application of photoelectric titration to various quantitative procedures. During the same period, Ringbom and Sundman (40) and Miiller (99) in Germany, del Campo and co-workers (3, 4) and Gonzales (9) in Spain, and Lur’e and Tal (88)in Russia have made contributions to the subject. In this country, Alyea (f), Boyer ( d ) , Goodhue (IO), Hickman and Sanford (fz), Miiller and his co-workers (30-34, 97-39), Rowland (41), Russell and Latham (4,9), and Styer (@) have described improvements in photoelectric titration technique. In the authors’ laboratories electrometric titrations have been used in the past where visual end points with colored indicators are difficult or impossible to obtain. However, in certain crtses, electrode equilibrium is slow and titrations are time-consuming. For example, in applicatioris involving two-phase systems, or in those carried out in certain organic solvent media, it is sometimes
saponification numbers of dark-colored resins is discussed in detail. A precision of *l per cent is easily obtained. Preliminary work has shown that precipitation reactions and reaction rates can be studied, and, in addition, the apparatus may be used as a chemical colorimeter.
necessary to wait for 10 to 20 minutes after each addition of titrant before equilibrium is established. Furthermore, in these cases, the electrometric end point is often not very sharp, whereas indicator color changes sometimes remain sharp, although they are often obscured by background color or turbidity.
MICRO BURET
I
d FILTERS
LENS \
LAMP
PHOTOCELLS
+w SOLUTION-/-’ TITRATED
GLASS PLATE
(4
PHOTOCELL NO.1
PHOTOCELL N0.2
GA
CIRCUIT
(b) DIAGRAM OF PHOTOELECTRIC TITRATION FIGURE 1. SCHEMATIC APPARATUS
ANALYTICAL EDITION
October 15, 1943
643
= current through resistance RI i n = current through resistance R( El = potential across resistance Rl Ea = potential across resistance Rp kl, kf,and k = constants depending on the characteristics of the hotocells, the spectral transmissions of the coloref glass filters, and the fractions of light transmitted and reflected by the 45' glass plate ,it
Since, for low light intensity and small external resistance, the current flowing in a barrier layer photocell circuit is known to be directly proportional to the intensity of illumination, = kJT1 and El = kllTIRl za = kJT2 and E2 = kJTnRp
A t balance where the galvanometer reads zero,
p,: W
El= E, or klITIRl = k21T2R2
from which
d
At the beginning of a titration, resistor R1 is set a t an arbitrary value, and R1 is varied until balance is obtained. Then R1 is kept fixed, and Rz is used for balancing after each addition of titrant. From E uation 1, we see that the ratio R*/RI,at balance, ispro20 30 10 portiona? to the ratio of the transmission of the solution for light corresponding in color to the color of the filter in front of photocell 1 to the transmission of the solution for light corresponding I in color to the color of the filter in front of photocell 2. The filters I are generally selected so that one transmits light in a region of $0 the spectrum where the transmission of the solution with indicator does not change appreciably upon addition of titrant, while FIGURE2. SPECTROPHOTOMETRIC CURVESOF BROMOPHENOLthe other filter transmits in that region of the spectrum where GLASS FILTERS BLUEAND SELECTED the transmission of the solution with indicator changes most ra idly upon addition of titrant. a. Bromophenol blue (acid form) is immaterial whether the filters are interchanged in the two b. Bromo henol blue (basio form) e. Blue fiier light paths. This is true because a ratio of the response of the d. Amber filter photocell having maximum sensitivity a t the absorption maximum of the solution to the response of the photocell having maximum sensitivity in the region where no absorption occurs is obtained at each reading. If the two filters are reversed, a reciprocal of In view of these facts, the authors decided to try the photothis ratio is obtained. This makes no difference for comparative electric method on those titrations which could not be carried measurements, out satisfactorily by visual or electrometric means. I n Nome cases indicators go through their end points with a decrease in transmission in one portion of the spectrum and an increase in another. I n such cases filters are chosen to transmit in Description and Operation of Instrument these spectral regions and sensitivity is considerably increased. An example of such a case is shown in Figure 2 where spectral Figure 1 (a) is a schematic diagram of a trial arrangement transmission curves of clear aqueous solutions of bromophenol which was used successfully in these laboratories for several blue are lotted. a is a spectral transmission curve of an acid months. Light from a concentrated filament lamp is collisolution, that of a basic solution, while c and d are the transmated by a lens. A circular diaphragm limits the essentially mission curves of the two filters which are used in the photoparallel beam to 2.5 cm. (1 inch) in diameter. This beam then electric photometer for titrations with this indicator. passes through a cylindrical flask containing the solution to be titrated and impinges on a glass plate set a t a 45" angle with the beam path. Approximately 8 per cent of the light passing After the feeasibility of the photoelectric method had been through the flask is reflected to a barrier layer photocell in front demonstrated, an attempt was made to design an apparatus The remaining 92 per cent is of which is a colored glass filter. based on the same principles as that shown in Figure 1, but transmitted by the glass plate and strikes another photocell in front of which is another colored glass filter. The two filters differ capable of being easily manipulated by the average technician, in color. and rugged enough to withstand hard daily use. Figures 3 and The photocells are connected with two variable resistors and a 4 show front and rear views of the instrument which was finally galvanometer in a Wheatstone bridge circuit originally used by constructed. Figure 5 is a circuit diagram of this instrument. Wilcox (JO), and shown in Figure 1 (b). This scheme was used previously by one of the authors (35) in a reflectometer. Muller (Sf),in an analysis of the circuit, has derived an expression for the It will be observed that a third photocell is provided. It is transmission of a solution interposed in one of the light paths for mounted on the back of the sample compartment with its axis a t right angles to the direction of the beam. When in use, it is the case in which no optical filters are employed. In the present connected in the bridge circuit with one of the photocells which arrangement in which an optical filter of one color is used with one receives the beam transmitted by the sample. I n this manner, photocell, while a filter of a different color is used with the other titrations may be carried out in which the end point is indicated photocell, the following analysis applies: by the formation or disappearance of a precipitate, or by means of a fluorescent indicator. The effect on the photocell due to Let Z = intensity of source light scattering is enhanced by painting the interior of the samTI = transmission of sample under test for light rorreple compartment white. An increase of turbidity or fluorescence sponding in color to color of filter in front of of the solution results in an increase in the response of the cell, photocell 1 1'7 = transmission of sample under test for light correwhile the response of the cell which receives the transmitted sponding iii color to color of filter in front of beam decreases. Since a quantity proportional to the ratio of the two responses is measured, great sensitivity is obtained. photoccll 2
-4-
it
644
Vol. 15, No.. 10
INDUSTRIAL A N D ENGINEERING CHEMISTRY
the 115-volt line stabilized by a voltage regulator and stepped down to 6 volts. .
~ I
~~~~~~~~
~
~~
.
~
with glass windows are used. This type of construction protects the photocell element from deterioration due to corrading fumes.
Application to DarkColored Resins The visual detection of the end point in the titration of dark-colored solutions by the usual indicator methods is, in most cases, u n c e r t a i n . A number of attempts t o overcome this uncertainty have been made.
Lamp rheostat Storage battery binding Posts A.C.-D.C. aritoh 4. Step-down transformer. 115-6.3 Volts 5. Fluorpsoenoe or turbidity ohotnaell 23. Extra filters 1.
2. 3.
Furthermore, the readings are practicdly independent of small color changes. A switching arrangement, shown in Figure 5, permits the interchanging of the three photocells in the circuit. The power supply to the lamp of the photometer may be either direct eurrmt from a storage batterv or alternating current from
,
These have included the use of adryexternalindicator ( 1 0 , the use of mixed indicators (67, and titration of dilute solutions in a thin cell (87). Coburn (6) employed a two-phase system where the end point was detected in the clear aqueous p h a s e . S m i t h (43') a n d Tingle (49) used a hand spectroscope to detect the appearance of the absorption band of phenolphthalein a t the end point. Pall (36) suggested a color-matching scheme employing a visual comparator, while Fratis and Condit (8) designed an Erlenmeyer flaskwith a side tube of small diameter through which the indicator change a t the end point was more readily obsenred. Xone of the foregoing methods has been entirely satisfactory.
The apparatus described ahove was applied in this Iahoratory to the titration of darkcolored rosins and Belro and Vinsol resins. Preliminaryellperiments indicated that 0.1 to 0.2 per cent alcoholic solutions of the various grades of resin which were studied were satisfartory. Since the color of such solutions is
6.
Galvanometer lampcord
8. 9.
Lens housing Sample compartment Clamp sampie Bask Main COm mtment Filter hol& (r?Uected beam) Filter holder (direct beam) Dial resistom (Rubiaon type B) Rheostat for coam adjustment (General Radio type 214A. 1000 o>m) Rheostat for fine adjustment (General R d i o type 2J4A. IOU phm) Snitches for interchsnging photocells Locking push button anitoh (galvanometer oiroult) Galvanometer (Rubiaon 3402B) Binding posts for galasnometer leads Galvanometer leads Microburet Stirrer
7. Lnmp houping 10. 11.
12.
13. 14. 15. 16.
17. 18.
19.
20. 21. 22.
23. 24.
ANALYTICAL EDITION
October 15, 1943 PHOTOCELL N0.I
PWTCCELL N 0 . 2
PHOTOCELL NO. 3
ADJUSTMENT RHEOSTATS ON
FIGURE 5. CIRCUIT DIAGRAM
of alklkaline thymol blue. The blue filter is a I-mm. thickness of Coming No. 511. Since the color change a t the end point is from yellow to blue, the relative current in the arm of the bridge containing the amber filtered photocell will decrease, necessitating insertion of greater resistance for balance. Therefore, the instrument is balanced with a low resiskanc-. g., 40 ohms-in the amber filtered photocell arm bv adjustment of the resistance in the blue filtered arm. I'h Ikter-n!.istanee is not rhangrd duriug t1.c wt!rse,of tlic titration. T m t h normal alcoholic potassiurii I.ydroxdr IS t l m added in rm,tll inrrmients from II 10-ml. ridcroburct. h i t t r endi addition of titrant the resistance in the smber filtered photocell arm necessary for balance is recorded. I n the neighborhood of the end point, the increments of titrant are made smaller. The titration is continued until a verv marked ehanee in color is obyved. l h logarithm of the resistaim iii thr amhpr filterrd photocell arm is then plotted against the volume of titrant. A Curve consistinc d t w o 1,rmoht.a at neilrlv d . t aneles to raeh otlier is ohtainea which mav be extrsuola?edras in; conductimetric t i t m tion, t o give an'end poin6. A tipieal curve, together with a blank on the solvent, is shown in Figure 7. Two points should be emnhasiaed: (1) It, is necesmm t o run a h l a k on the alcohol. ~~, (2) A relatively large volume 0; indicator must be used in order to get a sufficient number of points for accurate plotting on the vertical portion of the curve. Saponification numbers were run on several resins using 0.1500-mam samules and 10 ml. of 0.1 N alcoholic pot&881um ~~~~~~~~~~~~
orange, experiments showed that phenolphthalein with a color change of colorless t o red was not a suitable indicator. Thymol blue with a change of yellow t o blue wm found to be very satisfactory. This indicator WBS suggested in 1920 by Holde (96)for use in the titration of daxk oik and soaps. The following is a detailed description of the method that was developed for acid number determinations.
645
~~
~~
~~~~~~~~~
~~
~~~~
~~
~
~~~
~
and due t o the readiness withkhich atmospheric caGhon dioxide dissolves in the alcohol used for washing and transferring. These difficulties were overcome by saponifying in a silver flask, Using neutral alcohol for transferring and diluting, and conducting the titration with the solution blanketed under nitrogen. Table I gives the results obtained for acid numbers and saponification numbers on samples which had been analyzed previously by electrometric titration. The starred values in Table I were obtained by titrating the Samples with alcoholic hydrochloric acid t o the thymol blue end
Approximately 0.5 t o 1.0 gram of the resin was accurately weighed out, dissolved in 95 Der cent alcohol. and made UD to 500 i d . in a volumetne flask. Akluots of 150 mi.were then~pipetted into a titrntion cell, C drop" of I per cent aqueous th.mol blue w r r added. the lielrt was turned on., and the ~.~~... d m e r W I L ~started Figure 6 shows thi two t m e s of titration cells used. A for solutiom of moderate color a i d B for extrrmely dark or'turbid &tions. When thymol blue is used ari the indicator the filters are nmlipr and blue, the amber being mnde up of Corning filters Nios. MI (4.4 mni. thick) and 397 (2.8 mm. thick). Thrse tliirknriws ~~. . . arc such that the maximum of the transmi&ion of the eambination occurs a t 605 millimicrons for the particular melts of glass supplied. This coincides with the peak of the absorption band ~~
~~~
FIGURE 7. TYPICAL TITRATION CURVES For 0.150 gram of vinsol ~smpleand solvent blank
INDUSTRIAL AND ENGINEERING CHEMISTRY
646
TABLE I. ACID AND SAPONIFICATION NUMBERS Sample N wood rosin K gum rosin D urn rosin Befro No. 1 Belro No. 2 Vinaol No. 1 Vinsol No. 2 Vinsol No. 3 Vinsol No. 4 Dehydroabietio scid
Acid No. by Photoelectric Photometer
Acid No. by Electrometric Titration
Saponification No. N wood rosin N wood rosin* Belro No. 1 Belro No. 1* Vinuol No. 1*
173.2, 173.5, 144.7. 140.0; 136.5,
173.8 174.5 145.7 141.8 137.2
172.1, 172.9 172.1, 172.9 139.0, 142.5 139.0, 142.5 135.0
point, adding bromophenol blue, and, after resetting the instrument, titrating to the bromophenol blue end point. The difference between these end points minus the corresponding difference for a blank gives a direct measure of the saponification value, since the thymol blue end point corresponds to the neutralization of the excess alkali and the bromophenol blue end point to the neutralization of the soap. The authors have found that saponifications can be m d e in glass flasks when this technique is used, and that the effect of atmospheric carbon dioxide is lessened. Such titrations, however, should be made under nitrogen. Examination of data in Table I shows that the agreement between the two methods is good for a wide range of materiah and that checks by the colorimetric method generally agree within *l per cent, This method has been applied to research samples of very dark materials which were not entirely soluble in the solvents used. Despite the resulting turbidity and dark background color, the end point was sharp. I t is felt that this method is of use in the titration of darkcolored resins for the following reasons: The end point may be determined with considerable certainty. It is rapid, only 15 minutes being required for a titration and calculation. It is simple and easily handled by a technician. Small samples are used, 0.5 gram being sufficient for triplicate determinations. This makes it of considerable interest for research samples.
Other Applications The foregoing technique can be applied profitably to a number of different types of reactions, and such work is now in progress in this laboratory. As in all titrations, it is necessary to pick an indicator that corresponds to the end point desired. It is extremely important to choose the proper filters, taking into consideration the spectral transmission curves of the indicator used, as well as that of the solution. In general, this photometer appears to be adapted to all types of acid-base and oxidation-reduction titrations using colored or fluorescent indicators. Preliminary experiments indicate that it is also useful in studies of reaction rates involving changes of color, and in studies of precipitation reactions, including turbidity measurements as well as the determination of end points. By employing precision optical cells, the apparatus may be used as a sensitive chemical colorimeter.
Summary A photoelectric photometer has been used for titrations which cannot be carried out satisfactorily by the usual visual or electrometric means. The apparatus ordinarily employs two barrier layer photocells in a balanced Wheatstone bridge circuit. In front of each photocell is a filter, one of which has a maximum transmission in the spectral region where the absorption band of
Vol. 15, No. 10
the selected indicator develops, while the other has its maximum transmission in the spectral region where little or no change takes place in the indicator absorption. The instrument has been applied successfully to the hitherto difficult determination of acid and saponification numbers of dark-colored resins. Graphically determined end points are very sharp and can be obtained in approximately 15 minutes with precisions of * 1 per cent. The photometer is applicable to all types of titrations in which a colored or fluorescent indicator can be used. Precipitation reactions and reaction rates involving changes in solution color or turbidity can be studied. The apparatus may also be employed as a chemical colorimeter.
Literature Cited (1) Alyea, H. N., J . Chem. Education, 18, 57 (1941). (2) Boyer, W. J., IND. ENQ.CEEM.,ANAL.ED., 10, 175 (1938). (3) Campo, A. del, Burriel, F., and Garcih Escolar, L., A n d e s 80c. espafi, fts. qu{m., 34, 829 (1936). 14) Ibid.. 35. 41 (1937). Coburn,’H. H., IND.ENG.CHEM.,ANAL.E D . ,2, 181 (1930). Cohen, A., J . Am. Chem. SOC.,44, 1851 (1922). Field, J., 2d, and Baas-Becking, L. G. M.,J . Gen. Physiol., 9, 445 (1926). Fratis, J. E., and Condit, D. H., IND.ENQ.CHEM.,ANAL.ED., 9, 563 (1937). Gonzales, F., I X Congr. intern. quim. pura aplicada (Madrid). 6, 70 (1934). Goodhue, L. D., IND.ENQ.CHEM.,A a . 4 ~ .E D . , 10, 53 (1938). Hartmann, B. G., J . Assoc. Oficial Aor. Chem., 3, 410 (1920). Hickman, K., and Sanford, C. R., IND.ENG.CHEM.,ANAL.ED.. 5, 65 (1933). Hirano, S., J . Soc. Chem. I n d . ( J a p a n ) (Supplemental Binding), 37, 177B (1934). B i d . , p. 178B. I bid., p. 454B. Ibid., p. 561B. Ibid., p. 754B. Ibid., 38, 176B (1935). Ibid., p. 598B. Ibid., p. 646B. Ibid., p. 648B. Ibid., 40, 412B (1937). Ibid., 41, 266B (1938). Hirano, S., and Nakamura, Y . , J . SOC. Chem. I n d . (Japan) (Supplemental Binding), 37, 147B (1934). Holde, D., Seifenfabr., 40, 113 (1920). Kasai, Y., and Takii, S., Repts. I m p . I n d . Research Inat. (Osaka, Japan), 16, No. 3, 1 (1935). Kryz, F., Oesterr. chem. Ztg., 26, 94 (1923). Lur’e, Y. Y., and Tal, E. M.,Zavodskaya Lab., 9, 702 (1940). Muller, F., 2. Elektrochem., 40, 46 (1934). Muller, R. H., IND.ENG.CHEM.,ANAL.E D . ,7, 223 (1935). Ibid., 11, l ( 1 9 3 9 ) . Muller. R. H.. and McKenna, iM.H., J . Am. Chem. SOC.,58, 1017 (1936). Muller, R. H., and Partridge, H. M.,IXD. ENCI.CHEM..20, 423 (1928). Muller, R. H., and Partridge, H. M., I X D .ENCI.CHEM.,ANAL. E D . , 3, 169 (1931). Osborn, R. H., J . Optical Soc. Am., 31, 758A (1941). Pall, D. B . , Can. J . Research, 14B, 299 (1936). Partridge. H. M..I N D .ENQ.CHEM.,ANAL.E D . ,2, 207 (1930). Ibid., 4;315 (1932). Partridge, H. M., and Smith, R. A., Mikrochemie, 11, 311 (1932). Ringbom, A , , and Sundman. F., Z . anal. Chem., 116, 104 (1939). Rowland, G. P., IND.ENG.C H E MANAL. ., ED., 11, 442 (1939). Russell, W. W., and Latham, D. S., Ibid., 6, 463 (1934). Smith, W. C., Ibid., 6 , 122 (1934). Somiya, T., and Nakamura, Y., J . SOC. C h a . Ind. (Japan) (Supplemental Binding), 38, 262B (1935). Somiya, T., and Shiraishi, S., Ibid., 33, 300B (1930). Somiya, T., and Yamane, T., Ibid., 41, 422 (1938). Somiya, T., and Yasuda, Y., Ibid., 41, 314B (1938). Styer. C. A., U. S. Patent 1,977,359 (Oct. 16, 1934). Tingle, A., J . Am. Chem. SOC.,40, 873 (1918). Wilcox, L. V., IND.ENQ.CHEM.,ANAL.ED., 6, 167 (1934). P R E E ~ N Tbefore E D the Division of Analytical and Micro Chemistry at the 105th Meeting of the AMEFXCANCHEMICAL ~OCIETY, Detroit, Mich.
