and W. J. Levene The Israel Electro-Optical Industry Limited Rehovot, Isroel
Spectrophotometric Titrations for Students
The basic concepts of optical ahsorbance and transmission are readily demonstrated by the student in a series of colorimetric titrations carried out in a titration cell installed in a ranid scannine snectrocolorimeter. The student observes color changes the titration cell and relates them to the spectral transmission curve of the solution, wbich he sees simultaneously on an oscilloscope. The process of formation of new species a t each stage of the titration can be followed. The student can also readily demonstrate isosbestic points. A further method is presented for determination of the endpoint of a titration by simultaneously comparing the absorbance of the liquid a t any two wavelengths in the visihle region. introduction Spectropbotometric and colorimetric titrations are standard techniques in analytical chemistry, The liquid may be circulated through an absorbance cell ( I ) , or the titration vessel itself installed within the optical path of the spectrophotometer (2). Generally the solution becomes colored only when the titrant is added in excess, so that the transmission a t a selected wavelength decreases progressively after the endpoint. Satisfactory determination of the endpoint demands that there be a sharp break in the curve of transmission plotted against volume of titrant added. Instruments suitable for this and other techniques of optical ahsorbance spectrometry are described in a review article by Boltz and Mellon (3). Spectrometers in general use employ monochromators which may be slowly scanned through the spectral range of interest. Rapid scanning spectrometers, in which the spectrum is examined many times per second, have also been described (4, 5). The rapid scanning spectrometer used in this work2 enables instantaneous measurement over the whole of the visihle spectrum (400-700 nm) at 25 nm resolution, 30 timesls. It presents the spectral transmission curve of the specimen on a simplified oscilloscope, whose screen graticule is calibrated in percentage transmission (ordinate) and nanometers (abscissa). Spectra displayed in this way have been photographed in Figures 1,2, and 4-7. This technique for spectrocolorimetric titration is very instructive, emphasizing the individual characteristics of each titration. The species present a t each stage of the titration can readily he identified, and the process of formation of new species followed. Titration Procedure Standard 0.1 N solutions are introduced by means of a buret into a simple glass titration cell (6)of 200-ml volume, installed in the specimen chamber of the spectrocolorimeter. The cell is a 2-in. diameter horizontal cylinder with an optical path length of 100 mm (the solution is made u p to 150 ml with water to enclose the whole of the beam area). A 3-cm Teflon-coated magnetic stirrer, rotating a t 250 rpm, mixes an added drop in a few seconds.
6
Present address: Soreq Nuclear Research Center, Yavne, Israel. 2Manufactured bv the Israel Electro-Ootical fndustrv. Ltd.. P.O.B. 1 1 6 , Rehovot, Israel. Availnhle in Nonh nnd South America from Broomer Research Carp., 23 Sheer Plnm. I'larnvieu. S . Y . 11803. 136
/ Journal of Chemical Education
S Z
Z
'/ 400
600
W ~ ~ L E N G Tnrn H
700
Figure 1. Multiple-exposure oscillogram showing effects of adding single drops at l2 solution at endpoint of Na2SzOl determination using fast ,,,,,ingspectrocolorimster,
Thiosulfate-Iodine Titration In this titration (7) 0.1 N iodine solution containing KI is added from a buret to a solution containing 25 ml of 0.1 N thiosulfate in the titration cell. Figure 1 shows the traces as they appear successively on the scope during the course of the titration. A Polaroid camera was used to photograph the scope and all the traces were recorded on a single film. Trace 1represents the spectral transmission of the uncolored thiosulfate solution before the beginning of the titration. Trace 2, almost coincident with Trace 1, shows the spectral transmission after the addition of iodine to one drop before the endpoint. Traces 3, 4, 5, and 6 show the effects of adding successive single drops. Trace 1 is not perfectly flat because of irregularities in the spectral sensitivity of the spectrophotometer, which is a single-beam instrument with mask correction of the spectral sensitivity. Trace 2 shows that there is almost no change in spectral transmission until the endpoint is reached. The addition of one drop at the endpoint produces a reduction in transmission a t wavelengths below 600 nm. There is no cover over the titration cell, so that the very slight change in color a t the endpoint can just be seen. This change becomes more pronounced after the addition of further drops in excess. Thus Traces 4, 5, and 6 of Figure 1 show correspondingly greater absorhance in the blue, which the student will relate to the increasingly reddish tint of the iodine solution as seen in transmission. Ferrous-Permanganate Titration The second titration (7, p. 564) is the determination of ferrous ion by means of permanganate. The titration cell contains 25 ml of 0.1 N ferrous ammonium sulfate in 0.25 N HzSOn. In Figure 2, Traces 1 through 7 are, respectively, from the uncolored ferrous solution alone, for 1 drop before the endpoint, and for the further addition of 1, 2, 3,4, and 5 more drops of permanganate after the endpoint. point. Trace 1 shows that the Fez+ present before the start of the titration has very slight absorhance located in the blue. Fe3+, wbich absorbs blue light, is formed by reac-
which is pale yellow, due to its slight absorbance at wavelengths below 500 nm (see Trace 1). The second is the ferric ferrocyanide complex Fer[Fe(CN)& formed by reaction with the ferric chloride indicator from the start of the titration. Its deep blue color predominates throughout the titration. The color is due to a very strong absorbance in the red, a t wavelengths ahove 550 nm, which obscures a slight absorbance, below 450 nm, in the blue. The third colored species is the ferricyanide ion F e ( C N ) e 3 seen in Trace 6, which has a much stronger blue absorbance than the ferrocyanide from which i t is formed during the titration by the reduction Figure 2. Titration of ferrous ammonium sulfate with potassium perrnanganate. Oscilloscope photographed at the endpoint and after each of 5 further drops of KMnO,.
NUMBER Figure 3.
OF
DROPS
Below 500 nm, the transmission decreases continuouslv during the titration because of the formation of the ferricyanide, as can he seen in Fieure 4, Traces 2,. 3,. 4.. and 5. At wavelengths ahove 550 n k , the low transmission in Traces 2 through 5 is mainly due to the ferric ferrocyanide complex. At the end of the titration the ferrocyanide ion necessary to the formation of this complex has all been oxidized, and the complex disappears. The addition of a single drop at the endpoint produces a rise in transmission at 700 nm from 20% to 50%. The solution takes on the yellow color of the ferricyanide ion alone. lsosbestic Point-Acid-Base Titration The next example demonstrates an acid-base titration using an indicator. The 150 ml of solution in the titration cell contains 10 ml of 0.05 N NaOH, to which is added 3 drops of 0.1% aqueous Congo Red indicator. In Figure 5, Traces 1, 2, 3, 4, and 5 are the spectral transmissions of the solution after the addition of 29.0, 31.6, 32.0, 32.6, and 33.9 ml, respectively, of approximately 0.02 N HC1. The
Beer's Law curvesfram the dataof Figure 2 (at 550 nm).
tion progressively from the beginning of the addition of KMn04 at the start of the titration. At the endpoint, a transmission minimum appears in the yellow and the solution takes on faintly the typical purple color of Mn04-. After the end~oint.the color deenens as the vellow transmission decrezises. The huretted drops are &bstantially uniform in volume. Quantitative adherence to Beer's Law can therefore he demonstrated by plotting the logarithm of the transmission a t the wavelength of the minimum against the number of drops added. Thus, the Beer's Law curve for 550 nm for this set of results (Fig. 3) shows that the stoichiometric endpoint was reached about threequarters of the way through the first drop. Ferrocyanide-Ceric Titration Potassium ferracyanide can he determined by ceric ammonium sulfate (7, p. 583) using ferric chloride as an indicator. The 150 ml of solution in the titration cell contains 25 ml 0.1 N K4[Fe(CN)6]solution in 0.25 N HzS04, with a single drop of a 0.5% FeC4 solution. This is titrated with 0.1 N ceric ammonium sulfate solution. Figure 4 Trace 1, shows the spectral transmission of the ferrocyanide ion alone, before the addition of the indicatar. Traces 2, 3, 4, and 5 show the situation after the addition of 5, 10, 15, and 20 ml, respectively, of the titrant; and Trace 6, one drop after the endpoint, shows the spectral transmission of the ferricyanide ion alone. Only three of the species formed during the titration have appreciable color, and it is instructive to prepare solutions of them separately, and compare their spectral ahsorhance curves with those of the solution at the several stages of the titration. The first of these is the ferrocyanide ion, [Fe(CNkI4-
Figure 4. Determination of potassium fermeyanide by ceric ammonium Sulfate using FeCIs as indicator.
Figure 5. Acidlbase titration using Congo Red as indicator, lsosbestic point at 555 nm.
Volume 5 1, Number 2, February 1974
/
137
endpoint (at 31.7 ml in this case) can be determined with a repeatability of 1 drop a t this concentration by plotting a graph of transmission a t a selected wavelength (for example 650 nm) against titre, and finding the point of inflection. The graph can also be drawn out on a recorder. An isosbestic point occurs when the reaction products of a stoichiometric titration have the same absorbance a t a certain wavelength as the reactants. In such a case the absorbance a t the wavelength will not change during the intermediate stages of the titration. In the titration described above, the isosbestic point occurs at 555 nm, as seen in Figure 5. The isoshestic point may be observed without photography by following the transmission a t the appropriate wavelength on the scope. lsosbestic Point-Vanadate-Vanadyl Reduction A further example of the isosbestic point may be ohtained using the reduction of sodium vanadate by ferrous ammonium sulfate in acid solution (7, page 580). A solution of 0.1 N ferrous ammonium sulfate is added in 2-ml aliquots to 150 ml of strongly acid solution containing 10 ml of 0.1 N sodium vanadate in the titration cell. In Figure 6, Traces 1 through 6 correspond to 0, 2, 4, 6, 8, and 10 ml, respectively, of the ferrous solution. At the start of the titration the vanadate absorbs only at wavelengths below 520 nm, and the solution is yellow. The vanadate absorbance decreases when the ferrous ion is added, and the vanadyl ion formed absorbs strongly above 500 nm, so that the solution becomes blue. The isosbestic point is a t 505 nm. Two Wavelength Titration-Ferrous Ammonium Sulfate , and Potassium Dichromate The titration of ferrous ammonium sulfate with potassium dichromate, usually carried out with the aid of diphenylamine indicator, is not a good student experiment because the indicator sometimes oxidizes during the titra-
tion (7. n. 579) leadine to lack of c recision. The titration can, however, be pert'ormed spertn~colorimetr~cally without the indicator. A 0.1 .V ~otassiumd~chromatesolution is added from the buret into 150 ml of acid solution containing 25 ml of 0.1 N Fe(NH4)~(S04)zin the titration cell. Spectral transmission curves after the addition of 7, 14, 19, 24, (just after the endpoint) and 29 ml, respectively, of the dichromate are shown in Figure 7 (Traces 2 through 6; Trace 1is for the clear solution). During the titration the CrzO+ added oxidizes the Fez+ ion to Fe3+, and is itself reduced to C++. The curves of these four species may be inspected separately on the scope before starting the titration. The Cr3+ has a transmissi6n minimum a t 580 nm. At this wavelength the transmission decreases until the endpoint, whereafter it remains constant, since no more of this species is formed. At wavelengths be10.w 500 nm both the Fe3+ and the Cr3+ ions, formed before the endpoint, absorb. Furthermore, the CrzOrz- ion, present in excess after the endpoint, ahsorbs a t wavelengths below 560 nm. Thus at 485 nm, for example, the absorbance decreases slowly during the course of the titration and then more rapidly after the endpoint is reached. A straightforward method of determining the endpoint of this titration would be to follow the reduction in transmission a t the Cr3+ minimum (580 nm) as the dichromate is added dropwise from the buret until a further drop produces no further absorbance. This is rather difficult in practice, since the absorbance change due to a single drop even before the endpoint is quite small. Figure 7 shows that the absorbances in the two hranches of the C?+ curve are practically equal at the wavelengths 485 and 675 nm. During the titration a graph may he plotted of the difference between the transmissions at these two wavelengths against titre. At the endpoint, the dichromate in excess suddenly reduces the transmission at the lower wavelength only. The procedure can be carried out i n s t ~ m e n t a l l yas follows: The spectrocolorimeter can he fitted with a logarithmic amplifier (to provide photometric outputs in absorbance units) and withSpectral Line Followers (SLF's). The SLF is an electronic circuit which samples the transmission a t a selected wavelength during the scan. In Figure 7 the brightened points on the traces show where the sampling was made, a t 485 and 675 nm, by two SLF units. The difference between the absorhances (as measured using the logarithmic amplifier) at these two wavelengths was fed to a recorder. The titrant was delivered at a constant rate of 0.8 ml/min through a capillary tube attached to the buret, and dipping slightly below the solution sur-
Figure 6. lsosbasfic paint at 505 nm during the vanadate/vanadyi reduction bv ferrous ammonium sultafe.
Figure 7. Determination of ferrous ammonium sulfste by potassium diChromate using spectral line followers at 485 and 675 nm.
138
/
Journal of Chemical Education
Figure 8. Determination of the endpoint at the titration of Figure 7 by recording the difference between the absorbance$ af 485 nm and 675 nm using two spectral line follower circuits.
face. A by-pass tube of normal diameter with a tap enahled the major part of the titrant to he added rapidly first. Figure 8 shows the record of the difference between the ahsorbances at 485 and 675 nm plotted in this way against titre. Because this difference is small before the endpoint, a highly expanded scale may be used, and the curve slopes steeply after the endpoint, The has been found to be reproducible to 0.02 ml from a total of 25 ml of 0.1 Npotassium dichromate. Conclusion
The series of oxidation~reductionand acid-alkali titrations discussed provides examples of the way in which a
rapid-scanning spectrocolorimeter can help in the understanding of chemical reactions in solution. With the demonstration of isosbestic points, the method also gives insight into the phenomena of spectral absorbtion. Literature Cited (11 "hm, C.. Bodin. J. I.. Connom, K. A,. and Hipuchi. T.. Anol. Chem.. 31. 483. (19591. (21 odd^, R.F.,and H D ~ ~ ,M..AO~I. D. chpm.. 26,174011954). (31 Boltz,D.F., and Mellon, M. G.,lnol. chm.. 42. I ~ Z R ( L W O I . (4) Stmiek, J. W., Gruuer.G.A.. andKuuana.T.Ano1.. Chsm.. 4l,481(L9691. ( 5 ) Dolin, S.A., Kruegle. H.A., andPenriar, G. J ~ p p l upt, . 6,267 ,1967). (6)Sueetser, P. B., and Bricker, C. F., Anol. Ckm.. 25.253 (19531. (71 Kolthoff. I. M., and Sandell. E. B., "Textbook of Quanfitafiw Inorganic Analysia." 3rd Ed.. Macmillan Company. New York, 1961, p. 585. These titrations ill also h found in many otherstandard textbooboiana~yticatchemistry.
Volume51. Number 2, February 7974
'/
139
