Eugene D. O l r e n and Craig C. Foreback University of southFlorida Tampa, Florida 33620
II
Automated Spectrophotometric Titrations
The advantages of spectrophotometric titrations are well known (1-6, 18,19). Spectrophotometric titrations are especially useful for determining low concentrations of substances, for analyses involving reactions in which the equilibrium is unfavorable a t the end-point, and for determinations in which there are no suitable indicators but one of the species involved has an absorbance. The precision of spectrophotometric titrations can be greater than direct colorimetric measurement since the end-point is determined by the intersection of two straight lines, each of which is determined by the average of several points. Many people have automated spectrophotometric titrations (7-10, 1.3, 81) but all of these have used relatively expensive and complex equipment. I n this work a Spectronic 20 with a very simple titrant delivery system was used to give automatically recorded spectrophotometric titrations on a number of titration systems. The use of transmittance instead of absorbance curves for end-point determinations was shown to be very satisfactory, particularly at low concentrations.
would read 55010070 transmittance, s. two-fold scde expansion. As another example, a. 120 mV offset voltage in conjunction with a. recorder sensitivit,y of 40 mV full scale would give a full-scale recorder readout of 75-100% transmit,tsnce, or a four-fold scale expansion. Since the maximum sensitivity of the Heath model EUW-20A recorder is about 3 mV full scale, it would be possible to expand the transmittance scale a meximum of ahout fifty-fold, reading out transmittances between 98 and 100% over the 10-in. chart. In the titrations reportedin this paper themaximum scale expansion that was found to be necessary to use was about sevenfold, and in this case the copper titration was followed over the transmittance range of 85 t,o 10070 transmittance. The offset voltaee had no noticeable effect on the Spectronic 20 meter-reading. The ahsorption cell and titration-c.irculation apparatus were similar to that described previously (11) except that a 125-ml Erlenmeyer flask was modified and used in place of the beaker. The flask was modified by sealing outlet and inlet tubes into the side of the flask similar to the design of Beilby and Landowski (11 ) except that the return tube was bent downward 90" at the center of'the flask and the end of the return tube came within in. of the bottom of the flask. The solution to be titrated had to be kept a t s level which was above the end of the return tube so that no air bubbles could get in the circulation system. The glass tubing and rubber connecting tubing wrts '/,-in. diameter. The absorption cell was 3Vrin. high and 5f8-i-in.in diameter. A simple cover to keep light out of the absorption cell was constructed out of a block of styrofoam lafrin. long X I>/%-in.wide X 11/,-in. high, with a.groove l l / A . long X '/%-in.wide X 1-in. deep to accommodate the outlet and inlet tubes fram the absorption cell. After cutting the block to the proper shape with a hot knife, the entire block (inside and out) was wrapped with black Blectricsl t m e ta make it lieht nroof. A black niece of naner with
that the sverage drop-si~ewas about 0.07 ml. Reagents
Figure 1 .
Sehemotic of ~poctrophotometrictitrotion system,
Experimental Apparatus The Bausch and Lonib Spectronic 20 was adspted for this work. Recorder readout was obtained by simply tapping scross the meter of the speetrophotometer. To do this the cover was temporarily removed and leads from a recorder were' soldered across the meter in parallel as shown in Figure 1. A Heath model EUW-20A recorder was used to measure the meter voltage. A Heath model EUW-16 Voltage Reference Source was used as an offset voltme. and was nlsced in series between the recorder and input as shown in Figure 1. The purpose of the offset voltage was to give expanded scale transmittance readout fram the Spectronic 20. Full scale deflection of the Spectronic 20 meter (8. 10070 transmittance reading) results in s. 160 mV voltage drop across the meter. Thus, by offsetting the meter voltage by 80 mV and increasing the sensitivity of the recorder to 80 mV full scale, the full-scale recorder chart
206 / journal of Chemical Education
Elh~lenediaminetel~mcelic Acid (EDTA). A 0.2 M stock sclution was prepared by dissolving 74.450 g of the disodium salt of EDTA in 1 1 of water. Titrant solutions were prepared by appropriate dilution of this stock solution. E~iochromeBlack T. 1 g of the indicator was mixed with 100 g sodium chloride, and was stored and used as a dry solid. The indicator was stable indefinitely when prepared in this manner. Coppw(I1). 0.4 g copper metal was dissolved in concentrated nitric acid and then diluted to 1 I. Standardization was by direct titration with EDTA using pyrocatechol violet indicator (16). Barium(I1). A 0.10 M solution wasprepared by dissolving 1.97 g of reagent grade barium carbonate in dilute nitric acia. The solution was standardized by direct t,itrrttion with diethylenetriaminepentsscetic acid (16) (if EDTA were to be used a back
pH 10 Bufler, Potassium Dichmmate, I&(II) and Iodine. These solutions were prepared as described previously (14,17, $0 Procedures Barium. The recorder span was set to 100 mV. The concentration of EDTA titrant which was used depended on the concentration of barium to he titrated. For samnles containine 0.2-0.6 mmoles of barium 0.1 M EDTA was used and for samples con-
taining 0.03-0.06 mmales of barium 0.02 M EDTA was used. To the sample was added 0.1 g Eriochrome black T indicator mixtore, 0.5 g magnesium-EDTA rhelrtte, and 2 ml pH 10 buffer. The solution was then titrated a t 620nmuntil the transmittance reached a minimum. It was found convenient to titrate subsequent samples in the same flask simply by adding a new sample to the previously titrated sample. The end-point of the first titration was the beginning of the second. In this manner, a t least five different determinations could he made in the same flask without emptying. I t was usually necessary to change voltage spans on the recorder after three or four titrations to compensate for the change in absorbance with dilution. This "cumulative-sample" procedure for consecutive titrations represented a considerable savings in time. Copper. Copper was titrated at 625 nm with 0.2 M EDTA. A sample containing 0.2-0.4 mmoles copper(I1) and 2 ml pH 2 bufferwas titrated until the percent transmittance reached a minimum. Beceuse the absorbance increased with successive samples it was not feasible to tit.rate "eumi~lative-mmples"due to Ions of sensitivity. The 25 mV recorder spsn was nsed for this titration. Iron. Iron(I1) was t i t r a t ~ da t 390 nm and arecorder span of 50 my. A 50-ml Erlynmeyer flask, modified in the same manner as the 126ml flask, was nsed in order to increase the sensit3"ity. A sample containing 0.1-0.3 mmoles iron(I1) and 2 ml concentrated phosphoric acid were added to the approximetely 40 ml in the titratian flask. The solution was titrsted with potassium dichromate until the transmittance increased. Five or six increments beyond the end-point were added. About five successive determinations could he csrried out in the same flask without emptying. When more than five sainples had been allowed to accumulate in the flask without emptying, dilution and the weak %hsorhance of chromium(II1) beceme aproblem. Antimony. Antimony was titreted with iodine at 390nm and s. recorder span of 50 mV. About 0.2 g of solid sodium bicarbonate were added to 100ml of solutionin order to buffer the pH a t about 7. Samples containing 0.01-0.03 mmoles antimony were titrated with 0.02 M iodine while samples containing 0.002-0.004 mmoles were titrated with 0.004M iodine.
Discussion and Results Copper
Figure 2 shows the titration curve for the titration of copper(I1) with EDTA. Notice that transmittance decreases until the end-point is reached due to the formation of the blue copper-EDTA complex. After
all the copper is complexed no further decrease in transmittance occurs. The linear portions of the curve were extrapolated to locate the end-point. Rather than drawing the best straight lines directly over the existing curve the equilibrium transmittance data were replotted by offsetting the final transmittance reading one increment to the left, thus making the best straight line go through points placed at the beginning of each titrant-increment, rather than at the end of the increment. This method of locating the end-point was used for all titrations. The end-point in Figure 2 occurred a t 17.2 drops added. The average of four such titrations a t a copper concentration of 3 X M had an error of +0.6% and a relative standard deviation of 0.7y0. It was found t.hat if the above transmittance data were replotted in the form of absorbance versus number of titrant drops there was even more curvature in the region of the end-point, and thus the transmittance plot was deemed preferable. It has been reported (4) that greater sensitivity could be obtained by titrating at 745 nm, but this would require a red sensitive phototube and a red filter. Antimony ond lron(l1)
The titration of antimony with iodine gave a titration curve nearly identical in shape to the curve for the titration of iron(I1) with dichromate, except that the iron-dichromate plot shows a slight decrease in transmittance prior to the end-point due to the formation of chromium(II1) which absorbs slightly at 390 nm. After the end-point both curves showed large decreases in transmittance from excess titrant. In the antimony titration the solution remains transparent prior to the end-point. Of the various titrations illustrated in this paper, the antimony titration proved to be the most sensitive, enabling extremely low concentrations of antimony to be titrated. The results for antimony are shown in Table 1. Titration of 1.5 X lo-' 1M antimony gave a relative standard deviation of 0.25% and an average relative error of -0.2%. Titration of 4 X M antimony gave a relative standard deviatiou of 0.8% and an ayerage relative error of +0.3%. The results of the iron analysis are shown in Table 2. Table 1. Titration of Antimony at Two Different Concentrations
4 . 5 X 10- M ShSb found Sb taken
-1.5 X 10-' M SbSh taken Sh found
%
%
%
%
24.24 24.24 24.24 24.24 24.24
24.3 24.3 24.2 24.2 24.2
24.24 24.24 24.24 24.24
24.2 24.2 24.3 24.6
The antimony sample was obtained from Thorn Smith, Chemist, 1847 N. Main Street, Royal Oak, Michigan. Toble 2. Titration of lron(ll) a t Two Different Concentrations with Dichromate
-4 x 1 0 - W FeFe taken Fe found mmoles mmoles NUMBER OF TITRANT DROPS Figure 2. Automoticolly recorded titration curve for the titration of 3 lo-= M Cu(ll) with 0.2 M EDTA ot 625 nm.
X
0.200 0.200 0.200 0.200
0.203 0.204 0.204 0.203 Volume
-2
x
lo-"
Fe taken mmoles
FFR Fe found mmoles
0.100 0.100 0.100 0.100
0.102 0.102 0.101 0.102
49, Number 3, March 1972
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207
Table 3.
Ba taken
Titration of Barium with EDTA 4 X lo-'
M Barium Ba. found mrnoles
rnrnoles
NUMBER OF TITRANT DROPS Figure 3. Avtornoticdly recorded titration curve for the titrotion of 1 O-a M Ba with 0.1 M EDTA at 620 nm.
For 0.2 mmole samples the relative standard deviation was 0.1% and the relative error was +1.7%. For 0.1 mmole samples the relative standard deviation was 0.3% and the relative error was 1.8%. Barium
The results of the barium titrations are shown in Table 3. The relative standard deviation was 0.7% and the average error was -2.0%. Titration of higher concentrations of barium gave better precision. Figure 3 shows the titration of barium with EDTA. Notice that the transmittance shows little change until the end-point is approached after which it drops rapidly. After the end-point is reached the transmittance levels off. The end-point is determined by the extrapolation of the linear portions of the curve just before and after the end-point. It was found that if the above transmittance data were replotted in the form of absorbance versus number of titrant drops the relative shape was very similar to that of the transmittance plot, and no advantage resulted. Still (20) used transmittance curves in the titration of magnesium with EDTA with more accuracy than absorbance curves. Conclusions
It is standard practice in manual spectrophotometric titrations to plot absorbance versus volume rather than transmittance. It was shown in this study that re-
208 / Journal of Chemical Education
cording percent transmittance instead of absorbance versus volume gave satisfactory results. Miyake and Sakamato (21) have also shown that there is little error in plotting transmittance versus volume if the percent transmittance is greater than 60% near the end-point region. In all the titrations reported in this paper the transmittances were confined to the 85-100% transmittance range, except for the barium titrations, which covered the range of 60-9070 transmittance. The spectrophotometric titration of copper with EDTA with manual plotting of absorbance has been reported (.I),but it was shown in this study that automatic recording of transmittance gives sat.isfactory results. Likewise iron (4) and antimony (3) have been titrated previously by manual spectrophotometric methods using indicators, but in this study they mere titrated without indicators and a t lower concentrations than had previously been reported. Previous direct titrations of barium with EDTA have been unsatisfactory (15, 16). In this study good results were obtained by titrating low concentrations of barium directly with EDTA. It has been reported that the use of DTPA rather than EDTA might give even better accuracy in the titration of barium (15). Literature Cited
UNDERWO~D. A. L.. A n d . Chen.. 2 5 , l S l O (1953). U ~ o e n w o o oA . . L.. J. Cxew. Eouc. 31,394 (1954). U I u c m I t C. F.,A N D swzmm~, P.U., A n d . Chem., 24.409 (1952). SWEETBER. P. I?., A N D BRLCKI~R, C . Ti., Anol. Clmn.. 2 5 , 2 5 3 (1953). Donnu. R . F.. AND H u m . D. N . . Anal. Chem... 22.. 1314 11950). . . isj ibid..26,1679 (1954). (7) M A R P L ET. , L..*ND H ~ M ED.. N.. Anal. Chcm..28, 1116 (1956). E. C., A n d . Chem., 26, 442 (8) MA~nasrAoT. H . V., A N D DORRORINOT, (9) MALMBTADT, H. V.. A N D ROBERTS. C. V.. A n d . Chcm.. 27,741 (1955). (101 Ibid..28,1408 (1956). . I,.. A N D L A N D O W ~ X I C. . A., J. CHEM. EUUC..47. 288 (1970). (11) U s m n ~ A. . A N D RINCIOM.A,, Anal. Chin. Aclo, 36, 105 (1966). (12) S m m v ~ m13.. Anal. Chem..36,2461 (1964). (13) OLSEN.E.D., (1) (2) (3) (4) 151
,,"Z"V
F. S . , Anol. Chcm., 3 9 , 8 1 (1967). (14) OLSEN,E. D.. A N D ADAMO, R , J., Anol. Chem.. 38, 152 (1966). (15) OMEN.E. D., A N D NOVAK, , "Corn~lerometrieTitrations." Methue" and Co., (16) S c r w h n z e l m A C ~G., London, and Interseienoe, New York, 1957. "Element~ry Quantitative (17) BI.AEDDL.1V. d., A N D MBLOOHE,V. Analysis." Hamer and Row. New York. 1963, pp.460-2. J . , Talonto, 8 , 7 2 0 (19611. (18) FLAscna~,H., A N D GANCROFF, . AND BUTCHER, d . , Tolonta. 1 2 , 9 1 3 (1965). (IS) F L A ~ O H K AH., , , A d a Chem.Fenn., 41, 33 (1968). Annl. Abdr. 17, 586. (20) S T I ~ E.. (21) MIYAIE,S.. A N D SAKAIOTO, Jopan Anal., 16, 238. Annl. Abslr., 15,
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