Manual Differential Spectrophotometric Titrations Application to Aqueous a n d Nonaqueous Acid-Base Titrations and to Chelometric Tifrations STANLEY BRUCKENSTEIN and M. M. T. K. GRACIAS School o f Chemistry, University of Minnesota, Minneapolis 14, Minn.
b A differential end point technique based on the use of spectrophotometry has been used for aqueous and nonaqueous acid-base titrations, and cheloA suitable indicator metric titrations. i s a d d e d to the unknown and the change in indicator absorbance on adding a titrant increment i s determined using the unknown solution as its own high absorbance reference standard, i.e., employing the method of differential spectrophotometry. ApH/AV (or ApM/AV) can then b e calculated from the observed change in absorbance and other easily determined absorbances. Phosphoric acid (1st and 2nd end point, 0.08M), acetic acid (0.001M), L-phenylalanine (0.03M), L-leucine (0.03M), and DLalanine (0.05M) were titrated with sodium hydroxide in aqueous solution, and sodium acetate (0.001M) and o-chloroaniline (0.09M) were titrated in glacial acetic acid as solvent with perchloric acid obtaining results superior to those found with a potentiometric titration using a glass electrode. Also, dilute aqueous calcium solutions (0,001M) containing no magnesium were titrated with EDTA using Eriochrome Black T as indicator with an accuracy of O.4y0. Under optimum conditions the error in this method i s equivalent to a differential potentiometric procedure with an electromotive force error of 1 0 . 1 mv.
E
POIST L O C A T I ~ N in volumetric analysis can be based on any property of the solution which changes during the titration. \Then the most commonly used technique, visual detection, is unsatisfactory, physicochemical techniques are employed. I n recent years there has been considwable interest in the use of photonietric titration techniques and exellent revien-s of these techniques have been given by Goddu and Hume (7’) and Malmstadt (18). Kearly all photometric titrations employ data obtained well before and after the end point. I n some of the methods proposed an indicator is added and the absorbance data must be suitably transformed to yield linear plots (S 5, 8, 23). SD
Only very fen photometric procedures have been proposed which involve data obtained close to the end point. Among these are the Type I plot of Higuchi, Rehm, and Barnstein (8),the titration to the equivalence point using a reference solution and differential spectrophotometry (26), and the automatic differential photometric titration apparatus developed by JIalmstadt and Roberts
(19). The ~voik reported below describes a manual differential spectrophotometric end point location technique in n hich a p H or pl\I (metal ion) indicator is added to the titration cell. Using differential (precision) spectrophotometry, whose advantages have been described elsenhere (1, 9-11, 15), the change of indicator absorbance which occurs on adding a titrant increment is determined. Consider a solution containing a pH or pAI indicator to which 8, ml. of titrant have been added. At some wavelength, A. the absorbance of one of the indicator species is A I by conventional spectrophotometry, Le., with respect to pure solvent. If the titration cell is placed in the light path and the absorbance scale reset to zero, either by adjustment of the monochromator slit width or of the sensitivity of phototube current amplifier, the change in absorbance, AA, on adding a titrant increment, AV, can be read directly from the absorbance scale. While i t would be possible to prepare a reference high absorbance standard which exactly corresponds t o the equivalence point mixture using the method of Ringbom, Sundman, and Vanninen (25, B6), i t is simpler to use the titrated solution as its own reference. As is shown belon-, there is a simple relationship between the spectrophotometrically determined quantities and the change in p H or 1111. The end point is located using the “second derivative” method, Le., A2pH/AV2 is calculated from ApH and AV values, and plotted us. V . The volume of titrant for which A2pH/AVz is zero is taken as the end point. The only requirement our technique places on the spectrophotometer is t h a t
there is no amplifier drift of light source intensity change during the time required to add the titrant increment and measure AA. These requirements are easily fulfilled n-ith modern instruments. THEORETICAL
Aqueous Acid-Base Titrations. T h e hydrogen ion activity determines t h e color of an acid-base indicator i n a solution through the reaction HI=H+ +I(1) and the paH can b p determined spectrophotometiicall! from the expression paH = p K a ~-‘r log { [ I ] ~ I / [ H I ] ~ H( I2\) I n Equations 1 and 2 chsige types are not indicated; ICHI represents the thermodynamic acid dissociation constant for HI, and f represents the activity coefficient of the species indicated by the subscript. Near the end point of an acid-base titration the ionic strength of the titrated solution remains almost constant, and the change in p H on adding an increment of titrant is given by ApH = paHz - paHl =
log( [IIZ/[Illi - log( [HIIZ/F~IIlJ (31 because it is a good approximation t h a t (fJl ( ~ H I ) P / ~ ~ I ) z ( ~= B I )1. I ‘The subscripts 1 and 2 refer to the solution before and after the addition of the increment of titrant. At the wivelength chosen for the titration AI
=
Exb[I!i -k ~ H I ~ [ H I ] I (4)
n here A, is the absorbance ineasured prior to that of a titrant increment, E is the molar absoqJtivitj- of the species indicated by the subscript, b is the cell length in centinieteis, and [I],and [HI], are the equilibriuni concentrations of the two indicator forme. On adding the titrant increment, AT.’, the absorbance becomes Az =
E1b[1]2 -k
EHI~[HI]z
(5) The quantity A d = A 2 - A I is determined using the solution as its own high absorbance reference standard and the method of differential spectrophoVOL. 34, NO. 8, JULY 1962
975
tometry as is described under Esperimental. Assuming negligible volume changes throughout the titration, C = [HI], [I11 = [HI]* [I]z, and it follow from Equations 3, 4, and 5 that
+
+
and ~A-4 _ _ aw _ = l _ _[HI12 _ AI - d~ [HI11 [HI11
(7)
if €1 # €HI. The absorbance which the indicator would have if i t \\-ere present completely in the acid form in the same volume of solution is --1HI( = EHI~C) and AI( = elbC) is the analogous absorbance for the basic form of the indicator. Equation 8, the general expression for ApH A?', is obtained by substituting Equation. 6 and 7 into Equation 3
d:..I ant1 A 4 H I are easily calculated from €1. C H I , and the indicator concentration, C (or determined spectrophotometrically from measurements well before and after the end point), the measurement of A.1 and -Ii permits the calculation of ApH AT-. From a practical vien-point, the ivavelength chosen should be such that el and €HI differ as much as possible ii a wavelength cannot be found nliere either molar absorptivity is zero. I n the case where only I absorbs, Equation 8 becomes
n-hile if oiily HI absorbs
Nonaqueous Acid-Base Titrations. Application of Equations 8, 9, and 10 is not limited to aqueous solutions. I n nonaqueous solvents of high dielectric const'aiit, acid-base indicators are measures of hydrogen ion act,ivity a n d these equations apply without change. I n a low dielectric constant solvent, such as acetic acid, indicator bases arc iiieasurcw of the equilibrium concentration of the undissociated free ncaitl, HS;( I S )
+
I H S e HI+Xbasic color acid color
(11)
Near the end point of the t,itration of a base in glacial acetic acid, (aHT)l,'(aH+)? = [HITS]l,[HI+S-],, since the equilibrium concentration of S- remains practically constant. As a result, the derivative curve measured by this tech976
ANALYTICAL CHEMISTRY
nique will be identical to that obtained using a hydrogen ion indicator electrode. Complexometric Titrations. The titration of a metal ion, hI, b y a complesing agent can be performed using the differential spectrophotometric technique with the aid of a metallochromic indicator. If the indicatormetal ion reaction is
+
hI I color 1
eM I
(12)
color 2
AplI AT' may be calculated as described above by substituting RI for H in the appropriate equations. Error in ApH Choice of Indicator. When this differential method is employed, indicators of widely different equilibrium constant can be used for the same titration because Equation 8 does not involve KHI. It is of interest to discuss the criteria for choosing KHI so as to minimize the error in ApH, d(ApH), which is associated with the uncertainty in the spectrophotometric determination of A A , diA.4). Differentiation of Equation 8 yields ~ A P H= ) 4Ad)
i\
1
2F-X
-
must be used to obtain a specified value of d 2 than is required if [Hf]..,. = KHI. Assume that - 4 ~ 1= 0, so that the entire absorbance is caused by the basic form of the indicator, that A ? = 1.0 a t the end point, and that = 100KHI. ThenAI = 101dzandd(ApH) = 1.01 d(Ad). I n the limiting case when [HI,,, >> KHI,d(ApH) = d ( L 4 ) . If A? = 1.0 and d ( A d ) = 0.0016, the limibing uncertainty in ApH is 0.00 15. The above d(ApH) values of 0.0030 and 0.0015 units correspond to hydrogen ion indicat'or electrode errors of 0.2 and 0.1 mv., respectively. It is of interest to compare the predicted errors with those observed using the MacInnes differential potentiometric technique (17 ) . Unfortunately, no quantitative stat'ements concerning the uncertainty in the measurement of AE appear in the literature, and original data are reported only infrequently. K e estimate that the error on A E is approximately 0.1 rnv. from the data given by Clark and Kooten (4) and from Figure 3 of Bates and Kichers' (8)work. Thus under optimum conditions, the differential spectrophotometric procedure is coinparable in sensitivity to X d n n e s ' differential technique.
i, (13)
+
where Az = -41 A.4. The minimum value of d(ApH) occurs when Az = (&I A1)/2, i.e., when [ H I ] = [I]. Thus, by choosing the indicator such that KaI = [ H I e p (hydrogen ion concentration a t the equivalmcc point), we obtain
+
The difference (ARI - AI) should be as large as possible to obtain the minimum error. This condition is met, for a specified value of A z a t t'he end point, if either AHI or .-iI are zero. When a Beckman Model B spectrophotometer is used, no difficulties are found a t the 1.0. Under t'hese end point if A 2 = 0 (or conditions -41 = 2.0 when nice ziersa) and d(ApH) = 2d(A.l). In most spectrophotornetcrs the error in the nieasurcd transniitt'ance, T , is a constant, dT, therefore d(A-4) = dT T , and d(A-4) is not constant. An upper limit is easily plawd on d ( A A ) by choosing Tmi, = 1 0 - ( A - 4 ) m w = 0.67, yielding d(A.-l) = 0.0015 when dT = 0,001. -4ssi.iining that cl(A.4) is 0.0013, d(ApH) = 0.003. A tn-ofoltl decrease in d(ApH) results when the indicator used is almost coinpletAy in one form a t the end point. For example, in the titration of a weak acid n-ith sbrong base, if the indicator HI is chosen so that [H+]..,. >-> KHI, and €1 >> 6x1 a t the warelengt'li used, a much higher indicator concentration
EXPERIMENTAL
Reagents. A11 chemicals not described below were of analytical reagent grade a n d were used without furt'her purification. Conductivity water was used t o prepare all aqueous solutions. Buffers. Clark's and Lubs's buffers Tvere used for determining all indicat'or spectra while the p H 10 buffer used for the calcium titrations was prepared according to Schrarzenbach (30). Titrants. Sodium hydroxide (0.9646.1.1) was prepared from filtered, saturated sodium hydroxide solution and freshly boiled conductivity water and standardized against potassium biphthalate using phenolphthalein as indicator. EDTA (0.1001X) was prepared according to Schnarzenbach (26') from a recrystallized Eastman Kodak product, 11-hite label. Perchloric acid in acetic acid (0.05X) was prepared by dissolving the requisite amount of 7100 aqueous perchloric acid in acetic acid and adding bhe stoichiometric amount of acetic anhydride needed to remove the nater present. This titrant ivas standardized against pot,assium biphthalate using crystal violet as indicntor. Indicators. The indicator solutions used in aqueous acid-base titrations were prepared according to Kolthoff and Rosenblum (14), the Eriochrome Black T according to Schwarzenbach (2Q),and crystal violet and p-naphtholbenzein by dissolving the solid in glacial acid. The spectra of both forms of all indicators were determined, and the molar absorptivities determined a t wavelengths suitable for use in the dif-
fercntial t'itration. Table I presents the values obtained and compares these data with previously obtained results. Apparatus. -4 Beckman Model B spectrophotometer was used for all Sorenabsorbance measurements. sen 0.017, a.c. voltage regulator was used t o stabilize t h e spectrophotometer against line voltage fluctuations. [ il I k c k m a n Model DU spect'rophotonieter equipped n-ith a DU power supply ( S o . 23700) is also siiitable.] Tlir spectrophotonietric titration cell used x i s descrilml by Bruckenstein and Selson ( 3 ) . Gilniont ultramicroburets, 1.0 antl 0.1 nil., n-ere used to deliver all titrants ant1 the same 10-nil. pipet and niicroburet n-ere used in all titrations, including standardizations, to eliminate volumetric calibration errors. A I3ec~kman Uodel H-2 pH meter equipped with micro glass and calomel electrodes n-as used for all potentiometric bitrations which were ($onducted under ponditions identical to the si,ectrophotomctric titrations. Did erential Spectrophotometric Titration Procedure. 'Ten milliliters of the s:iiiipIc n-as pipetted into t h r
lndicator BCG BCP
Wavelength Maximum Absorbance, Acid Basic form form 440 615 430 580
'1'B .IYR
440 3i5
596 495
titration cell and a n appropriate volume of stock indicator solution n-as added. Titrant was added until visible indication of approach to the end point was seen, or -99% titrated. Solvent was added to bring the volume up to the mark. The spectrophotometer n-as standardized a t t'he required n-ax-elength by placing the blank cell filled with solvent in the light path and adjusting the absorbance scale reading to zero. The t,itration cell was inox-ed in the light pat'h and the absorbance determined. This absorbance value is .I1. The titration cell n-as allonwl to remain in the light path and the absorbance scale adjusted to zero. hn increment of titrant was thcn added to the titration cell and the stoppered cell vas inverted and shaken several times. When the cell JVRS replaced in the light path, the absorbance was determined. This absorbance is A d . The above procedure was repeated to obtain successive values of --ti and A A . The second derivative h2pH,: ApH2 \\-as calculated and the end point found by graphical location of the volume a t which A 2 p H ; A P = 0.
RESULTS AND DISCUSSION
Aqueous Acid-Base Titrations. T o compare the relative sensitivit'ies of t h e differential spectrophotometric (DS) method with the usual pot'entiometric glass electrode technique, phosphoric, acetic, and several amino acids were titrated under identical conditions using both methods. A summary of the results obtained in the t'itration of phosphoric acid is shown in 'fable 11. Difffrent aliquots of t h r same phosphoric acid solution (0.056JI) n-ere titrated to the first and second cnd points ((experiments l a to Id) 11-hile aliquots of a second sampk m r e titratrd only to the second end point jcxperinients 2a t o 2c). 13roniocresol ljurple (13CP) and bromocresol green ( W G ) were used to locate the first eiitl 1)oiiit. n-hile thymol blue (T13) and =Iliaarin YelloJv R (AYR) were used to locatc the second end point. The precision obtained bj- the DS method Tvas escrllent antl the agreement between the DS and glass electrode titrations
Table I. Properties of Indicators RIolar Absorbance Molar Absorbance at Acid Maximum at Basic Maximum .kcid Form Basic Forni Acid Form Basic Forin LiteraLiteraLiteraMolar Ahsorbance a t Calcd.. ture. Calcd.. ture. Litera- Calcd., ture, Other Wavelengths Ilsed x 10; x 104 x 103' x 103 Calcd. ture X 10' x 10' 2 , 0 2 5.55 1 54 2.97 4 . 2 4 (basic) 1.94 2.12 4.80 3.11 4 34 x 101 1.3'2
2.22 2 . 36
x 102 12 x 103
2.15
2.67
1
2.14
4.05
2.3 3.11
x
PNU
4(i0
640
2.86
3.0 1 , 4 7 x 102
BCG = Bromocresol green (6, 20, 2 7 ) ) Fisher Scientific Co. BCP = Bromocresol purple ( 1 6 ) , Fisher Scientific Co. T B = Th>mol blue (20, 21), LIatheson Co. A I R = Alizarin Yellon I t ( 6 . 22, S I ) , Xlatheson, Coleman and Bell. Table II. Titration of
-
4.46
590 mp
(acid) X = 540 mg
1 36 X lo4
0-S.1
=
1 31 x 102
2 88 X lo1 2 15
(basir )
X = 540 nip X =5 '6
mp
x 103(CaEBT)
X = 635 n1p 1 00 X lo4 (EBT) X = 635 nip I'SB = p-Saphtholbmzein ( 1 3 ) , Eastman Icodak o - S A = n-Xitroaniline, Matheson Co.
CBT
=
Eriochrome Black T, Satlonal Aniline.
0.1M Phosphoric Acid Solutions
First Lquivalence Point ____ Eyuivaleiits x 104
Second Equivalence Point Equivalents x 104 Glass DS Glass Electrode Espt BCPb BCGc electrode TBd AIXe hYRf electrode 20 58 2a 20.3'3 Id 8 392 8 363 8 324 16 7'2 16 76 16 755 16 67 20.62 2b 20,5Y 8 392 8 363 8 :B2 16 74 16 74 16 747 16 67 113 IC 8 . :302 ... 8.370 16.74 16,750 2C 20 58 Id 8.392 16 74 16,750 20.59 hv. 8 .:392 8 : 363 8.360 16.73 16.75 16 750 16.6T 0,003 Av. dev. 0.000 0.000 0,024 0.01 0.01 0,002 0.00 All experiments labeled l a to Id were taken from same phosphoric acid solution and similarly for experiments '2a to 2c. ml. of 1 M XaOH for DS titrations. A V = 0.010 ml. for glass electrode t'itrations except as noted. b X = 590 nip, hroniocresol purple concentration = 1.78 X 10-4.Tl,AHI = 0.036, A I = 38.5, (Az- A H I ) ~ = , ~ 0.54, . ( A . - l L P . = 0.24. X = 615 mp, hroniocresol green concentration = 9.7 X l0-eAl1> A H I = 0. -11= 1.38, (-42 - HI)^.,,. = 0.65 (A-41e.p. E 0.21. d X = 596 nip. thl-mol Hue concentration = 1.48 x lO-5.lI, . 4 ~ 1= 0.016, -41 = 1.98, ( A z - A H I ) ~ .= ~ .1.36, (A.4)e,p % 0.27. e A = 540 nip. illizariii I'elloiv R concentration = 4.26 X 10-5.11, .-lnr = 0.028, r l ~= 2.9, (d42- A H I ) ~ = . ~0.40, (1.4)e.p. 0.16. f Same as e, but A V = 0.0025 ml., ( A A ) ~ ,=~ 0.10. , dame as d , but = 1.28, ~ 4 ~= . 0 .~2 2 , _
118 _
_
~ Glass
(1
Q
VOL. 34, NO 8, JULY 1962
977
was within the accuracy of the experimental technique used. Figure 1 shows a plot of AAl(A1 - AHI), ApH/AV and A2V/3pH2 for one of the DS titrations using AYR as indicator. The titrant increment used, 0.0025 ml., is 0,3y0of the amount of sodium hydroxide required to titrate H2PO4- to HP04+, and is one quarter the size of the smallest volume increment, 0.01 ml., which could be used in the glass electrode titration. The titrant increment used in Figure 1 IS sufficiently small that the plot of A2pH/AV2us. V approaches the limiting d2pH/dV2us. V curve. The commonly used procedure of drawing a straight line betneen the t\vo points lying on either side of the abscissa is inexact, but, in this case, should not lead to an error in estimating the equivalence point greater than 20% of AV(O.O6%). The mean of four titrations (Table 11) performed under the conditions used in Figure 1 has a mean deviation of O . O l ~ o . This result is fortuitous because the volumetric technique used introduces an uncertainty of a t least this order of magnitude. I n the titrations shown in Figure 1, ( A 2 - A H I ) p~. = 0.4, (AA), p. = 0.10, and AV = 0.0025 ml. Xo attempt was made to attain the ultimate sensitivity of the DS method. However, since the optimum value of ( A 2 - A H 1 ) e p is 1.0, increasing the indicator concentration by a factor of 2.5 would permit reducing A V by the same factor without changing (AA4)e I n addition, a further decrease in A V by a factor of 0.055/0.10 is possible, because as shown below in Table V, adequate second
v
Three amino acid solutions, 0.03X L-phenylalanine (pK = 8.6), 0.03M leucine (pK = 9.6), and 0.05X DLalanine (pK = 9.05), and dilute ( - ~ i O - ~ ? / 1 ) acetic acid were titrated with sodium hydroxide spectrophotometrically using AYR as indicator and potentiometrically using a glass electrode. A comparison of the results obtained with these two methods is given in Table 111 and demonstrates the precision and accuracy obtained with the differential spectrophotometric procedure. The value of d(ApH),,. was calculated for these titrations from Equation 14 assuming d(4-4) = 0.0015 and is listed in the final row of Table 111. I n a11 titrations a significant decrease in error is found as compared to a glass electrode titration using a pH meter with a reading error of 0.025 pH unit. KO effort was made in these titrations to adjust the AYR concentration to obtain the minimum error in (ApH), ; only in the acetic acid titration could a significant improrenient be obtained by increasing the indicator concentration. Chelometric Titrations. In some chelometric titrations, visual end point detection is difficult when known metal ion indicators do not undergo sharp color changes a t the equivalence point because of overlapping of the spectra of the two indicator forms or because t h e ~ K , I is very different from (phf), p . . I n such cases the DS method can be applied if a wavelength can be found for which cMI differs appreciably from eI. For example, while calcium can be titrated visually with
10 0
-I 0
-20
I73 I74 mi I M NaOH
I75
Figure 1. Differential spectrophotometric titration of -0.1 M phosphoric acid to the second end point [AYR] = 4.26 X IO%, (Az - Aa11e.p. = 0.4, (AA)e.p. = 0.10, and AV, = 0.0025 ml. A. Ordinate = 100 AA/(Al - AHI); B, ordinate = 4pH/4V;C,ordinate = (h2pH/4Vzl/1 00
derivative curves can be obtained when (AA)e,p. = 0.055. Thus a volume increment 0.22 times smaller than 0.0025 ml., 0.00055 ml., Lvould have been feasible. This volume increment is l/lsth of the smallest AV possible using the glass electrode, This result agrees closely with that calculated from Equation 4, assuming p H errors of 0.025 and 0.0015 unit using the glass electrode and DS methods, respectively.
Table 111.
Titration of Aqueous Acids with AYR ( X = 540 mp)
Found .kid Acetic acidb
Taken 9.92
L-Phenylalaninec*d
321
L-LeucineCse
303
DL-AlaninecJ
499
DS 9.99 9.97 9.99 9.98 (mean) 321 323 324 323 (mean) 303 303 303 303 (mean) 498 502
500
500 (mean)
Equivalents X 106 Mean deviation Glass Glass electrode DS Electrode 10.17 0.01 0.06 10.19
Error
D9 S0.06
Glass electrode d( A p H ) , , p , a +0.21 0.01
+2
-7
0,004
-3
0.004
- 22
0,005
10.04
10.13 (mean) 311 320 311 314 (mean) 302 301 298 300 (mean) 487 467 476 477 (mean)
1
4
0
1
0
1.2
-
+1
I
Calculated from Equation 13 using d ( A A ) = 0.0015. [AYR] = 1 . 7 1 x lO-'M, AHI = 0.109, AI = 11.6, (A2 - A H I ) ~ .=~ .0.13, ( L ~ A )=~ 0.05. , ~ , Standardized -0. 131 acetic acid was diluted 100 to 1 and 10 ml. of this solution titrated. A V = 0.010 ml. both methods, V , p , 'V 0.53 ml. [AYR] .= 2.08 x lO-SM, AEI = 0.03, A I = 1.42. Equivalents taken calculated assuming 100% purity in solid acids. A v = 0.05 nil. using DS method and 0.100 ml. using glass electrode. d ( ~ ~ - i 4 ~ ~ ) e . p . ~ 0 . 6 6 , ( A A ) B , , ,V~, 0. p. ,3N10 . 8 9 m l . a (A% - A H I ) ~ .'V~ .0 71, ( A A ) ~ ,-U ~ ,0.21, Ve,p = 0.83 ml. f (Az - A H I ) ~ .r~ ~ . 1.04, (AA)e,p, 0.13. V..p = 1.38 ml.
978
ANALYTICAL CHEMISTRY
versene using Eriochrome Black T (EBT) as indicator in the presence of magnesium, in the absence of magnesium the color change is so ill defined that visual titration is impractical. Hon ever, this titration can be performed in the absence of magnesium b y using the DS method at 635 mp because, as shon-n in Table I, E C ~ E B T = 2.2 X lo3 and EFBT = 1.0 X lo4in p H 10.4 buffer. The results obtained in the titration of 0.01JE and 0.002M calcium solutions are shown in Table IV and illustrate the evcellent accuracy and precision obtained under rather unfavorable conditions.
Table IV.
Indicator
=
2.76
Moles X 105 Takend Found Mean dev.
Error
d
Titration of Calcium with Versene
x
lO-5M Eriochrome Black T, = 635 mp Calcium Concentration O.OlM"~* 0.002JP~C 10.24 2.054 10.25, 10.26, 10.21 2.061, 2.064 (mean 10.24) (mean 2.062) 0.003 0.002 0.00
+0.008
-4EBT = 1.40, ACoEBT = 0.32. (AS - A c ~ E B T )% ~ . 0.9, ~ . ( L L ~= )~ 0.15, . ~AV . = 0.01 ml., ( V ) e . D= , 1.02 ml. (Az - A c ~ E B T ) ~ . 0.6, ~ : (AA)e,p.= 0.055, AV = 0.0025 ml., (V)e,p,= 0.21 ml. Corrected for indicator blank.
TITRATION OF BASES IN GLACIAL ACETIC ACID
Two acetous sodium acetatc solutions, -0.05.1E and -O.OOl11I, were titrated with perchloric acid b y the DS method using p-naphtholbenzein as indicator. The results shown in Table V are excellent for the -0.05X sodium acetate solution, and are l.S% high for the 0.001M solution. I n the latter case, a blank correction for the amount of perchloric acid needed to obtain the absorbance observed at the end point n-as made and amounted to 8% of the acid required to reach the end point. The explanation for the high result in the titration of the 0.001M sodium acetate solution is not apparent since the second derivative curves were well defined and reproducible. The significant result of this experiment is t h a t the DS method can locate the end point under conditions of high dilution where visual methods fail and potentiometric methods arr usable only with great difficulty. I n acetic acid, sodium acetate is one of the most dissociated bases. On the other hand, o-chloroaniline is very much less dissociated. When o-chloroaniline is titrated with perchloric acid using a hydrogen ion indicator electrode, comparatively small changes in potential occur a t the end point and rather large titrant volume increments must be used. Both Reilley and Schweizer (24) and Hummelstedt and Hume (12) titrated this compound by photometric methods reporting e~cellentresults. I n Table T'I a comparison is made of the results obtained by titrating the same sample of o-chloroaniline using all the above methods and the DS method using o-nitroaniline as indicator. All the spertrophotometric procedures are in fair agreement and yield results significantly larger than calculated on a weight basis. It seems probable t h a t the o-chloroaniline sample used in these titrations contained some lorn- equivalent weight, basic impurity. If at least two bases of different strength were present, the differences betn-een the various photometric techniques are understandable. I n support of this impurity hypothesis, the conventional
Table V.
Titration of Sodium Acetate with Perchloric Acid in Glacial Acetic Acid
Indicator = p-naphtholbenzein, = 640 mp Moles Sodium Acetate Concentration x 106 0.05~5 0 001.w Takenc 522.3 9.80 522.3,521.8, 522. 3, 522.3 [mean = 522.21 9.98,9.96,10.00 [mean Found Mean dev. 0.2 0.02 Error -0.1 +0.18 AEI = 2.20,Ax = 0.00, ( A Z ) ~ = . ~1.05,(AA)..,. .
=
0.55, Ve,*.= 1.05,AV
=
9.98Id
=
0.005.
* AHI = 4.84, A I = 0.00, ( A Z ) ~ =. ~0.72, . (AA)e.p.= 0.15, Ve,,,.= 1.02, AV = 0.010. 0 05M sodium acetate solution can be titrated visually with PNB.
This solution
was diluted 50-fold to prepare the l O - 3 M solution.
Blank correction of 0.83 X equivalent has been subtracted from amount found to allow for perchloric acid required t o give the absorbance of indicator acid form found at end point.
Table VI.
Titration of o-Chloroaniline with Perchloric Acid in Glacial Acetic Acid
Moles X lo4 Method DS" Glass electrode* Photometric, X = 312 mpc Photometric, X = 320 mMd Calculated (from weight) a
(Az
* 0
d
Found 9.570, 9.58, 9.57 9.41, 9.41, 9.41 9.56, 9.56, 9.57 9.49, 9.52, 9.52
Indicator = 1 05 X 10-4M o-nitroaniline, X = 400 mp, A I = 23, -0 32, AV 'V 0 01, V , = 1 96 ml. - A E I ) ~ . ~ 1 0, ( A A ) , AV = 0 05. Method of Reilley and Schweizer (84). Method of Hummelstedt and Hume ( l a ) .
photometric titration curves a t 320 mp showed some curvature not reported in the original work. The potentiometric method yields the lowest results of all. It seems likely t h a t this is caused b y the large volume increment required to obtain a measurable electromotive force change a t the end point. As shown in Table V, this volume increment corresponds to 2.5y0 of the total volume required to reach the end point. Under such conditions, the second derivative technique cannot be evpected t o locate the end point very precisely, and the observed results are not surprising. On the basis of the sodium acetate and the o-chloroaniline titration, the DS method can be used to detect end points in glacial acetic acid m-ith high precision
Average deviation 0.003
Average 9.57 9.41 9.57 9.51 9 34
:
0 003
0.013
lIJ1
=
0 100,
and should be generally useful in nonaqueous acid-base titrations. CONCLUSIONS
The differential spectrophotometric method has three principal advantages compared to the potentiometric methods: visible warning of approaching end point, permitting minimum number of data to be taken; absence of problems arising from the high resistance of low dielectric constant solvents; applicability in situations where reversible indicator electrodes are not available. The three principal disadvantages of this method are: inapplicability to precipitation titrations; necessity of finding a suitable indicator; necessity of makVOL. 34, NO. 8, JULY 1962
979
illg indicator blank corrections in dilute solutions. LITERATURE CITED
i l 'i A a s t i m . R..
Weherline. R.. Pallila. F.. i).
(3) Bruckenstein, S., Xelson, D. C., ANAL.CHEW33, 439 (1961). 14) Clark. B. L.. Tooten. L. A.. J . Phus. ?'hein. 33. 1468 11929).' ( 5 ) Connor;, K.- Ak,Higuchi, T., ANAL. CHEM.32, 93 (1960). (6) Fort,une, W. B., Mellon, 11. G., J . Am. Chem. Soc. 60, 2607 (1938). ( 7 ) Goddu. R. F., Runie. D. Y.. ANAL. CHEW26, 16T9 (1954). 18) Himichi. T . Rehm. C.. Barnstein. --a C., Zbid., 28, 1506(195k). (9) Hiskey, C. F., Ibid., 21, 1440 (1949). (10) Hiskey, C. F., Rabinowitz, J., Young, I. G., Ih&, 22, 1464 (1950). (11) Hiskey, C. F., Young, I. G., Ibid., 23, 1196 (1951). \ - I
~~
~ - >
~J
~~
(12) ,Humnielstedt, L. E. I., Hunie, D. S., Ibzd., 32, 576 (1960). (13) Kolthoff, I. M., Bruckenstein, S., J . Am. Chem. Sac. 78. 1 119561. (14)Kolthoff. I. iI.. 'Ro&nbl&n. G..
(24) Reilleg, C. X., Schweizer, B., Zbid., 26, 1124 (1954). (25) Ringborn, 8., Sundman, F., 2. anal. Chem. 116. 104 11939). 126) Rinehoh. A'. 1-anninrn. E.. .?no?.
(15) Kortum, G., Anqew. Chem. 50, 193 (1937). (16) Loomeijer, F. J., .-Lna/. Chim. d c t a 10. 148 (1954). 1171 'hlacInnes.'D. A.. Jones. P. T.. J . ' A m . Chem. Aoc. 48, 5831 (lb26). (18) ?\Ialmst,adt, H. V., Rec. Chew. Piogress 17, l(1956): (19) Malmstadt, H. T. ., Roberts, C. B., ANAL.CHEZI.28, 1408 (1956). (20) Mrllon, l f . G., Ferner, G. IT-., J . Phus. Chem. 35. 1025 11931). (21) kellon, 31.G,, Mehlig, 'J. P., Zbid., 35, 3397 (1931). ( 2 2 ) Sichols, 11.L., Kindt, B. H., .%SAL. CHEM.22, 785 (1950). (23) Rehm, C., Higuchi, T., Ibid., 29, 369 (1957).
Schooley, M . R., J . ilk. C', 70, 732 (1948). (28) SchJLarzenbach, G., *'Coniplexonietric Titrations." 1). 5 5 . Intrrscience. Sew Tork. 1957. (29) Ibid., p.' 57. (30) Ibid.,p. 58. (31) JToodland, W. C., Carlin, R. B., Warner, J. C., J . A m . Chem. Soc. 75, 5835 (1953). RECEI\EDfor review Februarv 9. 1962. Accepted April 19, 1962. Takkn in part from a thesis submitted by 11. 11. T. K. Gracias in partial fulfillment of the requirements for the degree of master of science. Work sponsored by the Office of Ordnance Research, U. S.Army.
Spectrophotometric Determination of Microgram Quantities of DivinyI Sulfone in Aqueous Media C.
R. STAHL
Cenfral Research laboratory, General Aniline 8, Film Corp., Easton, Pa.
b An alkaline aqueous solution of benzenethiol exhibits an absorption maximum at 262 mp in the ultraviolet region. This method is based on the reaction of divinyl sulfone with benzenethiol to form bis (phenylthioethyl) sulfone which precipitates causing a decrease in the ultraviolet absorption of the benzenethiol solution. The decrease in absorbance i s a measure of the amount of divinyl sulfone present. Although benzenethiol reacts with many compoundsi this method i s specific for divinyl sulfone since this i s the only compound which i s known to precipitate under the conditions of this procedure.
D
studies of cross-linking agents for cotton, a method was needed for the determination of trace quantities of divinyl sulfone in n-ater. .1 literature survey produced no niethods for the determination of divinyl sulfone in the parts per million range, and only one method for determining higher concentrations. Ford-Xoore. Peters, and Kakelin ( 4 ) outline a method for assaying divinyl sulfone gravimetrically as the bicysteine derivative. This method cannot be adapted to the determination of small concentrations of divinyl sulfone because of the solubility of the bicysteine derii-atil-e and the gravimetric nature of the method. Corrections for solu-
980
URING
ANALYTICAL CHEMISTRY
bility must be made even when high concentrations of the sulfone are being determined. Stahmann and covorkers ( 5 ) in studying reactions of divinyl sulfone follon-ed the disappearance of the vinyl groups by determining the decrease in thiosulfate titer or by measuring the drop in benzenethiol by titration with iodine. S o details of their procedures are given. Divinyl sulfone is a very reactive compound, and its reactions have been 1%idely discuwed ( I , 3-5). The reaction with thiols appeared to be the most promising for our purpose since it is rapid and complete. Stahmann et al.( 5 ) found that the reaction of divinyl sulfone n i t h benzenethiol, in the presence of base, is 99% complete in i minutes. The method of Beesing et ul. ( 2 ) for acrylonitrile by reaction n ith dodecanethiol \\as tried for dkinyl sulfone and gave good results for relatively high concentrations of the sulfone with slight modification to prevent the precipitation of the partially reacted sulfone. 13y using 0.001S iodine solution as titrant for the e w e v thiol, i t is possible to determine dil inyl sulfone in the parts per million range; honever, the accuracy and precision are poor because of the small titrations. 13is(phenylthioethyl)sulfone is very insoluble in n ater and precipitates when microgram quantities of divinyl sulfone react with benzenethiol in aqueous sodiuni hydroxide solution. The re-
moval of phenyl groups from solution by this process causes a decrease in absorption in the ultraviolet region. A rapid, precise method for determining microgram quantities of divinyl sulfone was developed on the basis of this decrease in ultraviolet absorption. PROCEDURE
A sample, containing 50 to 200 fig. of divinyl sulfone, is diluted to 5 nil. with distilled nater in a 50-nil. voluiiietric flask. One milliliter of a solution of benzenethiol in I S sodium hydroxide containing approximately 60 mg. of thiol per 100 nil. of SaOH is added. The flask is stoppeied and allowed to stand for 15 minutes a t rooni temperature. The contents of the flask are then diluted to 50 nil. with distilled nater. The absorbance of the solution is determined at 340 and 262 nip in a 1.0-cni. cell. X blank of 5 nil. of distilled n ater is treated in t h r same manner. The difference in the absorbance a t 340 and 262 mp is subtracted from the blank absorbance to obtain the decrease in absorbance due to the precipitation of the bia(pheny1thioethyl)sulfone. Llicrograms of diviiiyl sulfone are read from a standart1 curve of decrease in absorbance 2's. micrograms of divinyl sulfone prepared from knomn quantities of the sulfone, and parts per niillion divinyl sulfone are found by dividing micrograms of sulfone by grams of sample. .1lternately, parts per million divinyl sulfone ran be calculated using the expression
