Spectrophotometric Determination of Dibutyltin Dichloride ROLAND T. SKEEL' and CLARK E. BRICKER Department of Chemistry, Princeton Universify, Princeton,
b A method has been developed for the determination of small amounts of dibutyltin dichloride in the presence of mono-, tri-, and tetrabutyltin compounds and several inorganic ions. Diphenylcarbazone reacts with the dibutyltin compound a t a p H of 1.8 to give a 3 to 1 complex with an absorption maximum a t 530 mp. Butyltin trichloride, the only one of the n-butyltin compounds to interfere a t this pH, is removed b y extraction with (ethylenenitri1o)tetraacetic acid (EDTA). The procedure developed is applicable in the range of 3 to 150 pg. of dibutyltin dichloride.
R
using organotin compounds as bactericidal agents in hospitals have made their analytical determination imperative, as these conipounds are toxic to humans (4) and other animals (2) as m-ell as to microorganisms (6-8). Farnsworth and Pekola have developed a titrimetric and colorimetric method for determination of the tin in organotin compounds after destroying the organic material (6). Polarographic methods for determining organotin compounds in mixtures have also been suggested (IO). I n order to separate a mixture of organotin compounds, Barbier et al. have suggested paper chromatography ( 2 ) . Aldridge and Cremer described the determination of di- and triethyltin chlorides with dithizone and mention that other organotin conipounds behave similarly (1). Snabk reports that diphenylcarbazone gives a red color with dibutyltin dichloride a t a pH of 8.4 (9). This paper deals with the quantitative determination of dibutyltin dichloride with diphenylcarbazone. ECENT EXPERIMESTS
N. 1.
grade chloroform mas used to dissolve the organotin compounds and the diphenylcarbazone. Stock solutions were made as follows. DIPHENYLCARBAZONE. About 0.5 gram was dissolved in 250 ml. of chloroform. ORGANOTIN COMPOUNDS. Four SOlUtions were made, each consisting of about 0.3 gram of the organotin compound per 250 ml. of chloroform. I n the case of D B T D the resulting solution was diluted 1 to 10 with chloroform before using. TRICHLOROACETIC ACID-SODIUMHYDROXIDE BUFFER. 32 grams of acid and approximately 6.5 grams of base were dissolved in 1 liter of water. The p H of the resulting solution was adjusted to a p H of 1.80 f 0.05 by the addition of either acid or base. EDTA. About 4 grams of the disodium salt of E D T A were dissolved in 1 liter of water. When the time came for its use, this solution was mixed 1 to 1 with the trichloroacetate buffer, and the p H of the resulting solution was adjusted to 2.05 f 0.02 with 0 . 2 5 trichloroacetic acid. All absorbance measurements except those used to determine absorption curves were made on a Beckman Model B spectrophotometer. The absorption curves were obtained with a Warren Spectrocord Standard matched 1.00em. Corex cells were used throughout. The p H measurements were made on a Leeds & Northrup Model 7664 p H meter. Absorption Characteristics. Dibutyltin dichloride and diphenylcarbazone react to give a colored complex
c
0.4
EXPERIMENTAL
Reagents and Apparatus. Butyltin trichloride (BTT), dibutyltin dichloride (DBTD), tributyltin chloride (TBTC), and tetrabutyltin (TBT) were obtained from Metal and Thermit Corp. and were used without further purification. ACS reagent 1 Present address, University of Cincinnati Medical School, Cincinnati, Ohio.
428
ANALYTICAL CHEMISTRY
Absorbance
a i
4 0 0 m*l
Figure 1. Diphenylcarbazone calibration curve for reagent correction
that has a n absorption mayimum a t 530 mp. Because diphenylcarbazone alone, which has a maximum absorbance at 560 mp, also absorbs strongly at 530 mp, some compensation must be made for the unused reagent. A high concentration of diphenylcarbazone must be used to get maximum color development with DBTD; therefore, both the reagent blank and the solution containing excess reagent and complex exhibit high absorbances at 530 mp. Thus, to measure accurately the amount of complex in a solution containing excess diphenylcarbazone, it would be necessary t o measure the absorbance of this solution against a reference solution that contained only the excess reagent. Because it is virtually impossible t o measure precisely these amounts of a chloroform solution of diphenylcarbazone, a n alternative method for correcting for the absorbance of the excess reagent was sought. When a 1 to 1 dilution of chloroform is made with ethyl alcohol, the absorbance of diphenylcarbazone alone is reduced to one eighth of the value it would have if the dilution had been carried out with chloroform On the other hand, the absorbance of the DBTD-diphenylcarbazone complex under identical dilution is reduced by only one half. With the absorbance of the reagent thus reduced, a calibration curve can be made of the absorbance of the reagent a t 530 mp us. the absorbance at some wave length A here the reagent but not the complex absorbs. h wave length of 400 mp suits this purpose well; a n absorbance reading of the sample a t this wave length allows one to find the corresponding absorbance of the reagent alone a t 530 mp from the calibration curve (Figure 1). Thus, one can subtract the calculated reagent absorbance at 530 mp from the one observed for the sample and find the net absorbance of the complex. Recommended Procedure. Add 1 t o 6 nil. of the organotin compounds in chloroform t o 10 nil. of the E D T A solution buffered as described above t o a p H of 2.05 i 0.02. If less than 6 ml. of the chloroform solution have been added, add sufficient chloroform t o bring the volume of the organic layer to 6.00 ml. Total D B T D concentration should be between 3 and 150 pg. and other butyltin compounds should not exceed 1.5 mg. each. Shake the mixture moderately about 30 times, draw off the chloroform layer into 10 ml. of fresh EDTA solution, and shake
L m
4
Q
00 ) 0
I
I
I
I
to
20
30
40
Reagent
C once niration
Figure 2. Dependence of diphenylcarbazone complex absorbance on reagent concentration Reagent concentration is expressed in moles per liter X DBTD concentration remained 2.03 X 10-5 throughout
again. Repeat a third time, finall:drawing off the chloroform layer into 5.0 ml. of 0.2X trichloroacetate buffered to a pH of 1.80 rt 0.05. .4dd 5.00 ml. of diphenylcarbazone solution and shake the reaction mixture moderately about 50 times. Dram- off a sufficient amount of the chloroform layer to fill a 10-ml. volumetric flask to which 5.00 ml. of absolute ethyl alcohol have already been added. Read the absorbance at 400 and 530 nip and determine the net reading a t 530 mp by subtracting the absorbance of the excess reagent determined from Figure 1. One EDTA extraction is sufficient to remove potentially interfering inorganic ions, if indeed they would be soluble enough in the chloroform to cause trouble. If neither the inorganic ions nor BTT is present, the EDTA extractions are unnecessary and much greater accuracy can be obtained.
work equally well, showing absorption for both the DBTD and TBTC, whereas only DBTD gave any color a t lon-er pH's. I n preliminary work with diphenylcarbazone, all four organotin compounds were tested with diphenylcarbazone a t p H 2 with a trichloroacetate buffer and a t pH 8 with a phosphate buffer. Approximately twice the molar concentrations of reagent as organotin compounds w r e used. Under these conditions only the dibutyltin dichloride gave any evidence of having reacted at either pH. Because the peak a t pH 2 (530 mp) was considerably stronger than the one a t pH 8 (520 mp)-an absorbance of 0.26 compared to 0.19-further study was carried out near a p H of 2. Since a pH of 1.8 gave increased buffering capacity to the trichloroacetate as well as a good reaction, it was used in all subsequent reactions. Because of the extreme sensitivity of dithizone to various inorganic ions, the difficulty of preventing its contamination, and the fact that diphenylcarbazone is nearly as sensitive as dithizone to DBTD (about 700/0), all further work n as done using only diphenylcarbazone as the complexing reagent. Interferences. Once the optimum p H was found, i t was necessary to
Table I. Variation of Absorbance of Dibutyltin-Dithizone Complex with Changes in Buffer Concentration at PH 5 (06 ,.g. of DBTD were reacted with 90 p g . of dithizone)
RESULTS
Determination of
Wave Length
Optimum pH.
Preliniinary irork with dithizone shon-ed t h a t the mavimum absorbance obtainable upon reacting dibutyltin dichloride with dithizone was dependent not only upon the p H of the buffer, but upon the concentration a n d the composition of the buffer as n-ell. At a p H of 5 with acetate buffer, the absorbance of the dibutyltin-dithizone complex was stronger a t buffer concentrations of 0.02N than of 0 . 2 S . The peak of the curve had also shifted from a maximum a t 510 mp to a maximum a t 496 mp. As shown in Table I, more dilute acetate buffers resulted in even greater color change. The dependence upon the composition of the buffer was shown using chloroaretate and acetate buffers a t a pH of 4. As shown in Table 11, the chloroacetatebuffered solution gave a stronger absorption than the one buffered with acetate. Similar results were noted at a pH of 2 where trichloroacetate-buffered solutions gave more color than those buffered with chloroacetate. With a phosphate buffer, pH's of 6, 7 , and 8
Buffer Concn.,
M
0.2 0.1 0.05 0.02 0,002
Table II.
PH 2 2
of
Maxi- Absorbmum, ance at hfp Maximum Color 510 0.26 Purple-pink 500 0.27 Purple-pink 498 0.295 Paleoink 496 0.31 PinkIorange 495 0.36 Orange
0.0 0
1
2
Reagent
3
4
5
Concentration
Figure 3. Dependence of BTT-diphenylcarbazone complex absorbance on reagent concentration Reagent concentrations expressed in moles per sample X 1.1 X 1 0 - 6 mole of BTT used throughout
determine the amount of reagent necessary to drive the reaction between the D B T D and the diphenylcarbazone to completion. This was done by taking a constant D B T D concentration, and adding successively larger amounts of reagent. The results, shou-n graphically in Figure 2, indicate that it is necessary to have about 100 times as much reagent as DBTD in the final solution. At this concentration of diphenylcarbazone the BTT also reacted. The absorbance a t a constant concentration of BTT and varying concentrations of diphenylcarbazone is shown in Figure 3. Apparently several different complexes are formed. TBTC and TBT gave negligible interference, allowing determination of the DBTD in the presence of a t least fifteenfold excesses of these compounds without using EDTA. Data illustrat-
Variation of Absorbance of Dibutyltin-Dithizone Complex with Different Buffers
(66 p g . of DBTD were reacted with 90 pg. of dithizone) Wave Length of Absorbance Buffer" Maximum, M p a t Maximum Color 505 0 30 Pink TC-4 485 0.18 Blue-purple CA
-".
447
4 4 6
7
A P P P
49 7
475
0.31 0.21
492 492 492
0 32 0.32
0.32 TCX, trichloroacetate; C h , chloroacetate; -4,acetate; 8
a
CA
Pink Blue-purple Pink Pink Pink P, phosphate.
VOL. 33, NO, 3, MARCH 1961
429
ing this type of determination are given in Table 111. The absorbances shon-n in Table 111 for TBTC and T R T alone could well be due to an impurity of DBTD in these samples. For example, the 0.01 absorbance observed from 0.720 mg. of TBTC would indicate less than 2 parts
Table Ill.
DBTD
per thousand of DBTD. The discrepancy between the observed and expected absorbance is never greater than 7%. EDTA \vas used to extract the BTT, which was found to be highly p H dependent. Both UTT and DBTD are extracted with phosphate buffers at p H values of 6 or above, while only BTT
Determination of DBTD in Presence of TBTC and TBT
Sample Weight, Mg. TBTC
Absorbance4 Exptd. Obsd.
TBT
0.132 0.106 0,066
1.10 0.89 0 60
0 . 720 0,132 0.106 0.106 0.106 0.066 0,066 0.066 0.026 0,026 0 .G26
1.11 1.10 1.11 0.895 0.89 0.89 0.61 0.61 0.608 0.225 0.225 0,225
:
0 692 0.553
0.575 0.360
..
0.346 0.138
0 :i44 0.720
o:i&
...
0.57R
0.346
0.360
0:553
o:ii+
0,692
I .08 1.107 1 05 0 83
0.85 0 83 0.65 0.63
0.62 0.248 0.215 0.22
a Organotin compounds used here vere dissolved in alcohol. Since the entire procedure used n-as somewhat different from the recommended one, the absorbances here cannot be compared nlith the absorbances in Tables IV or V.
Table IV.
Effect of pH on EDTA Extractions of BTT and DBTD
Buffer Trichloroacetate
PH 2.0 3.0
Chloroacetate
4.0
Acetate
6.0
Phoswhate
8.0
Phoephate
Organotin, M g . 1 55. BTT 0.137, DB' m 1.55, BTT 0 137, DB' I'D 1.58, BTT 0.137, DBTD 1.55. BTT 0.137, DBTD 1.55, BTT 0.137, DBTD
Net Absorbance 0 400
0,G o b 0.50 0.92 About 2.50 0.90 0.13 0.38 0.05 0.60
Untreated BTT after reacting with diphenylcarbazone has an absorbance of about
2.50.
Untreated DBTD after reacting with diphenylcarbazone has an absorbance of about
0.90.
Table V.
DBTD 0.134 0.134 0.134 0.134 0.096 0.096 0,096 0.096 0.041 0,041 0.041 0.041 0.014 0.014 0.014 0.014
430
Determination of DBTD in Presence of BBT, TBTC, and TBT
Sample Weight, Mg. BTT TBTC 1.55 1.08 0: 775 0.310 1.55
...
1.08 0.775 0.310
...
1.55 1.08 0.775
0 :3io
ANALYTICAL CHEMISTRY
0.935
;
0 si0 0.268 1.34 0:9i, 0.670 0.268
...
1.34 0.935 0.670 0 268 1.34
DBTD TBT
Found
Error, Mg.
...
0.136 0.126 0.126 0.129 0,096 0.108 0.090
$0,002 -0.008 -0.008
0.78 0.312 1.56
...
1.09 0.780 0.312
...
1.56 1.09 0.780
0:3i2 1.56 1.09
0.100
0,041 0.032 0 035 0.044 0.011 0.008 0.005 0.012
-0.005 10.000 $0.012 -0.006 +0.004 1 0 .000 -0.009 -0.006 +0.003 -0.003 -0.006 -0.009 -0.002
was extracted a t lower pH's. The data shoring the absorbance of B T T and DBTD complexes after first extracting the chloroform layer containing the organotin compound n i t h EDTA a t various pH's are given in Table IV. Because the DBTD did not appear to be extracted a t p H 2, a more detailed study was made a t pH's in that range. Chloroform solutions containing 0.137 mg. of DRTD, 1.55 mg. of BTT, 1.34 mg. of TBTC, and 1.56 mg. of T B T were extracted \yith EDTA solution buffered a t several pH's betmen 1.80 and 2.40. Complete removal of B T T and no loss of DBTD were realized a t a p H of 2.05 i 0.02 after three extractions n-ith equal volumes of fresh EDTA. Varying the p H of the EDTA by as little as 0.1 from pH 2.05 caused the apparent recovery of DBTD to be 15 to 207, high, indicating incomplete extraction of BTT. Since at a pH of 2.05 the EDTA is likely to precipitate, it is beet to make the properly buffered solution immediately before using. If on standing the EDTA does precipitate out, heating to about 85" C. nil1 dissolve it, and the supersaturated solution obtained on cooling to room temperature is suitable for extraction. Even slight amounts of precipitate may hurt the extraction, so care must be taken to have none present. Some typical results obtained from the analysis of mixtures of butyltin chlorides using the recommended procedure are shonm in Table V. These values indicate that small amounts of DBTD can be determined even in the presence of greater than 100 times as much of other butyltin chlorides. The amount of DBTD found tends to be generally low by about 5 pg., nhich is undoubtedly due to manipulative losses in the procedure. Beer's Law and Properties of Complex. T h e absorbance of the dibutyltin dichloride-diphenjlcarbazone complex obeyed Beer's law with samples between 2.7 and 137 fig. On the basis of the molecular weight of DBTD and considering that only 5/11 of the sample in the recommended procedure is actually taken for measurement of the color, the molar absorptivity is calculated to be 44,300. The empirical formula of the organotin diphenylcarbaxone complex was determined by the slope ratio method and n-as found to be (DBTD)(diphenylcarbazone)3. An effort to determine the dissociation constant of the complex in chloroform was made. From the concentrations of reagent and DBTD present a t the three points marked on Figure 2, from the empirical formula of the complex and from the absorbance observed at these points compared to the maximum absorbance that the weight of DBTD could produce, values for the dissocia-
tion constant were calculated. These varied from 2 X 10-l2 to 18 x lo-'*, the most reliable figure being about 9 x lo-'*. DISCUSSION
The greatest obstacle to be overcome in the BTT extraction is the carrying over effect of the TBTC and the TBT. For example, a t a pH of 2.2, BTT is extracted well by itself, but when the higher butyltin compounds are present, they are extracted into the chloroform layer and tend to pull along the BTT, From a practical standpoint, it should be possible to remove and concentrate butyltin chlorides from various materials by a chloroform extraction. Even though it is unlikely that inorganic ions as such would be extracted appreciably with organotin chloridcs, a brief study showed that one extraction with EDTA was sufficient to keep the following ions from interfering with the determination
of DBTD: Zn(II), Mn(II), Fe(I1, III), Pb(II), Cu(1, 11), Cd(II), and Sn(I1, IV) . The success of the recommended procedure in being able to determine only DBTD in the presence of a mixture of the various butyltin chlorides is due to the selective extraction of BTT with EDTA. As noted in Table IV, DBTD as well as BTT is extracted in varying amounts a t the higher pH's. Conccivably the TBTC would not be extracted within this range and it could then be determined with dithizone by the method of Aldridge and Cremer ( 1 ) . These methods should be investigated for other series of alkyltin chlorides. ACKNOWLEDGMENT
One of us, R. T. S., gratefully acknowledges the financial assistance received from a National Science Foundation Undergraduate Research Assistant-
ship. The organotin compounds were supplied by Metals and Thermit Corp. LITERATURE CITED
(1) Aldridge, W. K., Cremer, J. E., Analyst 82,37-43 (1957). ( 2 ) Barbier, R., Belluco, U., Tagliavini, G., Ann. chim. (Rome)48, 940 (1958). (3) Barnes, J. M., Stoner, H. B., Brit. J. Ind. Med. 15 ( l ) ,15-22 (1958). (4) Brit. M e d . J . 2,639 (1954). (5) Farnsmorth, M., Pekola, J., ANAL. CHEM.31,410-14 (1959). (6) Faulkner, C. J., Brit. Patent 743,119 (July 27, 1955). (7) Hudson, P. B., Sanger, G., Sproul, E. E., J . Am. Med. Assoc. 169, 1549-66 (1959). (8) Osmundson, J. A,, N . Y . Times 1, March 28, 1959. (9) Snable, G. L., senior thesis, Princeton University, May 1959. (10) Toropova, V. F., Saikina, bl. K., Sbornik Statei Obshchei Khirn. Akad. Nauk. S.S.S.R. 1, 210 (1953).
RECEIVEDfor review June 17, 1960. Accepted November 7 , 1960.
N e w Spectrophotometric Method for Molybdenum ARTHUR H. BLACK and JAMES D. BONFIGLIO' Department o f Chemistry, University o f Toledo, Toledo 6, Ohio
,b A spectrophotometric method is presented for the rapid determination of molybdenum in low-alloy steels. Molybdenum is separated from the bulk of the sample b y use of Amberlite IR-l20(H) resin, after which the complex of molybdenum and phenylfluorone is formed. The red-orange complex follows Beer's law and exhibits a maximum absorbance at 550 mp over a range of 0.33 to 1.67 pg. of molybdenum per ml. This method produces excellent results for low-alloy steels containing between 0.1 and 0.5% molybdenum.
C
OLORIMETRIC METHODS for
the determination of molybdenum have been reviewed recently (1, 5, 8, 9). Khere applications to lon--alloy steels have been made, either precipitation or extraction procedures were involved in the separation of the molybdenum from the bulk of the sample. This investigation was instigated to develop a new and different colorimetric procedure for molybdenum. Luke (3) and Luke and Campbell (4) elucidated photometric methods for tin and germanium with the reagent phenylfluorone. The molybdate ion in each 1 Present address, National Cash Register Co., Dayton, Ohio.
of these procedures was an interfering color-producing substance. As a result of the literature review, this investigation became concerned with the feasibility of an ion exchange method for the separation of molybdenum (6, 7 ) and the subsequent determination of the molybdenum as the phenylfluorone complex. APPARATUS AND REAGENTS
Colorimeter, Spectronic 20, Bauhch
8: Lomb.
I O N EXCHANGE COLUhfN. A Jones Reductor tube, 18 mm. in inside diameter, packed to a height of 15 em. with Amberlite IR-12O(H) cationic resin of 50 to 100 mesh. Prior to use the column was eluted with 250 ml. of 4iv hydrochloric acid. The excess acid was removed from the exchanger with deionized water until the eluent was neutral t o litmus. ~IOLYBDEXUII SOLUTION.A standard solution of molybdic acid equivalent to 1 mg. of molybdenum per ml. was prepared by dissolving 1.8i50 grams of molybdic acid monohydrate (Climax Molybdenum Co., Climax, Colo.) in deionized water and diluting to 1 liter. The solution was standardized gravimetrically by precipitation with 01benzoinoxime ( 2 ) . PHENYLFLUOROSE SOLUTIOK.A solution was prepared by dissolving 0.075 gram of the solid (Eastman Organic Chemicals) in 75 ml. of methanol and 1 ml. of 12N hydrochloric acid. The
solution was diluted to 500 ml. with methanol. Guhi ARABIC. One gram of gum arabic was dissolved in 100 ml. of boiling deionized water, filtered, and stored for use. PROCEDURE
For steels containing between 0.1 and 0.5% molybdenum, weigh a 1-gram sample, place in a 250-ml. beaker, and dissolve in 50 ml. of 1 t o 5 sulfuric acid. If a standard calibration curve is being run, a t this point add 1, 2, 3.5, and 5 ml. of the molybdic acid standard solution to each of four 1-gram samples of a low-alloy steel which contains no molybden urn. Add a minimum of nitric acid (1 ml. for a 1-gram sample) to the hot solution. Boil gently to remove the oxides of nitrogen, dilute to 100 ml. with deionized water, and add 1 gram of ferrous ammonium sulfate to reduce any vanadic or chromic acids present. Add 50 mg. of citric acid ( 7 ) , and transfer the sample to a 250-nil. volumetric flask. Dilute to approximately 200 ml. Adjust the pH to 2 with 257, sodium hvdroxide. using a narron- range pH paper. Dilute t o volume Ind mix thoroughly. Pipet a 25-m1. aliquot into the ion exchange column. Place a 500-ml. glass-sGppered graduated cylinder containing 30 ml. of phenylfluorone and 5 ml. of gum arabic beneath the column. Allow the solution to stand several minutes in the column, then let it drain VOL. 33, NO. 3, MARCH 1961
rn
431
