Add one scoop (approximately 200 mg.) of Cal Ver I and titrate the clear yellow solution with EDTA to a purple end point. (Approximately 0.5 ml. before the end point, the solution becomes redviolet. The end point is reached when one drop of EDTA produces the last visible color change.) P a r t I1 (in presence of interfering iron and cobalt). Weigh a 0.1-gram sample into a 400-ml. borosilicate glass beaker and dissolve in 10 ml. of 1 t o 1 nitric acid. When most of the sample is in solution, add 5 drops of hydrofluoric acid and 10 ml. of perchloric acid. Heat t o strong fumes of perchloric acid. Dilute with 150 ml. of mater, add 5 ml. of hydrochloric acid, and heat to complete solution of the salts. To the hot solution, add small portions of sodium sulfite until the yellow ferric iron is reduced to the colorless ferrous state. Filter through a No. 31, 11-cm. Whatman paper, collecting the filtrate in an 800-ml. beaker, and wash the precipitate two or three times with n arm water. To the filtrate add 9 ml. of mercaptoacetic acid, 5 grams of sodium sulfite, 330 nil. of water, and 20 ml. of ammonium hydroxide. Heat the solution to 60 O C. and, while vigorously stirring, add 50 ml. of dimethylglyoxime disodium salt. Let stand for 20 minutes at 45” C. Filter through a No. 31, 11cm. Whatman paper, and wash the precipitate with hot water until the nashings are colorless. Add 25 ml. of concentrated hydrochloric acid and 5 nil. of water to the 800-ml. beaker and heat to boiling. Place a 400-ml. beaker
under the funnel and dissolve the precipitate through the paper with the hot mixture. Wash the paper with hot water. Dilute the filtrate to 150 to 200 ml. and continue as in Part I (fifth paragraph) , beginning with the addition of 5 ml. of tartaric acid solution. DISCUSSION
Application of Procedure. The proposed procedure has been applied to a Fide variety of nonferrous standard samples and mixtures of standards issued by the Xational Bureau of Standards and the Bureau of Analyzed Samples, Ltd. (British). Typical results are shown in Table VI. (Each result represents a single determination.) The total elapsed time for a single determination is approximately 11/2 hours. Although this does not represent a large saving of time when conipared with the time required for the cyanide method, the method is useful because of its other advantages. These include use of a stable titrant and elimination of hydrogen sulfide. Results. E w e p t for samples which contained less than 0.16% nickel, the results obtained over the entire range of nickel tested (66.3S70 maximum) are in good agreeinent with the values sought. The results obtained for samples n-ith less tlian O.l6Y0 nickel tended to be lon-; therefore, the method is not recommended for this percentage range. Precision and Accuracy. *4 statis-
tical analysis was made of 25 results for 14 different samples in two percentage ranges--0.16 t o 0.60% and 30.08 to 66.38y0. Only results obtained by the application of the procedure to certified standards from the Yational Bureau of Standards and the British Bureau of Analyzed Samples were used. For the 0.16 to 0.60% range, on the basis of 16 determinations, the limits of error based on the $57, confidence level are =!=0.03%. For the 30.08 to 66.38% range, on the basis of 8 values, the limits of error based on the 9570 confidence level are 10.217,. h test for bias shon-ed no constant error. (Samples containing less than 0.16% nickel were not included in the tests because of a constant bias towards lo^ results.) LITERATURE CITED
(1) Flaschka, E., Chemist A n a l y s t 42, 84-6 (1953). ( 2 ) Harris, W. F., Sweet, T. R., ASAL.
(3) Hillebrand, W.F., Lundell, G. E;. F., Bright, H. A., Hoffman, J. I., “Applied Inorganic Analysis,” 2nd ed., p. 289, Wilev, New York, 1953. (L
(:
I
A n a l y s t 44,‘50 ‘( 1955). (6) Kinnunen, J., Wennerstrand, B., Ibid., 44, 33 (1985). ( 7 ) Trepka-Bloch, E., Ibid., 43, 63-5 (1954).
RECEIVED for reviev June 26, 1959. Accepted November 4, 1959.
MuItif uncti o n Automatic Recording Photometric Titrator P. W. MULLEN and ANTHONY ANTON lexfile Fibers Department, E. 1. du Ponf de Nemours &
b Photometric equivalence point detection methods are assuming an important role in titrimetry because of their generally wide applicability, particularly where theoretical or practical conditions prevent the use of potentiometric or arnperometric methods. A versatile automatic recording photometric titrator, suitable for colorimetric, nephelometric, and chemiluminescent indicator titrimetry, has been assembled from a Precision-Dow recording potentiometric titrator, a modified Beckman Model B spectrophotometer, and necessary intermediate electronic components. The operation of the titrator is described and typical titration curves are shown. ITRATION
Tmetric
systems utilizing photomethods of equivalence
Co.,Inc., Wilmington, Del.
point detection are rapidly becoming important in the analytical laboratory. The theory and important applications of the method, including use of the ultraviolet region and of chemiluminescent indicators, have been given by Goddu and Hume (3, 4), Headridge (6),and Underwood ( 1 2 ) . Instruments heretofore described for photometric equivalence point detection have utilized laboratory-constructed or comniercially available filter photometers such as those made by Sargent (?‘), and spectrophotometers including the Beckman llodel B (3, 4),Model DU ( I ) , and the Cary ( 2 ) . These instruments were modified to permit insertion of a titration assembly into their cell compartments. The equivalence point is indicated in some cases by merely a sharp change in the photocell current n liich can be used to trigger
a solenoid-operated buret valve and in others by continuous indication of the absorbance of the solution a t a n appropriate wave length. The titration plots have been recorded manually and automatically including means of electronically producing first ( I S ) , second (Y), and even third derivative (6) curves as an indication of the equivalence point. However, no integrated photometric titration unit has been described which is equally adaptable to the detection of equivalence points using visual, nearultraviolet, and chemiluminescent indicator systems, which directly plots absorbance of the solution vs. milliliters of titrant added, and \\hich is easily assembled from readily fabricated or commercially available units. Such a unit (Figures 1 and 2 ) has been available in this laboratory for over a year VOL. 32, NO. 1 JANUARY 1960
0
103
and fornu ti portion of a vcrsstilc niultifunction titnition console now under tlevelopinrnt. Its design, operation, ani1 use form tlir subjcct of this pnpcr. TITRATION SYSTEM
A Precision-Dow recording titrator, a coinniercial version of the instrument described by Robinson ( I I ) , provides the titrant-delivery s p t c n i , recorder, and a portion of the electronic circuitry. The deliwry system normally comprises t,n.o scpamt.c! I-! B-, or 50-ml. precisionhore gl:iss feed punips, glass titrmt ciclivery lines with buret tips, tlirecivay stopcocks, and sir’ion tubcfi. Tlic right or lcft fcwl sj.stcrn is selochl by n i ~ ~ n i i ~shifting i l l j ~ the sclrctor knob on the cont,rol pnnel. I n tlic present applic.ztion, oiily 0110 iiiilivcry u n i t is utilized and the coiiibiiiation titrnnt dclivcry tube : m i buret t.ip norninlly strpplrcd with tlic titrxtor is rcp1:icctf b y one which 1c:irls direetly from tlic fced puiiip to the titr:ttion :issenibly located i n thc modified cc.11 wr:ip:rtmcnt of the spectro~~hoto~iietcr (scc below). ‘i’his arlniigrincnt is estrcniclg usefiil, liccniisc: it pcrinits me of tlic PrccisionDon. instriiinmt RS n norind potentionictric t8it,r:it,ion unit utilizing t!ic nonnior.1i ficd feed ii ni t \vi thou t disfissernhly of t!ic I)liotoiiictric cquipinent. ‘ I * h pluiiper of the Eceti ~ I U I Y I Jis~ nutoiii:itit>:illy c.Iiiven by :L rclny-opcmted motor ivitli t,ke Fced rntc beiiiy adjustable to n I I I R S ~ I I I ~ I I IofI 3 1i11. JXT riiinirte using tile !@-nil, Ecccl pump. Tlic upper limit of pl~ngcrtjravol is controllcd by a limit switch which protects t l i c feed p u n i p :\rid o t l w glnss\v:~rcfroin tlaiiiago. If tit,r:iiit, niiw lo\\ during a tittxtion, thc feed punip !nay be rc~illcdrind the titration resumed without’ loss of the samplc. 13eca:ise t’hc titrant feed and chart advsnciiig s ~ s t e m s are mechnnicnlly linked, the chart rcflcct.s the nctual amount of titrant added. One major chnrt division (approximately 23 mm.) equals 1, 0.2, or 0.05 ml, of titrant, depending on ivliich feed pump is med. The 22-second, 1-mv.Brown Eleetronik potentioineter furnished with the Precision-Dow instrument is an especitrlly modified, 6-range unit. Only the 0- to 1-volt range is cmployecl in the present application. I: bnlanccd tube vacuum tube voltmeter links the titrator circuit to t,hc potentiometer and allows only electrornot~ivc force cquilibrium valucs to be plot,kd. When an e m f . change of as little as 5 mv. occurs in the input to the titrator, the potentiometer is instantancously unbalanced and a fced control relay cuts off the feed motor and the chart drive until both the input e,m.f. and t.he otentiometer are sgnin in 1,alance. Pn t.his ninnner,. fairly constant titrant addition is riiaintaincd during the early stages of a titration, when buffer action is strong, while additions become more widely spnced as buffer action is reduced. The motor driving the feed system is equipped with a friction brake which allows close control of the incremental reagent additions. e
ANALYTICAL CHEMISTRY
Figure 1. 1, 2. 3. 4. 5. 6. 7.
Automatic recording photometric titrator
Kintel Model 1 1 1 .A differential direct current amplifier Beckman M a d e l B spectrophotometer Logarithmic and linear attenuators Constant voltage transformer for spectrophotometer Hydrogen lamp-magnetic stirrer assembly PrecisiowDow recording titrator Beckman power supply for hydrogen lamp
Figure 2.
Block diagram of recording photometric titrator
OPTICAL SYSTEM
A niotlified Beckinan Model B spectrophotometer provides the radiant encrgy sources, monochromator, and energy detection system. The cell compartment and cover nornialiy furnished with this instrument have been removed and replaced by an integrated unit (Figure 3) containing a cover. magnetic stirrer, and hydrogen lamp. The stirrer motor and magnetized bar were taken from a commcrcinlly available unit (A. H. Thomas Co., Catalog No. 9235-R) and placed in a black aluminum enclosure. The metal in the monochromator housing imparts sufficient drag to the rotation of the magnetized bar and rhe stirring bar to avoid the use of a speed-controlling rheostat. The Iainp normally supplied with the Beckman DU spectrophotometer is energized by a conventional Beckman power supply and is positioned normal to the optical axis of the phototube. Metal clips are attached to the lamp housing for the insertion of band isolation filtera. An ultraviolet absorption filter may be inserted in the
filter plate provided in the spectrophotometer phototube compartment to reduce the background signal due to scattered light. ELECTRONIC SYSTEM
l’lie electronic circuit of the spectrophotonieter wns modified to make the phototube output compatible with the input circuit requirements of the titrator. Electrical connection to the titrator is made through the electrode jack receptncles located on the side of the titratbr. A Kintel Model 111-A diffcrcntial direct current nniplificr serves to amplify and reverse the polarity of the titrator output voltagr, (0 to 50 mv,) to a level coniimtible with the rcauirenients of the iognrithniic attanit:tt,dr (0 t o -15 volts). Althouyli not indicatcd in Figure 1, this nmplifier should be physically isolnted from tlie remainder of thc titration unit. The logarithmic attenuator (Figure 4) serves to produce an output voltage proportional to the absorbance, rather than the transmittance, of the solution
003 YFD
I
005 MFD
Figure 3.
TITRATOR GJTPUT
Hydrogen lamp-magnetic stirrer unit
i n thc spectropliotoiiictcr. It is esscntially a modification of tho circuit proposed by hlliiler (9) and by Mnrple arid Hume (8), and utilizes a 6SK7G T electronic tube in which, at plate operating potentials in escess of 60 volts, the plate currcnt represents a good spprosiniation of thc logarithm of the grid bias; thus, plate current should be proportional to absorbance. The average dcviation based on the difference betn ccn the theoretical and expcrimcntnl response 1\38 0.005 absorbance unit for this attenuator as compared to 0.003 for the attcnuntor described by Marple and Humc ( 9 ) . As thcsc authors pointed out, serious deviations frotii logarithmic attcnuation arc espericnced a t high nhsorbnnce Icvels-greater than 0.7-but this drawback, oncc recognized, docs not diminish t l i e utiIity of tlic titrator. OPERATION
Thc espccted clcctronic, opticnl, :iiid mec1ianic:il adjustments are required to prrpnre thc titrntor for operation. With tlic spcctropl~otoinctcr,the direct current ainplifier, and the lo nrithinic and linear attenuators c s c l u L ~froin the circuit, the recorder span is adjvsted using the zero and full scale controls on the Precision-now unit; ho\vcvcr, contrary to normal operation, thc 50-niv. range of the rrcorder is coniprcsscd so that it lies betivcen thc 1.0 ai:d 0.0 division marks on the 0 to 10 division scale. S e s t , tho Precision-Don. selector switch is positioned to nccept signals from the spectrophotometer (throuf; the electrode jack rcceptnclcs), linear attenuator is incl!icied in the circuit antl, with the shutter closcd, the spectrophotometer ncctlle is brought to 0% T . uifing the dark current control (sensitivity of 4). The recorder pen should rest on 1.0 on thc recordcr scale. The shutter is openrd, the ncegle is adjusted t o 100% T by the slit control nnd the pen is brought to 9:b’ on ‘the recorder scnlc using the linear nltcnuator. In this manner, the full rnnge output of the spectrophotonicter is nttenuated to a range acceptable by the titrator detector circuit. Finally, the direct current amplifier and the logarithmic attenuator arc included in the circuit, the spcetrophotometer needle is brought to 20% T by the slit control, and the recorder pen is brought to 7.99 on the recorder scale by the
Figure 4.
Titrator output and logarithmic attenuator circuits
logarithmic attenuator. A t this point, s’cale readings between 1.0 and 7.99 are directly proportional to absorbance values and may be directly converted to absorbance values by subtracting 1 and dividing the remninder by 10. The required optical nnd mechanical adjustnicnts arc self-evidcnt; a suitable wave lcngth must be chosen (7, IO), an optically transparent titration vessel must be provided (a 100-rnl. beaker made of Pyrex glass No. 7740 is suitable down to 320 mp; a silica or Yycor glass So. 7710 wssel is suitable below this wave lcnyth and for chemiluminescent titrations), and the buret tip should be carefully positioned out of the spectrophotometer light path 2nd close to one end of the mngnetic stirring bar. Ordinary analytical precautions should be observed with the titrant feed system on the titrator. I%utbIes sliouId be careiully cliniinated from the titrant fccd line, titrant should be 5s concentrated ns possible to avoid dilution effects :md consequent curvature of the lines, antl the feed pumps should be calibrateti. The nuthors’ experience is that the 60-mI. pumps suppIied with the Precision-Dow unit will be accurate to 2 t o 3 parts per thousand; the 5-ml. pumps, however, mny be in error by 1 to 2%. Accuracy and Precision. Espcriencc with a numbcr of titration systems hns indicated that the accuracy :inti precision lewis achieved with this recording photometric titrator , a s compaicd with those achieved by ni:iriunl titrtition, iiie a t least equivalent in tho case of straightfor~+nrd tituition systems and are superior in thc cnsc of difficult systems involving (lark solutions or indistinct indicator colur trP nsitions. TITRATION SYSTEMS
The photometric titrator has been employed thus far in a vnriety of titration systems: acid-base titrations using simple, mixed, and chemihminescent indicators, and precipitntion, redox, and coniplexometric titrntions. Typital titration plots are shown in Figure 5 and are includcd to illustrate the variety and form of obtainable plots. Plot A
W
Y
P 2 ML.TITRAH1
Figure 5. curves
Typical photometric titrator
is n curw obtained during the titration of 0.03 meq. CU+*in a pyridinc-water buffer with 0.0055di EDTA a t 460 nip using Pyrocatechol Violet iadicntor. Plot U represents thc titration of 0.5 meq. of hydrochloric acid with O.1N sodium hydroxide using 1-naphthol, an indicator which hns a bluish white fluoresccrice above pH 9. Plot C represents titrations of 0.26, 0.36, and 0.50 nieq. of hydrochloric ncid with 0.lN sodium hydroxide a t a n’ave lcngth of 610 nip, using bromothymol blue indicator. Plot D is a first derivative type curve obtaincd in the titration of 0.5 ineq. of hydrochloric acid with 0 . W sodium hydroxide at a wave Icngth of 565 nip using a methyl purplem-Cresol Purple mixed indicntor. Plot E represents a curve obtained during the prccipitntion titration of 0.46 mg. of CI- with 0.01 and 0.0033h‘ silver nitrate, The “first derivative” type curve obtained in plot D can be obtained with this apparatus b y carefully selecting the proper pair of indicators. Success of the titration is based on the fact that one of the indicators changrs color a t the proper pH to dctect thc end point, while the second indicator changes aftcr the end point t o bring the ~ 0 1 tion back to its initial color, The wave length is set for the intcrmcdiate color to ensure maximuni absorption. The curve will drop nftcr the end point, because there is very little contribution to the nbsorption of the original color. VOL 32, NO. 1, JANUARY 1960 a
105
~ -
The peak formed is therefore the end point. ACKNOWLEDGMENT
The aid given this work by C. P. Krula, Central Research Department, who designed the electronic circuitry, is gratefully acknowledged. LITERATURE CITED
(1) Bricker, C. E., Sweetser, P. B., ANAL.CHEM.24,409 (1952).
(2) De Ford, D. D., Miller, J. K., Ibid., 29,475 (1957). (3) Goddu, R. F., Hume, D. S . , Zbid., 26, 1679 (1954). (4) Zbid., p. 1740. (5) Headridge, J. B., Talanta 1, 293 (1958). ( 6 ) Malmstadt, H. v.1 Roberte, '2. B., ANAL.CHEM.28,1408 (1956).
(7) Malmstadt, H. V., Vassallo, D. A,, Zbid., 31,862 (1959). (8) Marple, T. L., Hume, D. S . , Ibid., 28, 1116 (1966). (9) Muller, R. H., J . Opt. icoc .h25, . 342 (1935).
(10) Reilley, C. M., Schmid, R. W., ANAL.CHERI.31,887 (1959). (11) Robinson, H. A., Trans. Electrochem. SOC.92,445 (1947). (12) Underwood, A. L., J. C h m . Educ. 31, 394 (1954). (13) Weilley, C. A Chalmers, R. A., Analyst 82,329 (19k7).
RECEIVEDfor review June 10, 1959. Accepted October 20, 1959. Division of Analytical Chemistry, Beckman Award Symposium on Chemical Instrumentation Honoring Howard Cary, 135th Meeting, ACS, Boston, Mass., April 1959.
PoIa rogra p hic Dete rminati o n of AI pha-Methy I- ~ ~ - ctiynes R. J. THIBERT and R. M. OTTENBRITE Department of Chemistry, Essex College, Assumption University of Windsor, Windsor, Ontario, Canada
b Because of a recent synthesis of a substituted amino acid, a - m e t h y h cystine, it became necessary to have a method of determining this compound. The polarographic determination of the substance in 0.1N hydrochloric acid, using thymol as a maximum suppressor, was investigated. The relationships of concentration of the compound and temperature to the diffusion current were studied. The influences of thymol concentration and pH on the apparent half-wave potential were determined. A linear relationship of the diffusion current to the concentration of a-methyl-DL-cystine was observed in the range of 5 X to 2 X 1 O - W . The system is not reversible.
A
synthesis O f a-methyl-DLcystine by Arnstein ( I ) , which has been confirmed in this laboratory ( 8 ) , made it desirable to establish a method of analysis for this compound. Cystine can be determined colorimetrically by reduction to cysteine and by using reagents which form colors in the presence of free sulfhydryl groups ( 2 , 9). Cystine can also be determined polarographically, and studies of this type have been reported ( 7 ) . Because the compound under investigation has a structure similar to cystine, it was thought feasible to attempt a polarographic estimation. RECEST
EXPERIMENTAL
Apparatus and Materials. T h e Sar-
gent ( E . H. Sargent & Co.) Model XXI Polarograph was employed for this study. A Heyrovsk3 polarographic cell was used during most of this work; a n H-cell with S.C.E. was used t o determine Eliz. The capillary
106
ANALYTICAL CHEMISTRY
Table I. Effect of Concentration on Diffusion Current
2 x 15 1x 7 5
5
x
10-3 X 10-8 X 10-4
8.66
8 _ ._ M
6 42 4 19 3.64 2 28
6 i8
4 60 3 53 2 36
8.03 6 03 3 98 3 06 2 01
Average of three determinatious. Table II.
O
c.
0.0 11.5 19.0 22.5 25.0 30.0 35.0 40.0 45.0
Diffusion Current, 4.28 5.28 5.66 6.00 6.22 E.54 l.06
7.3ti 8.00
*
RESULTS AND DISCUSSION
Effect of Temperature on Diffusion Current
Temperature,
DL-cystine and thymol, and diluting to volume with 0.1N hydrochloric acid or buffer. The solutions were transferred to a polarographic cell and nitrogen (purified by assing through ammoniacal cuprous ciloride) was bubbled through for 5 minutes. Polarograms were run through the range of 0.0 to -1.0 volt. The drop rate was adjusted to 3 seconds and the temperature was controlled to 0.1" c.
pa.
4.26 5.28 5.66 6.00 6.21 6.64 i.04 7.36 8 00
constant, m z W 6 ( m = 2.57 nig. per second; t = 3.00 seconds). was 2.253 at 25' C. and -0.555 volt. All pH measurements were made with a Beckman Model G p H meter. Buffer solutions employed were prepared according to Clark and Lubs ( 3 ) . The a-methyl-DL-cystine was recrystallized three times from absolute ethyl alcohol prior to use in this study. A stock solution of 1 X lO-*Jf was prepared in 0.1i17 hydrochloric acid. A 1.2 x lO-3M thymol solution in 0.1N hydrochloric acid was prepared for use as a maximum suppressor. Procedure. The solutions used for analysis were prepared in a 100-ml. volumetric flask by adding t h e appropriate concentrations of a-niethyl-
Effect of Concentration on Diffusion Current. The effect of concentration of a-methyl-nL-cystine (using 1.2 x 10-4M thymol as t h e maximum suppressor a t 25O, 30°, and 37.5' C.. respectively) on t h e diffusion current results in a linear relationship (Table 1)* The diffusion current was measured from the top of the first wave to the top of the second wave as there occurs a prewave with thymol. This type of prewave also occurs with cystine and has been studied by Kalousek, Grubner, and Tochstein (6). Figure 1, curve 2, is a typical example of a polarogram obtained under the above conditions. Except for the study of diffusion current dependence on temperature, where the height of the diffusion current represents the prewave plus the top wave, the diffusion currents and half-wave potentials are based on the top wave only wherever a prewave occurred. The half-wave potential ( E M ) x i s measured us. S.C.E. at 20' C. using 1 X 10-3M a-methyl-DL-cystine. E1 2 was observed to be -0.555 and -0.718 volt for thymol concentrations of 1.2 X and 4.8 X 10-4M, respectively.
