1.0 L
---0 :“C
2
3 E z
as
1300 V SENSITIVITY
RATIO
-to-
-I,-
*
: 3”
0
: D I F F E R E N C E B E T W E E N SENSITIVITY RATIOS
-
----.
c z
cn
W VI
for the relative errors. We have found it easier to choose the discriminator value which maximizes the determinant D = hlcl(cz/cl - hz/hl). Sensitivities cz and hz, in our case, are proportional to the area under the pulse distribution curves to the hght of the discriminator level (Figures 3 and 4). This procedure is the equivalent of finding the maximum “contrast ratio” (4). Numerical Examples. We have felt it justifiable to be general in our discussion, since each setting of the detection unit and choice of radionuclide-scintillator system will have its individual figures. Applying the screening method, we have found for our system, described above, the following data: At 900 volts all the 3H pulses are screened out and B1
=
5 cps
c1
=
90 cps, (nCi)-l
From Equation 1 it is seen that to detect 0.5 nCi of 14C with 10% accuracy, it is necessary to detect over a period of 2.7 seconds. To detect 5 nCi of 3H with the same accuracy, one needs to detect for 10 seconds (Equation 2). The presence of 1 nCi of 14C will, according to Equation 2, increase this detection time to 15 seconds, provided one detects for equal periods of time in the two samplings. When using the discriminator-radio method, one will have to choose the discriminator level in register 2 to optimize the method. In Figure 6 is shown how sensitivity ratios cz/cl and hn/h, vary with the discriminator level of register 2 for two different operating high voltages of the photomultiplier (1100 and 1300 volts). The difference (cz/cl - hz/hl), being proportional to the determinant, is also shown in Figure 6. The optimum discriminator level is found to be lower at 1100 than at 1300 volts (Figure 6). The maximum value of D is about the same for 1100 and 1300 volts, taking into account the reduced 3H sensitivity at 1100 volts (Figure 5). Hence, the operating voltage should be between 1100 and 1300 volts.
At a higher voltage (1300 volts), we have Bz = 130 CPS hz = 12 cps, (nCi)-l
RECEIVED for review January 27, 1970. Accepted April 17,
cz = 100 cps, (nCi)-l
1970.
Potentiometric Titration of Halides in Their Mixtures Theodore S . Prokopov Department of Chemistry, Upper Iowa College, Fayette, Iowa 52142
WHILEDETERMINATION of iodide in a mixture of halides does not cause problems and may be conveniently performed iodometrically, the determination of bromide or chloride in a bromide-chloride mixture is not so convenient. All known nonelectrometric methods (I-5) are laborious and tedious, involving many reagents and requiring special conditions. ~~
(1) I. M. Kolthoff, 2.unul. Chem., 60, 344 (1921). (2) J. H. Van der Meulen, Chern. Weekblud, 28, 82 (1931). ANAL. (3) I. M. Kolthoff and H. C. Yutzy, IND. ENG. CHEM., ED.,9, 75 (1937). (4) P. L. Kapur, M. R. Verma, and B. D. Khosla, ibid., 14, 157 (1942). (5) R. Belcher and R. Goulden, Mikrochim. Acta, 1953,290. 1096
The electrometric methods are more convenient but are not accurate. The theoretical error in potentiometric determination of bromide in a mixture which is 0.1F in Br- and C1- with silver nitrate varies from 0.19 to 0.28% of the initial bromide ion concentration (6, 7). In practice, however, neither the titration error nor the shape of the curve is quite identical with what is predicted by theory. This is because silver halides
(6) J. J. Lingane, “Electroanalytical Chemistry,” 2nd ed., Inter-
science, New York, 1958, p 128. (7) L. Meites and H. Thomas, “Advanced Analytical Chemistry,” McGraw-Hill, New York, 1958, p 55.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
readily form mixed crystals or solid solutions (8), so that an appreciable amount of the chloride is precipitated before the first end point. The relative errors in determination of halides at equimolar amounts are usually about A 3 %. Naturally, this error will be far greater in determination of either bromide or chloride present in smaller amount. The coprecipitation phenomena, of course, do not affect the total amount of silver nitrate used to titrate the two or even three halides. The coulometric, “coulogravimetric” (9), and amperometric analyses (10) of bromide-chloride mixtures yield no better results. It was, therefore, highly desirable to develop a potentiometric method of chlorine or bromide determination in bromide-chloride mixtures with accuracy of not less than ~ k 0 . %. 3 For this purpose a study was undertaken to investigate the possibility of quantitative oxidation of bromide to bromine by an acidic solution of hydrogen peroxide with subsequent “absorption” of bromide with 8-hydroxyquinoline (oxine); the chloride being freed from interference may then be titrated potentiometrically with silver nitrate. The bromide may be determined then by difference from the potentiometric titration of a second aliquot of the same bromide-chloride mixture. EXPERIMENTAL,
Apparatus. A Fisher pH meter Model 210 was used. A Beckman billet-type silver electrode and a calomel electrode in which a saturated solution of K N 0 3 was substituted for solution of KCl, were employed. A Corning hot platestirrer was used to warm the reacting mixture. The titrant was delivered from a 5-ml microburet. Reagents. An acetic acid solution of oxine was prepared by dissolving 4 grams of oxine in 6 ml of glacial acetic acid and diluting the solution with water to 100 ml. Aqueous ammonia was then added dropwise until a turbidity appeared, which was dissolved by dropwise addition of acetic acid. Two parts of 6% H20zand one part of glacial acetic acid were mixed to prepare the acetic acid solution of hydrogen peroxide. All chemicals used were of analytical reagent grade. Procedure. The size of sample depends upon the amount of sample available and upon the expected concentration of halides. Hydrogen peroxide is added first, then oxine and nitric acid (6N). The latter is added as much as is needed to dissolve the precipitate which may form. The approximate amount of oxine and hydrogen peroxide which should be added to the sample can be calculated from the corresponding equations. An excess of them will not do harm. The mixture is warmed for 5 minutes on the hot plate at approx. 90 “C. At this point, the mixture will appear to be of honey-yellow color. The cooled preparation is made approx. 90% in acetone, the pH meter is set to desired “zero” point, stirrer is started, and titration is carried out by addition of appropriate increments up to the end of titration. DATA OF THE TITRATIONS
The data of typical titrations are given in Tables I-N, which are self-explanatory. DISCUSSION OF RESULTS
In the above described reactions, the oxidation of bromide to bromine and iodide to iodine proceeds according to the (8) F. Vaslow and G. E. Boyd, J. Amer. Chem. SOC.,74, 4691 (1952). (9) W. M. McNevin, B. B. Baker, and R. D. McIver, ANAL. CHEM.,25, 274 (1953). (10) H. A. Laitinen, W. P. Jenings, and T. D. Parks, IND.ENG. CHEM.,ANAL.ED., 18, 355 (1946).
Table I. Titration of 3 Milliliters of 0.1016N C1- in 50 Milliliters of Acetone with 0.1034N AgN03 Ag+, ml
E , mV
AEjAV
AaE/AV2
2.8 2.9 3.0 3.1
255 280 580 605
25 300 25
275 275
+ 275 X 0.1 = 2.95 ml. 550
V = 2.9
-
Meq of C1Error
=
0.3048; meq of Ag+ = 0.3050.
(0.3050 - 0.3048) 100 = +O.O6z. 0.3048
=
Table 11. Titration of a Mixture of 3 Milliliters of CI(0.1016N), 3 Milliliters of Br- (0.1018N), 1.0 Milliliter of HQ, 3 Milliliters of Hz02,and 1 Milliliter of HN03 in 50 Milliliters of Acetone with 0.1034N Ag+ Ag+, ml E , mV AEjAV AaE/AV2 2.8 105 55 2.9 160 110 165 3.0 325 145 20 3.1 345 110 V = 2.9 x 0.1 = 2.94 ml. 255 MeqAg+ = 2.94 X 0.1034 = 0.3040. (0.3040 - 0.3048) X 100 = -0.26z. Error = 0.3048 (0.3040) [ci-1 = 64.5 = 5 x 1 0 - 3 ~ .
+
~
Table 111. Titration of a Mixture of 0.5 Milliliter of CI(0.1016N), 3 Milliliters of Br- (0.1007N), 3 Milliliters of I(O.lOllN), 2 Milliliters of HQ, 6 Milliliters of H202,and 4 Milliliters of H N 0 3 in 50 Milliliters of Acetone with 0.1084N A m 0 3 Ag+, ml E , mV AEIAV A aE/AV a 0.40 25 45 0.45 70 95 140 210 0.50 100 40 0.60 250 95 V = 0.45 X 0.05 = 0.47 ml. Meq = 0.1084 X 0.47 = 0.0509. (0.0509 - 0.0508)lOO = +0.2%. Error = 0.0508 0 0508 = 7 x 10-4~. [ci-1 = 69
+ s5
Table IV. Titration of a Mixture of 1 Milliliter of CI(0.1016N), 10 Milliliters of Br- (0.1007N), 5 Milliliters of HQ, 10 Milliliters of H2O2,2 Milliliters of H N 0 3 in 50 Milliliters of Acetone with 0.1084N AgN03 E , mV AElAV AaE/AV2 Ag+, ml 0.8 330 50 0.9 280 115 165 1.0 115 155 10 1.1 105 115 V = 0.9 F~ x 0.1 = 0.94 ml. Mea of Aa+ = 0.94 X 0.1084 = 0.1018. - (6.1018 - 0.1016)lOO = +0.20z. Error = 0.1016 0.1016 [Cl-1 = -- 1 x 1 0 - 3 ~ . 79
+
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9,AUGUST 1970
1097
reactions
+ 2Br- + 2H+ HzOz + 21- + 2H+
HzOz
+ -+
+ 2Hz0 Iz + 2Hz0
Brz
The result of this experiment show a possibility of a convenient determination of three halides in their mixtures by use of the following steps : Titrate potentiometrically an aliquot with silver nitrate, Titrate another aliquot with silver nitrate to determine the C1- ion, using oxine to eliminate the interference of iodide and bromide ions, Determine iodide iodometrically in a third aliquot, Calculate the concentration of bromide ion by difference.
and bromination of oxine takes place according to the equation
+ 2Br-
C9H70N
-+
+ 2Br-
C9HsBr20N
The bromide formed in this way is oxidized again and again until all is consumed by the oxine. The mechanism of iodine reaction with oxine is not known, but the fact that after its reaction with oxine, AgN03 forms no precipitate, allows us to assume that the iodine may react similarly to bromine. The above data of titrations show that 8-hydroxyquinoline quantitatively eliminates interference of iodide and bromide in titration of chloride in acetone mixtures of halides. The fact that titration of C1- in these mixtures produces smaller A E / A V ratio than in the absence of Br- and I- ions, cannot be attributed to incomplete elimination of ionic bromide and iodide. That these ions are completely eliminated by oxine can be easily proved by the fact that AgN03 in the absence of C1- in a mixture of halides with oxine and H202 forms no precipitate of AgI or AgBr. The decrease of potential break can be attributed only to the presence of reagents in the mixtures of halides. The above data of titrations also show that it is possible to determine C1- even in the case when iodide and bromide ions exceed the amount of chloride tenfold. ~
In a case of bromide-chloride mixture, Titrate an aliquot with silver nitrate, Titrate another aliquot to determine chloride, using oxine to eliminate the interference of bromide, and calculate the concentration of bromide by difference. Halogens in organic iodo-bromo-chloro-compounds and in bromo-chloro-compounds, after conversion to ionizable form by closed flask combustion (11, 12) or by other methods (12), also can conveniently be determined by this method.
RECEIVED for review April 1, 1970. Accepted May 26, 1970. (11) W. Schoniger, Mikrochim. Acta 1955, 123; 1956, 869. (12) T. S. Ma, Quantitative Microchemical Analysis, in “Standard Methods of Chemical Analysis,” F. J. Welcher, Ed., 6th ed., Vol. I1 A, Van Nostrand, Princeton, N.J., 1963, p 357.
~~
Spectrophotometric Studies of Complexes of 2-Benzoyl pyridine-2-Pyr idylhydrazone John E. Going1 and Ronald T. Pflaum Department of Chemistry, University of Iowa, Iowa C i t y , Iowa
THECOLORIMETRIC REAGENTS that are among the most widely known are the ferroin and terroin chromagens with the functionalities, -N=C-C=N-, and -N=C-C==N-C-C= N--. The examples, beginning with bipyridine, appear to be endless. Recently some new reagents have become available which can be considered as slightly modified terroin chromagens. These include certain of the substituted hydrazones, -N=C-NH-N=C-C=N-( I, 2). The secondary amine proton in this grouping has been shown to be quite labile when the ligand undergoes coordination with a metal ion. The complexes formed upon elimination of this proton are intensely colored and are amenable to analytical application. The purpose of this paper is to introduce 2-benzoylpyridine2-pyridylhydrazone (BPPH) as a useful and extremely sensitive reagent for certain metal ions. A detailed study of the iron(II), cobalt(III), nickel(II), copper(II), and zinc(I1) complexes has been undertaken. A procedure for the determination of cobalt in the presence of large amounts of iron, copper, and palladium, based upon a simple extraction, will be disPresent address, Department of Chemistry, University of Wisconsin, Milwaukee, Wis. (1) J. F. Geldard and F. Lions, Inorg. Clzem., 2, 270 (1963). (2) R. W. Green, P. S. Hallman, and F. Lions, ibid., 3,376 (1964). 1098
cussed. In addition, a study of the determination of zinc in a phosphor bronze and in the presence of a large excess of aluminum will be presented. EXPERIMENTAL
Apparatus. All absorption measurements were made with a Cary Model 14 recording spectrophotometer at room temperature in 10-mm matched silica cells. Conventional ion exchange columns were used for the isolation of zinc from the NBS sample. A 1.3- X 15-cm column containing 50-100 mesh Dowex 1-X8 anion exchange resin was used for the separation. Reagents. The reagent, 2-benzoylpyridine-2-pyridylhydrazone was prepared in this laboratory from commercially available materials, Stoichiometric amounts of the reactants, 2-benzoylpyridine and 2-pyridylhydrazone (0.22 mole), were dissolved in 250 ml of ethanol. The mixture was heated under reflux from 8 hours. Upon cooling at dry ice temperatures, the hydrazone crystallized out of solution. The light yellow product, recrystallized from ethanol, yielded 40 grams (67%), mp 113-114 “C. A reagent solution 2.00 X 10-2M was prepared by dissolving 0.55 gram of BPPH in a minimum amount of hydrochloric acid with heating and diluting to 100 ml. Solutions of metal ions approximately 0.05M were prepared
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
