the 25 ml of 0.1M K N 0 3 electrolyte. The current was manually controlled to better than 1 0 . 1 % over the duration of the generation, and the time measurement was accurate t o within +0.05%. The efficiency of generation was calculated by analysis of the resulting electrolysis solution. The analysis was performed using the technique of “analate additions” potentiometry ( 2 ) in which aliquots of the sample (analate) are delivered into a known volume of a standardized fluoride solution, and the analate concentration is calculated from the change in emf of a fluoride activity electrode. The precision of this technique at the millimolar level is 0.5% (expressed as the 95 % confidence limits for 35 determinations). RESULTS
The electrochemical generation of fluoride was performed nine times with an average efficiency of 99.21 %. The 0.51 precision (95 confidence limits) corresponds to that for the measurement technique and therefore cannot be used as an estimate of the generation reproducibility. (2) R. A. Durst, National Bureau of Standards, Washington, D. C., unpublished data, 1967.
The 0.79% inefficiency in the generation of fluoride by this method is assumed caused by an inefficiency in the transport of fluoride ions across, and/or an undetected leakage around, the LaF3 membrane. In this preliminary study with a relatively unsophisticated apparatus, the attainment of 99.2 current efficiency and an irreproducibility of, at most, 0.5% indicates the potential use as a technique for the accurate generation of fluoride ion in situ for coulometric analysis in the laboratory or for automated fluoridation of water supplies, where the water supply itself would be substituted for the electrolysis cell and K N 0 3 solution in the figure. In a practical sense, because the generation efficiency can be established for a particular set of operating conditions, the current efficiency need not be 100% as long as it is reproducible. It should also be noted that, although the reported results are for one specific set of experimental conditions, considerable latitude is available in terms of the variables of the techniquee . g . , generation current, membrane area and thickness, cell geometry, and the concentration, pH, and stirring rate of the catholyte.
x
RECEIVED for review March 11, 1968. Accepted April 15, 1968.
Spectrophotometric Determination of Nickel with 2-Amino1-Cyclopentene-1-DithiocarboxyIic Acid Masataka Yokoyama and Tatsuo Takeshima Department of Chemistry, Chiba Unicersity, Yuyoi-cho, Chiba City, Japan
IT WAS FOUND that 2-amino-1-cyclopentene-1 -dithiocarboxylic acid (ACDA) was a n excellent colorimetric reagent for the determination of Ni(I1) ion. The compound reacted especially sensitively with Ni(I1) ion in aqueous solution to form a pink-red compound. The visual sensitivity was 20 ppb. ACDA is more sensitive toward the Ni(I1) ion than dimethylglyoxime, diethyldithiocarbamate, and other compounds (1). Further, the relationship between absorbance and concentration of Ni(I1) ion obeyed Beer’s law between 0.5 to 5 ppm. ACDA was synthesized from cyclopentanone and carbon disulfide in the presence of ammonia. Details concerning the determination of the structure of ACDA will be reported in a separate paper.
Sorensen buffer solution was made by adding 400 ml of 0.1 M disodium citrate solution to 600 ml of 0.1N hydrochloric acid. All other reagents were analytical grade or of comparable purity. Apparatus. The absorbance measurements were made with an Hitachi EPU-2 type spectrophotometer for the visible region and an Applied Physics Corp. Cary 14 recording spectrophotometer for the ultraviolet region. The p H was measured with a n Hitachi, Horiba type M-4. Procedure. To each 100 ml of solution containing 50 t o 500 fig of nickel ion, 10 ml of Sorensen buffer was added to give a p H of 3.0, and then 0.2 ml of ACDA reagent was added (0.64 gram of ACDA in 100 ml of ethanol). The solution immediately produced a pink coloration and was
EXPERIMENTAL
2-Amino-1-cyclopentene-1-dithiocarboxylic acid (ACDA) was prepared as follows. A mixture of cyclopentanone (25 g, 0.3 mole), carbon disulfide (30 g, 0.4 mole) and 145 ml of aqueous ammonia (28%) was stirred at 0 “C for several hours. The yellow solid product which was collected consisted mainly of the ammonium salt of ACDA. Recrystallization from ethanol gave ca. 40 grams of yellow plates (ammonium salt of ACDA): yield ca. 80%. The yellow plates were dissolved in water and to this was added dilute hydrochloric acid until neutralization. The crude ACDA which separated from the solution was collected. Recrystallization from ethanol gave ca. 15 grams of pure ACDA: yield ca. 30z. Reagents.
(1) E. B. Sandel, “Colorimetric Determination of Traces of Metals,” Interscience, New York, 1950, pp 469-76.
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ANALYTICAL CHEMISTRY
Table I. Visual Limit of Identification and Color of Different Ions Visual limit of
Ion Ni(I1) Fe(I1) Fe( 111)
identification (mg/lOO ml)
Color Pink-red Dark green Yellowish brown M n( I I) Yellowish green Zn(I1) Yellowish green Cu(I1) Brown Co(I1) Brown When mixed with 5 X lo3ppm of AI(III), Sr(I1). Cd(II), Ca(II), Mg(II), K, Ba(II), Sn(Il), Sn(IV), Na, and Cr(II1) ions, ACDA reagent did not form coloration. 0 002 0 05 0 1 0.1 0.1 1 1
100
90 c L
e
t
ao 70 60
U w
2 50
4
:\
0,BO
I Y
~
2
-r-
3
4 5 6
7
8 4
IO
PH
Figure 1. Effect of pH on the transmittance a t 530 m p of a solution 2.5 X 10-8 mole in Ni(I1) and 1.2 X 10-7 mole of ACDA, and a solution 4 X 10-4 mole in Fe(I1) and 3.2 X 10-3 mole of ACDA
4 2 0.4 0,6 0,8 LO
O O
aP
3 060
-1
1
1
00
[ACDAIi [Ni(1I)l
+ IACDAI
Figure 2. Absorbance vs. mole ratio Concentration of Ni(I1) was held at constant mole: 5 X 10-5 mole
extracted within 30 minutes after the addition of the reagent by shaking with two 50-ml portions of chloroform for 0.5 minute each. The chloroform extract was diluted with the same solvent to 100 ml. The absorbance was determined immediately after the extraction. The wavelength for measurement was 530 mp and the slit width was 0.03 mm. The experiment was repeated with various amounts of nickel and a plot of absorbance cs. concentration of nickel was prepared. The color reaction obeyed Beer’s law between 0.5 and 5 ppm. Nickel from the unknown sample can be directly determined from the curve by measuring the absorbance after the reaction.
0
110
200
3,O
P’
i”
4,O
5,O
Concn of Ni(ll) Ion, pprn
Figure 3. Absorbance curve 0 Absorbance measured 5 minutes after adding reagent A Absorbance measured 30 minutes after adding reagent
range 2.0-8.0 and that of Fe(II), in the p H range 4.0-7.5. From the solution which is adjusted to a pH of 3.0 with Sorensen buffer solution, Ni(I1) ion can be separated by the chloroform extraction without the accompanying Fe(I1) ion. The color of Ni(I1)-ACDA in the chloroform remained unchanged when mixed with about 0.5 ppm of Mn(II), Co(II), Zn(II), Fe(III), and Cu(1I) ions. The absorbance coincided with results obtained with nickel only. The presence of 50 ppm of Cu(I1) ion led to error. To determine the constitution of Ni(I1)-ACDA complex compound, the mole ratio of Ni(I1) ion to ACDA was studied by the method of continuous variation. The results given in Figure 2 reveal that Ni(I1) ion and ACDA combine in a ratio of 1 mole:3 mole. The structure of the complex compound was tentatively proposed as an octahedron using d2sp3 hybrid orbitals of Ni(1I) ion. The stability of ACDA was verified by comparing the ultraviolet spectrum of a solution which was exposed to a scattered light for 24 hours with the one of freshly prepared solution. The results showed that ACDA was not changed, When the reagent which had been kept for one month was used, the color reaction of Ni(1I) ion revealed no change. Figure 3 shows the absorbance curve which was prepared according to Procedure. The results indicate that the color intensity of the Ni(I1) complex compound did not change within 30 minutes.
4
RESULTS AND DISCUSSION
It was found that ACDA produced various colorations with other metals. The color and the visual limit of identification are given in Table I. Nickel(I1) ion can be sensed at 2 ppb and the reaction mixture is easily extracted with chloroform. Fe(I1) ion is easily detected, but other metal ions are much less sensitive. The effect of p H on color intensity was studied for Ni(I1) ion and Fe(1I) ion. The results given in Figure 1 indicate that the color intensity of Ni(I1) ion is constant in the p H
RECEIVED for review January 26, 1968. Accepted February 26, 1968. VOL. 40,
NO. 8, JULY 1968
1345
