4
25
-
5 20w
v g
15-
g
IO
I 5
~~
100
Figure 3.
n=i
crl
Figure 2. F M R of H F A
I 83.5
400
Molecular weight distribution profile of Carbowax 200
2
e\
I 84.0
200 300 MOLECULAR W E I C M
I 83.0 PPM
I 82.5
adduct of Carbowax 200 in ethyl acetate
Figure 2 illustrates this advantage rather dramatically. This is a spectrum of poly(ethy1ene glycol) oligomer (Carbowax 200) containing a trace of water. The hydrate signal is a strong singlet a t the far right (at 81.25 ppm) and is not included in this figure. The peak a t the lower field (83.68 ppm) is due to a trace of ethanol in the ethyl acetate solvent. The Carbowax 200 sample gave a t least 8 different singlets. Each can be identified by the addition of a known component. The identification of the rather closely spaced components ( n 2 6) was done on a spectrum recorded at a highly expanded scale (0.4 Hz/cm). In this analysis, the water content was found to be 0.43 wt %. The total hydroxyl end groups was found to be 16.82 wt YO which corresponds to a number averaged molecular weight, of 202.2, in good agreement with the value of 200 reported by the manufacturer. From the abundance of each individual component, the weight averaged molecular weight, M,, was found to be 226.5. This material therefore has a polydispersity of 1.12. A molecular weight
mn,
distribution profile derived from FMR is shown in Figure 3. The large difference between the chemical shifts of the components of this low molecular weight polyether sample revealed that these hydroxyl end groups are in different structural environments. These data suggested that this high resolution technique could be useful in studying the structures of the polyethers in solution, in addition to the quantitative measurements of water and hydroxyls. This study is in progress and shall be discussed in a later communication. In addition to the simultaneous quantitation of water and hydroxyl groups, we had also observed in a spectrum of aminoethyl ethanolamine, H2NCH2CH2NHCH2CH20H, widely separated signals due to primary and secondary amines (peaks a t 85.12 and 88.52 ppm, respectively). This technique therefore shall be useful in following reactions involving polyfunctional compounds.
ACKNOWLEDGMENT The authors thank George A . Ward for many kelpful discussions, Mrs. Claire L. Carey for the Karl Fischer titrations, and c. A. Genge for the GPC work. Received for review January 7, 1974. Accepted March 25, 1974. Contribution Number 1620 from Hercules Research Center.
Interference of Aluminum in the Atomic Absorption Determination of Cadmium Using Sodium Diethyldithiocarbamate as Chelating Agent E. E. Kaminski Analytical Research Department, Abbott Laboratories, North Chicago, ///
Atomic absorption is a viable technique for measuring cadmium because of its specificity and sensitivity (I). A technique that has gained widespread acceptance for increasing this sensitivity is to chelate the metal ion and extract the chelated species into an organic solvent (2, 3). One chelating agent commonly used is sodium diethyldithiocarbamate (NDDC) which has the advantage of being operative over a wide pH range for some metals (4, 5 ) . However, studies in this laboratory have shown that care 1304
must be exercised in pH adjustment of cadmium solutions if aluminum is present, because cadmium will coprecipi( 1 ) W. Slavin, ' Atomic Absorption Spectroscopy," interscience Publishers. New York, N.Y., 1968. ( 2 ) W. Slavin, ~ tAbsorpt/on . Newsiett., 3, 141 (1964). (3) R E. Mansell. At. Absorption Newslett.. 4,276 (1965). (4) Kazuhiro Kuwata. Keiji Hisatomi. and Toshio Hasegawa, At. Absorption Newsiett.. 1 0 , 111 (1971). ( 5 ) E. Berman. ~t Absorption Newsleft.. 6 , 57 (1967).
A N A L Y T I C A L C H E M I S T R Y , VOL. 4 6 , N O . 9 , AUGUST 1974
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Table I. Effect of Aluminum on Cadmium Absorbance AI, ppm
Cd, ppm
0.0
1.0 1.0
10.0 100.0 300.0 600.0 1000.0
1.0
1.0 1.0 1.0
Ratio AI/Cd
Absorbance
. . .
10/1 100,/1 300/1 600/1
1000/1
Table 11. Absorbance of Solutions of Cadmium and Aluminum Prepared for Extraction AI, ppm
0.144 0.144 0.144 0.145 0.144 0.147
0 0 10 0 100 0 300 0 600 0 1000 0
tate with aluminum hydroxide. The problem is eliminated by complexing the aluminum with tartrate prior to p H adjustment. EXPERIMENTAL Apparatus. Atomic absorption measurements were made with a Perkin-Elmer Model 403 Atomic Absorption Unit. Cadmium absorbance was measured at the 228.8-nm line using a Cadmium Intensitron hollow cathode lamp, an air-acetylene flame with a Boling burner head and a slit width of 7 A. Reagents. Sodium diethyldithiocarbamate was obtained from Eastman as the acid-sodium salt. A 1% (w/v) solution in water was prepared and any insoluble material was filtered off. Cadmium metal was of sufficient purity for atomic absorption analysis. A 1000 part per million stock standard was prepared by dissolving the metal in nitric acid. Thymol Blue was obtained from Matheson, Coleman and Bell. Norwood, Ohio. A 0.5% (w/v) solution in water was prepared and any insoluble material was filtered off. All other chemicals such as sodium acetate. sodium potassium tartrate tetrahydrate, and methylisobutylketone (MIBK) were reagent grade. Procedure. This procedure was developed to determine the cadmium content of an organic sample. Therefore, the material was thoroughly wet ashed with nitric acid and the resultant white ash was dissolved with several drops of concentrated hydrochloric acid and about 10 ml of water. After quantitatively transferring the solution to a 125-ml separatory funnel, the following reagents were added: 0.5 ml of the 0.5% thymol blue solution, 40.0 ml of 0.1M sodium acetate, L V ammonium hydroxide dropwise until a green to blue color developed ( p H 8-9), and 1.0 ml of 1% sodium diethyldithiocarbamate. The solution was extracted with 20.0 ml of MIBK by equilibrating the phases with moderate shaking for two minutes. The phases were allowed to separate and the lower aqueous layer discarded. The cadmium is then back-extracted into an equal volume of 0 . 5 N hydrochloric acid.
RESULTS AND DISCUSSION It was during the addition of ammonium hydroxide that cloudiness and precipitation occurred. Hydrous oxides of cadmium do not precipitate a t this pH but standard addition-recovery experiments showed that cadmium was not being recovered and thus was being lost to the analysis somewhere in the procedure. The extraction method is valid so the ash was submitted for spectrographic analysis. The spectrum indicated large amounts of aluminum were present.
Cd, ppm
Ratio AI/Cd
10 1 1 1 1
0 0 0 0
10
1011 100/1 300 '1 600/1 1000 11
Absorhance
0 0 0 0 0 0
114
100 019 009 005 005
Table 111. Comparison of Cadmium Solutions (Values Given as Concentration Readoat) Actual concentration
Standards in 0.5N HCl'
413 p p b 80 ppb 120 ppb
0.42 0.83 1.25
Samples in 0.5N HC1 after extraction and back-extractiona
0 42 0.85 1.29
a Appropriately prepared blanks were used for these measurements. The back-extraction procedure was used because these aqueous samples are stable for at least eight days.
The effect of aluminum on cadmium extraction was investigated. First, however, the effect of aluminum on cadmium absorbance was checked. The results shown in Table I indicate that up to a ratio of a t least 1000/1, aluminum has no effect on the absorbance of cadmium. Extraction solutions were prepared containing ratios of aluminum to cadmium as shown in Table I. After the precipitate settled, the absorbance of the supernatant solution was measured and the results shown in Table I1 indicate considerable loss of cadmium. The problem is easily eliminated by adding a complexing agent such as tartrate. Therefore, in place of' the 40 ml of sodium acetate. 40 ml of 1M sodium potassium tartrate was added. Synthetic samples were prepared accordingly, containing 40, 80. and 120 ppb of cadmium and 1000 times as much aluminum. The cadmium was extracted and then back-extracted into the aqueous phase and compared to standard solutions of cadmium in 0.5N hydrochloric acid. The results shown in Table I11 indicate extraction of cadmium was complete with tartrate present to prevent precipitation of aluminuni. ACKNOWLEDGMENT I would like to thank Victor Rauschel for preparing the ashed organic samples and Suzanne Krogh for performing the emission spectrographic analysis. Received for review January 14. 1974. Accepted March 29, 1974.
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