Table I. Effect of Foreign Ions, Concentration of Nitrite iOpg/ml Concn Ion
Mg?' Ca2+ Sn2+ co?+
cuz+
~13+ N a'
c1Croi2Mn0,NO?-
permitted, ppm
48 10 600 1000 50 1000
Table 11. Recovery of Nitrite Amount of nitrite added
Amount of nitrite founda
Concn
Ion
S2104-
0 3 50
CNSO12-
22 200
F-
1000
C103'
20 600 600
1000
c,o,2-
1000
37 0 600
IPO,?c 12
1000
103'
0.50 pg 1.0 p g 3.0 p g 10.0 pg
permitted, ppm
1000 0
ing to the recommended procedure. When interference was excessive, the experiment was repeated with a reduced quantity of foreign ions. A 2% relative error in the determination of nitrite was considered tolerable. Table I indicates the permissible quantity of foreign ion. Beer's Law, Precision, a n d Sensitivity. Two standard series containing 0.1,0.2, 0.5, 0.7, 1.0; and 1.0, 2.0, 5.0, 10 pg nitrite/ml were tested according to the recommended procedure. Each series was found to obey Beer's law. The relative standard deviation for 6 determinations a t the 0.5 pg nitrite/ml level was 1.5% and a t the 5 pg nitrite/ml level 2.3%. The sensitivity of the method according to Sandell (5) for ( A = 0,001) is 8.4 X pg/cm2. The molar absorptivity based on nitrite is 5.5 X lo4. Application of Method. Samples of nonchlorinated tap water were spiked with known quantities of nitrite and the
a Based
NO,'/ml N02'/ml NO,-/ml N02'/ml
0.55 pg N02-/ml 0.98 pg N02-/ml 3.0 pg N02-/ml 10.3 pg NO,-/ml
on three determinations.
nitrite concentration determined according to the recommended procedure. The results are tabulated in Table 11. In each case, the amount found was based on 3 determinations. The amount found was obtained from a calibration curve prepared by adding known amounts of nitrite to distilled water and determining the absorbance according to the recommended procedure. LITERATURE CITED (1) E. Sawicki, T. W. Stanley, J. Pfaff. and A. D'Amico, Talanta, 10, 641 (1963). (2) Fritz Feigl. "Spot Tests in Organic Analysis", Elsevier. New York, 1966, p 65. (3) P. Griess, Ber., 12, 427 (1879). (4) Louis Meites, "Handbook of Analytical Chemistry", 1st ed., McGraw-Hill, New York. 1963, p 11-3. (5) E. B. Sandell, "Colorimetric Determination of Traces of Metals", 3rd ed.. Interscience. New York, 1959, p 83.
RECEIVEDfor review November 1, 1974. Accepted February 3, 1975. This paper was presented in part a t the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, OH, March 4-8, 1974. Paper number 135.
Improved Automated Extraction Method for Atomic Absorption Spectrometry F. D. Pierce, M. J. Gortatowski, H. D. Mecham, and R. S. Fraser Utah State Division of Health, Bureau of Laboratories, Salt Lake City, UT 84 113
In trace metal analysis by atomic absorption spectrometry, the sensitivity is increased by chelating the ions and extracting the chelates with an organic solvent. Extraction by conventional methods, using separatory funnels or volumetric flasks ( 1 ) is the most time-consuming step in the analysis. An automated extraction procedure would therefore be a valuable adjunct to already existing automated sampling systems. We attempted to use the approach described by Goulden et al. ( 2 ) ,but found their method to be ineffective for our particular application. For our purpose, an automated extraction method would have to be applicable to a variety of trace elements. For this reason, we found it necessary to develop our own system which is described in this paper. We employed a parallel extraction system so that more sample volume can be handled with greater extraction efficiency. This approach permits the handling of a large number of samples (60 per hour) and provides greater analytical sensitivity. The apparatus, which includes a standard 1132
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
Figure 1. Standard Technicon 60-2/1 c a m altered for system
AutoAnalyzer (Technicon Instrument Corporation) has been adapted to an atomic absorption instrument which does not offer easy alteration of aspiration rate. EXPERIMENTAL Apparatus. A Beckman Atomic Absorption Spectrophotometer
Flgure 3. Altered sampler as the secondary separator
(A) Sample inlet from solvent overflow of primary separator. (B) Aspirator tube to atomic absorption spectrophotometer. (C) Reservoir for waste. (D) MlBK wash reservoir. (E) MlBK wash reservoir inlet. (F) Empty sample cup to receive MlBK ion chelate mixture. (G) Cup containing sample to be aspirated. (H) Waste outlet
Figure 2. Primary separator (1 c m i.d. X 1 0 c m )
(A) Solvent overflow to secondary separator. (B) Aqueous underflow to un-
derflow retention reservoir. (C)Inlet from extraction coils (Model 440) was used in conjunction with a variable span and variable speed strip-chart recorder. T h e proportioning pump was a Technicon 11, 20-channel type (Technicon Instrument Corporation). Two Technicon I samplers were used. T h e first sampler was adapted for sample introduction by altering the sampling cam (Figure 1) in such a way that it allowed the sampler probe to withdraw a sample from two consecutive cups each containing the same water sample. T h e second sampler was used for secondary phase separation. Separation of the aqueous phase from the organic phase was accomplished by a two-stage system (Figures 2 and 3). T h e primary separation stage tube (Figure 2) allowed the organic phase t o be totally conserved while the major portion of the aqueous phase was discarded, T h e secondary separation stage unit (Figure 3) had a
modified probe and reservoir configuration which received the solvent overflow from the primary separation stage tube and also acted simultaneously as an aspirator for the previously received sample. The cam used in the secondary separator was a standard 60-2/1 cam, the operation of which was manually synchronized with the operation of the first sampler's modified cam. This synchronization permitted the secondary stage separator to receive the water sample as the sample reached the secondary separation phase. The effectiveness of the synchronization process can be visualized by employing a water soluble dye (Methyl Orange) in the sample sequence. T h e manifold (Figure 4) was constructed in such a way that parallel units were used. This enabled the sample to be split, mixed with reagents, extracted, then recombined prior to entering the organic phase separation stage. The chelant and sample were mixed in two parallel Technicon mixing coils. Each mixing coil was followed in series by a Technicon extraction coil (COIL, 30.00TS3.4 mm EXTRACTR). In this system, the sample to solvent ratio was 7.21.
J
K Figure 4. Manifold diagram
Sample inlet. (B) APDC inlet. (C 8 D) H20 inlet. (E) Large mixing coils. (F) Large extraction coils. (G) MlBK displacement reservoir. (H) Primary separator. (I) Secondary separator. (J) Waste. (K) Atomic absorption spectrophotometer.(L) MlBK reservoir inlet. (M) Underflow retention reservoir. (N) Wash to first sampler
(A)
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
1133
Table I. Comparison of the Automated Extraction Method vs. Manual Extraction .Methoda Cd Sample No.
(1
cu
MC
Ab
A
M
Mn
Pb
Fe
M
A
M
A
5 6 4 4 6 5 1 5 4 12 9 12 10 16 15 2 12 12 26 24 19 18 20 22 3 18 20 34 35 38 40 42 44 4 42 40 65 70 5 64 67 58 62 60 64 90 97 6 92 100 94 98 98 106 Values are expressed in pg/l. h Automated method. Manual method.
Ni
Zn
'49
A
M
A
M
A
5 10 24 40 54 90
5 12 24 42 58 95
4 9 17 36 51 86
4 10 18 38 55 95
4 11 20 35 56 90
M
A
M
3 12 22 36 60 99
6 10 16 42 60 93
4 8 16 40 64 98
('
Table 11. Recovery of Cations Added to Water Samples of Known Cation Concentrations Cd
Cation added gg/l.
Tb
cu
F C Dd
T b FC
.. ...
..
Dd
Tb
.
FC
Mn
Pb
Fe
Dg
Tb
... ...
FC
.
Dd
FC
T b
.
Ni
Dd
FC
Tb
.. .. .
Zn
A9
d
T b FC
. ..
.
Dd
.
T b FC
Dd
. . .. .
0" 2" , 15" , ., 20" 4" . . . . 24" . 8" . , , 2" . . . , 18" . 5 7 8 +1 20 20 0 25 26 +1 9 11 +2 29 28 -1 13 12 -1 7 7 0 23 24 +1 15 17 15 -2 30 3 0 0 35 35 0 19 18 -1 39 38 -1 23 22 -1 17 16 -1 33 34 +1 20 22 20 -2 35 36 +1 40 41 +1 24 25 +1 44 44 0 28 26 -2 22 24 +2 38 40 +2 50 52 1 2 34 34 0 54 53 -1 38 36 -2 32 32 0 48 46 -2 30 32 34 +2 45 44 -1 40 42 40 -2 55 57 +2 60 62 +2 44 43 -1 64 62 -2 48 50 +2 42 40 -2 58 5 8 0 a Concentration (pg/l.) of cation determined by manual method prior to cation addition. Total calculated cation concentration (pg/l.) analyzed by the manual procedure. Cation concentration (pg/l.) analyzed by automated procedure. d Micrograms difference between cation concentration found and the total.
Table 111.Precision of Automated Extraction Analysis Replicate determinations, u g l l . Cation
1
2
3
4
5
6
7
8
9
10
11
12
13
Mean
Std dev
Cd
10 24 42
12 26 40
12 24 41
10 25 42
11 24 40
10 24 42
12 25 44
12 26 42
13 25 42
12 25 42
13 25 42
10 24 42
12 25 42
11.5 24.8 41.9
1.12 0.72 1.18
cu
10 18 34
10 20 38
10 20 36
9 17 36
10 18 38
9 18 38
9 18 38
8 19 37
9 20 37
10 19 35
9 19 36
9 20 36
8 19 37
9.2 18.9 36.6
0.72 0.98 1.26
Fe
13 25 40
13 26 41
12 24 40
12 25 41
13 25 41
13 25 41
13 26 41
13 25 42
12 25 40
12 24 42
13 25 43
12 24 40
12 24 42
12.5 24.9 41.1
0.51 0.68 0.95
Pb
10 18 36
12 17 38
12 18 39
13 18 38
12 19 36
11 20 37
11 20 35
12 .18 36
13 17 38
12 17 36
13 18 38
13 19 38
11 17 38
11.9 18.2 37.2
0.95 1.06 1.21
Polyethylene sample cups of 8-ml capacity (Evergreen Scientific Corporation) were used in the secondary separation stage sampler. Silicone tubing (Cole-Parmer) or glass connections were used beyond the MIBK addition stage. Reagents. Analytical reagent-grade chemicals were used throughout. Ammonium pyrrolidine dithiocarbamate (Fisher Scientific Company) was prepared daily in a 1%solution (w/v) and filtered prior to use. Procedure. All working standards and samples were adjusted with 0.3N HNOs to pH 2.3 f 0.1 which is optimum for extraction ( I , 3). The operation of the atomic absorption spectrophotometer was in accord with the manufacturer's instrument operation manual. T h e parameters chosen for analysis are documented in reference (3).
RESULTS AND DISCUSSION In our laboratory, the factors which seemed to limit the usefulness of the apparatus described by Goulden et al. were the extraction efficiency and solvent separation. We found that the most effective extractor for the volume of sample handled, was a parallel system of beaded extraction 1134
ANALYTICAL CHEMISTRY, VOL. 47,
NO. 7,
JUNE 1975
coils. This arrangement minimized pressure in the constant-flow system. In the present system, the separation of the organic phase from the aqueous phase was accomplished in two stages. The first stage consisted of a primary separation tube (Figure 2) which was physically separated from the secondary stage unit (Figure 3). The primary separation tube (Figure 2) was closed and the MIBK-water interface level relative to the outlet port "A" was maintained by continual removal of the aqueous phase through port "B". The secondary separator unit (Figure 3), which receives the organic phase, employed two sampler probes which are attached to the same sampler arm. One probe (Figure 3, A) delivered the MIBK containing the ion chelate t o the receiving cup "F". The second probe (Figure 3, B) acted as an extension of the spectrophotometer's aspirator and served as a means of drawing the organic solvent into the atomic absorption instrument. One of the virtues of this two-stage separator system is the time-delay between sample deposition into the sample cup "F" and aspiration of the sample
from cup “G”.This approach permits complete separation of the two layers. Added features of this system are that sample vials instead of sample cups could be used in the secondary separator and that the organic phase can be saved for use in other analytical procedures. A comparison of the results obtained by automated extraction vs. manual extraction in volumetric flasks is presented in Table I. The data show an acceptable correlation between the two methods in the concentration range from 0 to 40 pg/l. Although the extraction efficiency of the automated method gradually decreases a t concentrations greater than 40 pgll., its efficiency is still within 90% at the 95106 pg/l. level. Because of the principles attending an automated system and barring interfering substances in the sample, extraction efficiency at the higher concentrations becomes of secondary importance since both standard and sample are treated identically and thus are affected equally by the extraction process. Studies in our laboratory have shown that exceptionally high cation concentrations do not significantly increase ion carry-over from one sample to the following sample. Samples containing an ion concentration of 100 pg/l. or greater can be analyzed by direct aspiration of the sample. Recovery experiments were conducted by adding known amounts of metal cations to water samples which were previously analyzed by the manual extraction method. The results recorded in Table I1 show a maximum difference of 2 pgll. even a t the higher concentrations.
Data regarding the precision of the automated extraction method are presented in Table 111. Water samples containing three concentrations of each metal were analyzed for cadmium, copper, iron, and lead. Each sample was analyzed thirteen times, allowing a five-minute interval between determinations. The mean and standard deviation calculated for samples containing the highest concentrations was found to be: cadmium, 41.9 f 1.18 pg/l.; copper, 36.6 f 1.26 pg/l.; iron, 41.1 f 0.95 pgll., and lead, 37.2 f 1.21 pg/l. The sensitivity for each of the cations analyzed was: cadmium, 0.2 pg/l.; copper, 0.4 pg/l.; iron, 1.0 pg/l.; lead, 1.5 pgll.; manganese, 0.4 pg/l.; nickel, 1.0 pgll.; silver, 0.4 pg/l., and zinc, 0.2 pg/l.
ACKNOWLEDGMENT The authors thank R. E. Isaacs for his support. LITERATURE CITED (1) T. T. Chao, M. J. Fishman, and J. W. Ball, Anal. Chim. Acta, 47, 189 (1989). (2) P. D. Gouiden, P. Brooksbank, and J. F. Ryan, Amer. Lab., 5 (8), 10 (1973). (3) C. R. Parker, “Water Analysis by Atomic Absorption Spectroscopy”, Varian Techtron Ry. Ltd., Springrale, Australia, 1972, pp 28, 29, 42, 43, 48-51, 57, 58, 82-65, 72, 73.
RECEIVEDfor review October
2, 1974. Accepted February
3, 1975.
Determination of the Solubility of Manganese Hydroxide and Manganese Dioxide at 25 OC by Atomic Absorption Spectrometry H. A. Swain, Jr., Chris Lee, and R. B. Rozelle Department of Chemistry, Wilkes College, Wilkes-Barre, PA 18703
A number of methods have been employed to measure the solubility product constant of Mn(0H)Z and are documented in the text. This paper contains measurements which have for the first time measured the K,, of Mn(OH)2 by atomic absorption spectrometry. Also, the solubility of MnO2 was measured as a function of pH and graphical results used to calculate the K,, for the solubility of MnOz. The MnO2 used had a surface area of 1.0 m2/g and had the rutile crystal form.
EXPERIMENTAL Fisher Scientific Company reagent grade MnS04. H20 and MnO2 were used. The distilled water used was boiled and cooled under Nz. Carbonate-free NaOH solutions were prepared and used. The experiments were carried out under N2 and necessary precautions taken to avoid CO2 contamination of samples. The Mn02 samples were washed with distilled water and dried a t 110 “C before use. A Perkin-Elmer 290 atomic absorption spectrometer was employed for manganese analysis and found to have an ultimate sensitivity of 0.02 ppm manganese. The instrument was calibrated before each analysis using MnS04. H20 solutions of known concentrations in distilled water.
The Mn(0H)Z solubility measurements were made under a NZ atmosphere in a flask maintained a t 25.0 f 0.2 “C by means of a constant temperature water bath. Carbonate-free NaOH was added to 1000 ppm manganese(I1) solutions which were then equilibrated for varying periods of 24-72 hours and filtered under Nz. The data showed only random variation when the equilibration times were greater than 24 hours. The pH was measured before and after filtration and, in each case, the pH dropped by about 0.5 pH unit. The MnOz solubility measurements were made under a N2 atmosphere by mixing 0.25 g of washed and dried MnO2 and 50 ml of carbonate-free distilled water. The pH was varied from 2.1 to 10.6. The washed and dried MnO2 was found to have the rutile crystal structure and to have a surface area of 1.0 m2/g, increasing from about 0.7 m2/g before washing and drying. The solutions were allowed to equilibrate for approximately l to 4 hours a t 25 f l “C. Further precision in temperature control was not merited by the relatively poor precision for the small concentrations being measured. Samples from the MnOz, as well as from the Mn(0H)z runs were filtered through 50-mp millipore filter sheets. As a check, 100-mp millipore filters were used occasionally, giving results equivalent to those from the 50-mp runs. The pH was measured before and after filtration and, if the difference in the two pH’s was greater than 0.5 pH unit, the data were ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
1135
