with those measured by a gradient technique ( 5 ) .The calculated values are all within l% of the measured ones, and the standard deviation is onlv 0.0021e/cm3. Good agreement is also obtained-when calculated and measured densities are compared in determinations of methyl-group content. Table I1 shows results for seven different polyethylene films having from 2 to 40 methyl groups per 1000 carbons. A correlation of the two sets of data shows the following characteristics: Correlation coefficient, 0.9997; Intercept, 0.0258; Slope, 0.997; Standard error of slope, 0.0029; and Standard error of intercept,
Table 11. Effect of Density Correction on the Determination of Methyl Groups in Polyethylene Methyl groups/lWO carbons
Measured density
Calculated density
2.85 7.02 15.6 26.5 29.8 30.1 41 .O
2.86 7.01 15.7 26.4 29.7 29.9 41 .O
0.072.
Methyl Group Determination. Methyl groups are determined on a film sample of known thickness in a doublebeam spectrometer which has been calibrated with a known compound (Le., n-hexadecane) to convert absorption at 1378 cm-' to number of methyl groups per 1000 polyethylene carbons. The sample is first scanned from 1430 to 1250 cm-'. Then absorption at 1304 cm-' is measured and the dial setting made using Equation 3. The compensated spectrum is obtained, absorptions a t 1378 cm-' and 1304 cm-', if any, are measured and, along with the dial setting, are substituted in the equation: Number of Methyls 1000 c 1;4
(
Ai318 c m-1
1.003 - 0.0671
Actually, the high precision is inherent in the compensation of amorphous polyethylene. Because the entire range of 0 to 100% amorphous polyethylene corresponds to densities ranging from 1.00 to about 0.85 g/cm3, compensating to within 3% of the correct amorphous value corresponds to a precision of 0.5% in the measurement of density. Therefore, by eliminating the need for a separate density measurement, the calculation procedure permits faster methylgroup determinations with an accuracy equivalent to that of the ASTM procedure. ACKNOWLEDGMENT I gratefully acknowledge the assistance of W. T. Kaminsky, R. R. Hopkins, E. M. Banas, and J. M. McConnell in the design and development of the circular wedge.
- 0.5 A,X,, ((Ai304 cm-i
-1
+
0.000981 (D)+ 0.0067
- 0.0203)/0.0068 + Q0.329 + 2.25 F
If density is measured separately, the following equation is used: LITERATURE CITED
Number of Methyls 441 - F 1000 c
("
7 8 om-i
(1) "Absorbance of Polyethylene due to Methyl Groups at 1378 cm-'"
+
- 0.5 A1304 cm -1
0.000981 (0) + 0.0067
)
P
l3
(9)
Comparison of Calculated with Measured Densities. As shown in Table I, densities calculated from absorption data for 16 different polyethylene films compare favorably
Am. SOC. Test. Mater., Spec. Tech. Pub/., D2238-84.
(2) A. H. Willbourn. J. Polymer Sci., 34, 569 (1959). (3)E. M. Banas and R . R. Hopkins, Appl. Spectrosc., 15, 153 (1961). (4) "Density of Plastics by the Density Gradient Technique", Am. SOC. Test. Mater., Spec. Tech. Pub/.. D-1507. ( 5 ) C.W. Bunn, Roc. R. SOC.London, Ser. A, 180,88 (1942).
RECEIVEDfor review November 1, 1974. Accepted February 18,1975.
Spectrophotometric Determination of Nitrite Ion with 4,4'-Bis(dimethylamino)thiobenzophenone James A. Dougherty and Gilbert A. Laban College of the Virgin Islands, St. Thomas, USVlOO80 1
In our search for very sensitive methods for the determination of nitrite, we have tested the indicator 4,4'-bis-(dimethy1amino)thiobenzophenone which was first suggested by Sawicki and coworkers ( I ) as a reagent for the determination of nitrite ion. Feigl (2) also used this indicator to detect trace quantities of halogens in the gas phase. We have found 4,4'-bis-(dimethy1amino)thiobenzophenone to be more sensitive for nitrite ion than the diazo compound formed in the Griess method (3). We have also found the use of this indicator requires less time for color develop1130
*
ANALYTICAL CHEMISTRY, VOL. 47,
NO. 7, JUNE 1975
ment than the latter method. The sensitivity of the propg/cm2 as composed method for ( A = 0.001) is 8 X pared to the Griess method which for ( A = 0.001)is 1 X 10-~ pg/cm2. Sawicki et al. ( I ) pointed out that a green color develops when a solution of the indicator, 4,4'-bis-(dimethylamin0)thiobenzophenone dissolved in 2-methoxyethanol, reacts with an aqueous solution of nitrite ion. Following these authors' procedure, we found that if the green partially aqueous solution is extracted with chloroform, the ex-
0 5
i
0.4
1.2
L 0 3 4
a m 0
;0 2 a C
B 0.I
400
500
600
A
700
NM
Figure 1. Absorption spectra of (A) indicator: (B)indicator, nitrite reaction product: (C) aqueous indicator nitrite reaction product sohtion
I 4
8
12
16
20
24
28
TIME I N MINUTES
Figure 2. Effect of time on formation of reaction product
cess indicator is extracted into the chloroform layer leaving a stable blue reaction product in the aqueous phase. We base our method on the spectrophotometric measurement of the aqueous phase a t 650 nm. Sawicki et al. ( I ) proposed an auto-catalytic reaction mechanism for the color forming reaction. We plan to investigate the mechanism of the reaction in the light of our present findings.
EXPERIMENTAL Apparatus. All absorbance measurements were made using a Beckman D.U. spectrophotometer. Matched 1.00 f 0.01-cm silica cells were used throughout. All spectrophotometric measurements were made a t 25 O C . A Leeds and Northrup Model 7413 pH meter was used for all pH measurements. Weighings were done on a Mettler Type H 20 balance. Reagents. The 4,4’-bis-(dimethy1amino)thiobenzophenone was an Eastman Kodak White Label reagent. The reagent grade 2methoxyethanol used was obtained from Fisher Scientific. Both reagents were used as received without further purification. A solution which was 0.03% w/v in 4,4’-bis-(dimethylamino)thiobenzophenone and 0.2% w/v in concentrated hydrochloric acid was prepared in 2-methoxyethanol. This solution was stable for approximately 48 hours. A stock solution of nitrite ion was prepared by dissolving 0.3756 gram of sodium nitrite in 250 ml of distilled water. One-half ml of chloroform was added as a preservative. A fresh stock nitrite solution was prepared after one month. Portions of this stock solution containing 1.0 mg NOdml were diluted to give the desired concentrations. The buffer solution used was 0.025M in sodium monohydrogen phosphate and 0.025M in potassium dihydrogen phosphate ( 4 ) . Fisher reagent grade chemicals were used throughout. The double distilled water used was stored in a polypropylene container. Procedure. High Range. A 4.0-ml sample containing from 1 to 10 wg nitrite per ml was transferred to a 50-ml beaker. One ml of buffer solution and 5.0 ml of the indicator solution were added, the solution was mixed and transferred to a separatory funnel. After five minutes, 5.0 ml of chloroform was added, the funnel shaken, and the layers allowed to separate. One ml of the aqueous layer was transferred to a 10.0-ml volumetric flask which was then diluted to volume with distilled water. The absorbance was read in a 1cm cell a t 650 nm vs. a reagent blank within 30 minutes. Low Range. A 5.0-ml sample containing from 0.1 to 1 wg nitrite per ml was treated as described above. In the low range, 5.0 ml of the aqueous layer was transferred to a 10.0-ml volumetric flask and diluted to volume with distilled water. The absorbance was read in a 1-cm cell a t 650 nm vs. a reagent blank within 30 minutes.
RESULTS AND DISCUSSION Absorption Spectra. Figure 1 curve A shows the absorption spectrum of the indicator. One ml of the indicator solution 0.03% w/v was diluted to 10.0 ml with distilled water and read in a 1-cm cell vs. a distilled water blank. Curve B is the absorption spectrum of the green solution
produced by reacting a solution containing 5.0 pg nitrite/ml with 5.0 ml of indicator solution and diluting the resulting solution 1 to 10 with distilled water. Curve C is the spectrum of the blue solution produced by 4.0 ml of a test solution containing 5 pg nitrite/ml prepared according to the recommended procedure. Curves B and C were recorded using a 1-cm cell and a distilled water blank. The slight absorbance of the indicator at 650 nm indicates the need of using a reagent blank. The excess indicator was extracted into the chloroform by the small absorbance a t 450 nm in Curve C. Since the indicator solution is a dark amber color before reaction and the reaction product is blue, this would give rise to the green color observed in the presence of excess indicator. Effect of pH. A study was made of the effect of pH on the color reaction. In this study, the pH of samples each containing 25 pg of nitrite were adjusted by adding a dilute solution of hydrochloric acid or sodium hydroxide and diluting to 5 ml with distilled water. The color was developed according to the recommended procedure. The results indicate the pH should be adjusted to pH 7 f 1 for maximum color development. Effect of Time. A study was made of the effect of time on absorbance of the blue reaction product. Measurements were made a t 650 nm. Figure 2 shows the absorbance of a series of solutions containing 5 pg nitrite/ml prepared according to the recommended procedure but with varying reaction times. Figure 2 indicates the color develops within five minutes after addition of the indicator. The blue reaction product was stable for 30 minutes. Effect of Concentration of Indicator. A study was made to test the effect of increasing amounts of indicator on a fixed amount of nitrite ion. In this study, solutions containing 5 pg nitrite per ml were treated with varying amounts of indicator and the color developed according to the recommended procedure. The maximum absorbance occurs when 5 ml of indicator solution was added to the nitrite. A decrease in absorbance was observed on addition of larger amounts of the indicator solution. This behavior can be explained on the basis of the increased volume of the aqueous 2-methoxyethanol layer when large amounts of indicator were added. The increase in volume would decrease the absorbance of the blue reaction product in the partially aqueous phase. Effect of Foreign Ions. Table I lists the ions tested. Varying amounts of foreign ions were added to fixed amounts of nitrite ion, and the color was developed accordANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
*
1131
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
