Trace Inorganics In Water

14

Downloaded by UNIV OF MICHIGAN ANN ARBOR on October 27, 2014 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0073.ch014

Analysis of Industrial Waters by Atomic Absorption J. A . P L A T T E Calgon Corporation, Hall Laboratories Division, Pittsburgh, Pa.

Simplicity, rapidity, and specificity have caused adoption of atomic absorption as a standard method in water analysis. Often solutions must be concentrated prior to measurement. Freezing, evaporation, ion exchange, and solvent extraction techniques have been reported. This paper describes a method for concentrating ferric iron, copper, zinc, cadmium, and lead using sodium diethyldithiocarbamate and methyl isobutyl ketone. Data shows increase in sensitivity caused by (1) concentrating effect of extraction, and (2) choice of the ketone solvent in preference to water. Recovery data on various industrial waters indicate that the method is reli able, reproducible, and accurate.

Τ η the field of water analysis, an increasing number of laboratories are adopting atomic absorption as a standard method for the determina tion of many metals. Simplicity, rapidity, and specificity are the predomi nant factors for its wide acceptance, particularly in areas where the num ber of water samples analyzed is large (3). However, the concentra tion of certain metals in many industrial water applications must be maintained and controlled below the practical limit of detectability by atomic absorption. For instance, in modern high pressure boiler opera tions, the desired limit of iron, copper and zinc i n the boiler feedwater cycle is usually less than 5-10 parts per billion (p.p.b.). The concentra tion of lead in drinking water should be less than 50 p.p.b. Therefore, when the metal or metals of interest are present in the low p.p.b. range, the constituents to be measured i n the sample must be concentrated i n some manner before analysis by atomic absorption. Concentration by freezing, evaporation, ion exchange, and solvent extraction of metal organic complexes has been reported bv others. The solvent extraction

247 In Trace Inorganics In Water; Baker, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.


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technique was chosen for investigation because of its simplicity and the definite possibility that several metals could be simultaneously extracted in one operation. Allan ( I ) has shown that when copper or the copper complex of ammonium pyrrolidene dithiocarbamate is dissolved or extracted into methyl isobutyl ketone ( M I B K ) , sensitivity is increased 3.7 to 4.7 times over the same concentration of copper in aqueous solution depending on the type of burner employed. A further increase in sensitivity can be gained by extracting metal ions from a large volume of sample into a much smaller volume of an organic solvent. The total relative gain in sensitivity is equal to the gain caused by the organic solvent times the number of concentrations obtained by organic extraction. Malissa and Schoffmann (2) describe the preparation of ammonium pyrrolidene dithiocarbamate and show it to be advantageous because it is a broad spectrum complexing agent, and that its metal complexes can be extracted over fairly broad p H ranges. However, we found that the commercially available material would not dissolve when prepared according to directions given in the literature. Sodium diethyldithiocarbamate (4) forms complexes with many metals which are more or less soluble in various organic solutions. Pre liminary tests showed that at p H 2-3, the carbamate complexes of ferric iron, zinc, cadmium, copper, and lead are extractable from water with methyl isobutyl ketone. Experimental

Work

Comparison tests shown in Table I were run on small amounts of iron, copper, zinc, lead, and cadmium in methyl isobutyl ketone to deter mine the increase in sensitivity over the same concentration of metals in water solutions. The organic solutions were prepared by adding known volumes of acidified aqueous standards by means of a micro-buret to 5 m l . methyl alcohol and diluting to 100 m l . with methyl isobutyl ketone. The absorbance of each metal was measured by a Perkin-Elmer 303 Atomic Absorption Spectrophotometer using the standard instrumental parameters suggested by the manufacturer for each metal. The absorbance values were taken either at 3X or 10X range expansion using a recorder. When spraying organic solvents into the flame, the volume of acetylene must be reduced until the flame becomes nonluminous. The zero reference solu tion contained a quantity of reagent grade water equal to that used to prepare the standards and the same volume of methyl alcohol and methyl isobutyl ketone. The relative increase in sensitivity for lead is quite surprising. The test was repeated several times with higher concentrations of lead and the same values were obtained. The values found for iron and copper agree fairly well with those reported by Allan (2). However, the increase for zinc does not agree with that reported in the same article. H e found that zinc had the same relative increase as that for copper.

In Trace Inorganics In Water; Baker, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

14.

P L A T T E

Table I.


Metal

Absorbance Values of Water vs. Methyl Isobutyl Ketone ConcentraWavelength tion, p.p.m.

Fe

3+

2483.3

Cu

2 +

3247.5

Pb

2+

2170

Zn

2+

2138

Cd

2 +

2288

Extraction

249

Analysis of Industrial Waters

0.25 0.50 0.25 0.50 0.5 1.0 0.25 0.50 0.25 0.50

Water

MIBK

Relative Increase in Sensitivity

0.018 0.037 0.036 0.072 0.009 0.018 0.035 0.071 0.022 0.050

0.067 0.140 0.168 0.331 0.056 0.114 0.105 0.201 0.055 0.130

3.7 3.8 4.6 4.6 6.2 6.3 3.0 2.9 2.5 2.6

Ahsnrhanca

Procedure

Several tests were run to determine the efficiency and precision of extraction of metal complexes with sodium diethyldithiocarbamate and methyl isobutyl ketone by the following procedure. A single liter solution was prepared to contain 0.25 p.p.m. of the following: ferric iron, copper, zinc, cadmium, and lead in 1 % hydrochloric acid ( V / V ) . The solution was split into four 250-ml. portions. The concentration of each metal was determined i n the normal manner prescribed for aqueous solutions on one portion. The remaining three portions were adjusted to p H 2.5 with an ammonium acetate-acetic acid buffer solution and transferred to 500ml. separatory funnels. Twenty m l . of a 5 % sodium diethyldithiocarba mate solution was added to each sample and shaken. Each solution was then extracted with two 20-ml. portions of methyl isobutyl ketone. The extracts were combined for each sample and diluted to 50 m l . with methyl alcohol. The absorbance of each metal was determined in the three 50-ml. organic solutions. By this procedure, each metal was theoretically concentrated five times. Using the factors given in Table I, the total theoretical increase in sensitivity of absorbance values over those for aqueous solutions can be calculated as shown in Table II. For instance, the total increase in sensi tivity for lead would be 31 (6.2 X 5 ) . B y calculation, 31 X 0.25 p.p.m. taken for analysis should be equal to 7.75 p.p.m. lead when the absorbance Table II. Metal Fe Cu Zn Pb Cd

3+ 2 + 2+ 2+ 2 +

Recovery of Various Metals by Complex Solvent Extraction p.p.m. Found Concentration Actual Theoretical Factor 19.4 23 15 31 13

4.85 5.75 3.75 7.75 3.25

4.6,4.8,4.7 5.6, 5.6, 5.7 3.6, 3.5,3.4 7.8, 7.8,7.7 3.0, 3.0, 3.1

% Recovery 93-97 97-99 91-96 99-101 92-95



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value for the organic solvent is read from a curve prepared with aqueous standards, providing the metal was completely extracted. Table II com pares the theoretical and determined values. The rate of recovery was checked again b y comparing the absorbance of the three test solutions against calibration curves prepared by adding known amounts of stand ards to 5 m l . methyl alcohol and diluting to 50 ml. with methyl isobutyl ketone. The buffer solution used above was prepared by dissolving 62.5 grams reagent grade ammonium acetate (NH4C2H3O2) i n about 200 m l . deionized water. A d d exactly 17.5 ml. glacial acetic acid and dilute to 250 ml. Remove possible meal contaminants by adding 20 m l . of a 5 % sodium diethyldithiocarbamate solution and extract the buffer solution with two 50-ml. portions of methyl isobutyl ketone. Analysis of Samples Recovery tests were run on samples of various industrial waters by the above proposed atomic absorption-solvent extraction procedure. H o w ever before extraction, all samples were acidified with 1 ml. concentrated hydrochloric acid per 100 m l . sample and boiled for five minutes to dis solve precipitated metals. The samples were then cooled and filtered to remove any remaining particulate matter which could clog the atomizer. F o r the determination of iron, 0.1 gram potassium persulfate was added in addition to the acid and then boiled to oxidize ferrous iron to the ferric state. The values shown in Table III are typical of the results that can be expected. Table III.

Type of Sample Condensate Saturated Steam Boiler Feed Condensate Saturated Steam Condensate Treated Plating Waste River Water Surface Water Tap Water Lake Water

Recovery Tests on Various Types of Water Metal Determined

Amount Present p.p.b.

Amount Added p.p.b.

Total Recovered p.ph.

Iron Iron Iron Copper Copper Zinc Zinc Cadmium Cadmium Lead Lead

8 12 27 5 8 5 22 18 7 8 12

5 10 10 5 5 5 10 15 5 5 10

12 24 35 8 12 11 30 31 11 13 19

M u c h of the iron present in most condensates exists as finely divided suspended iron oxide particles. Since most operators of high pressure boiler plants are concerned with the total concentration of iron in the vari ous water cycles, solubilizing the suspended iron (usually hematite) is a necessary part of any procedure for this determination. Tetlow and Wilson (5) have shown that the suspended iron oxide is not completely



14.

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Analysis of Industrial Waters

251

dissolved when condensates are acidified with hydrochloric acid and boiled. They recommend thioglycolic acid and show it to be much more effective than hydrochloric acid. W e prefer ascorbic acid because of the odor problem with thioglycolic acid and the fact that comparative tests made in our laboratory indicate that ascorbic acid is comparable to thioglycolic acid for effectively dissolving iron oxide compounds. H o w ever, when either of these reducing acids is used, very low iron values are found by the proposed extraction method. Evidently, either ferrous iron forms no complex with sodium diethyldithiocarbamate or the complex is not extractable with methyl isobutyl ketone. This observation was con firmed from tests made with standard solutions containing ferrous iron and the recovery by the prepared solvent extraction technique was almost nil. Therefore, we do not recommend solvent-extraction for condensates treated with either thioglycolic acid or ascorbic acid. Perhaps the excess reducing acid could be destroyed and the iron oxidized to the ferric state, but this approach would not be practical for a number of obvious reasons. Table I V .

Comparison of Detection Limits

Gain in Sensitivity

Metal Fe Cu Zn Pb Cd

Concentration by Extraction

Caused by Organic Solvent

Overall

Aqueous

By Proposed Method

5 5 5 5 5

3.8 4.6 3.0 6.2 2.6

19.4 23 15 31 13

50 5 5 150 10

2-3 0.2 0.3 5 0.7

3+ 2 + 2+ 2+ 2 +

Detection

Detection Limit, p.p.b.

Limits

Table I V compares the present reported limit of detectability of iron, copper, zinc, lead, and cadmium in aqueous solutions with that obtainable with the proposed solvent extraction procedure. Detectability is defined as that concentration of metal which gives a signal twice the variability of the background. Since the amount of metal recovered as shown in Table II was not 100% for most metals, the limit of detectability by this method should be increased by about 5-10%. Conclusion The results of this investigation show that the method described can be effectively used when the concentration of metals in water is too low for analysis by normal atomic absorption procedures provided the metals form solvent extractable complexes with sodium diethyldithiocarba mate. W e use this method routinely for the determination of iron, lead,


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and copper in high brines and phosphate compounds and for lead and cadmium in most industrial waters. B y increasing the ratio of sample to organic solvent, the practical level of detectability should be even less than that shown in Table I V .


Literature

Cited

(1) (2) (3) (4)

Allan, J. E., Spectrochim. Acta 17, 467 (1961). Malissa, H., Schoffmann, E., Mikrochim. Acta 1, 187 (1955). Platte, J. Α., Marcy, V. M . , Troc. Am. Tower Conf. 27, 851 (1967). Sandell, Ε. B., "Colorimetric Determination of Traces of Metals," 2nd ed., p. 304, Interscience Publishers, Inc., New York, 1950. (5) Tetlow, J. Α., Wilson, A . L . , Analyst 89, 447 (1964).

RECEIVED

April

24,

1967.


Trace Inorganics In Water

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