727
Anal. Chem. 1981, 53, 727-731
AIDS FOR ANALYTICAL CHEMISTS Delivery of Acid Preservative for Trace Metal Determinations in Waters Ronald L. Guest State Rivers and Water Supply Commlssion, 590 Orrong Road, Armadale, Vlctoria, 3143, Australia
Harry Blutsteln" Environment Protectlon Authorlty, 240 Victoria Parade, East Melbourne, Victoria, 3002, Australia
The accepted method for preserving water samples for heavy-metal determinations involves the addition of sufficient concentrated nitric or hydrochloric acids to reduce the pH below 2 (I). This inhibits adsorption of metals ions onto the container surface and the removal of dissolved metals through hydrolysis. When water samples are collected in the field for heavymetal determinations, it is essential that there is little or no delay in preserving the sample, SQ that losses can be minimized. It has been common practice to add redistilled acid, in the field, using a glass or polyethylene measuring cylinder or other volumetric device. This procedure has two disadvantages. Transporting concentrated acid preservative in a glass container to each sampling site is hazardous and manipulating it under field conditions presents very definite hazards. Second, contamination of the sample may occur during the preservation step. It may arise from the acid added, the measuring cylinder or other device used, or prolonged exposure of the sample to the atmosphere with the possibility of contamination from dust-borne heavy metals.
EXPERIMENTAL SECTION Procedure. A new approach to adding acid preservatives in the field has been developed in which the redistilled acid is sealed in prewashed 10- or 20-mL glass ampules and placed in the empty sample containers, which are then capped. The container and ampule can then be safely trrlnsported from site to site. To keep blanks to a minimum, it is necessary to clean the ampules prior to their insertion into the sample containers. The glass ampules are filled with 2 N nitric acid and immersed in the same for a week. They are then drained, rinsed with distilled deionized water, and drained again. Following this the ampules are immediately filled with the required amount of redistilled nitric acid. At this stage it is essential to protect unsealed ampules from airborne dust, which may provide a source of contamination. The ampules are then sealed by use of an OceanographicSealing Unit, and the sealed ampules are stored in 2 N nitric acid for at least 24 h and rinsed with distilled, deionized water prior to being placed in acid-washed polyethylene containers, using polyethylene acid-washed tongs or disposable surgical gloves. At the sampling site the inside of the sample bottles and the outside of the ampules are rinsed several times with the sample water by half filling the container and then empyting it, with the cap partially covering the container mouth 1.0 keep the ampule in the container. The empty capped sample bottle is shaken to break the glass ampule and release the acid. The container is immediately filled with the water sample and capped. The container needs to be inverted several times to thoroughly mix the acid and water. Analysis. Zinc, cadmium, lead, copper, nickel, and chromium were determined simultaneously by using a solvent extractionatomic absorption spectrophotometric method. The sample was extracted into diisobutyl ketone by using a mixed chelating solution of ammonium pyrrolidine dithiocarbamate (CAS no, 5108-96-3),diethyl sodium dithiocarbamate (CAS no. 148-18-5), 0003-2700/81/0353-0727$01.25/0
and cupferron (CAS no. 135-20-6) at pH 4.9 f 0.1. The nonaqueous solvent was aspirated directly into a Varian Techtron AA6 atomic absorption spectrometer, and metal concentrations were determined by double standard addition. The blank is treated in the same way using the same batch of reagents. When samples were delivered to the laboratory for analysis, a sample bottle containing only an ampule was also included and a blank was made up by adding MilliQ (Millipore Corp.) water to the sample bottle.
RESULTS AND DISCUSSION The blank determinations of zinc, cadmium, lead, chromium, copper, and nickel over a 29-week period are shown in Figures 1-6. During the first 14 weeks (up to point I in the figures) acid preservative was added in the conventional manner (hereafter referred to as method A). Between points I and I1 in the figures the ampule preservation system underwent a trial period in which inspectors were trained in its use. During the remaining period (point I1 onward) the ampule system was fully operational (hereafter referred to as method B). A statistical analysis of the two preservative delivery methodp was done (excluding the trial period), and the results are presented in Tables 1-111. T o determine whether methods A and B differ from one another the hypothesis
H,:
PX
= Py
z
u,2
was tested against using the assumption ax2
A Student's t test was carried out to test the difference between the two means for the six metals involved, while an F test was used to determine the equality of the variances. The results are presented in Tables I and 11. Table I gives the means, standard deviations, and sample sizes for methods A and €3. In every case the mean blank value decreased from method A to B. The largest percentage decrease occurred with zinc. Chromium underwent the least improvement. The results in Table I1 show that for lead, nickel, and chromiw, both methods give results that are not significantly different at the 95% significance level. As for cadmium, zinc, and copper, the results show that the two methods do differ significantly and the use of ampules result in a distinct improvement in the blank level. The F test was carried out to verify the assumption that the variances are not equal. At the 95% significance level, results show that the assumption of unequal variances (ux2 # u;) is valid and therefore the conclusion obtained from the t test for testing the null hypothesis may not be attributed 0 1981 American Chernlcai Society
728
ANALYTICAL CHEMISTRY, VOL. 53, NO. 4, APRIL 1981 18-
*
171615'
1413-
TIME (DAYS)
Figure 1. Plot of cadmium blank concentrations (in pg/L) vs. time (In days).
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Table I." Means and Standard Deviations of Blank Values for Six Metals by Using Two Methods (A and B) for Delivering Acid Preservative
--
cadmium B
A
zinc A
X S, n,
0.23 0.10 8.79 0.27 0.11 6.91 66 32 66 r a n d S , expressed in og/L.
lead
nickel
B
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2.33 1.43 32
1.98 3.10 66
1.10 1.20 32
1.24 1.85 66
0.78 0.38 32
to the fact that this assumption has not been satisfied. The data best fit a lognormal distribution, that is X=lnY with a mean p and variance u2 and Y lognormally distributed. The graph of the natural logarithm of values y of the random variable Y vs. the empirical cumulative frequency function is approximately a straight line. An estimate of p and u may
chromium B
A
2.13 1.17 66
2.11 0.87 32
copper A
B
1.10 0.82 66
0.76 0.46 32
be obtained by carrying out a method of least squares. Having characterized the distribution, it is possible to identify outlying blank values. This is the measure of gross contamination which may be due to either the reagents used in the extraction step of the determination or the nitric acid preservation. The distribution over the entire population (method A + B) has been taken to calculate the lognormal distribution. Outlying values indicative of gross contamination have been defined
ANALYTICAL CHEMISTRY, VOL. 53, NO. 4, APRIL 1981
0
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Figure 3. Plot of
zinc blank concentrations (in pglL) vs. time (in days).
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11.0
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TIME (DAYS)
Figure 4.
Plot of nickel blank concentrations (in pglL) vs. time (in days).
Table 11. t and F Tests to Compare Two Methods for Delivering Acid Preservative statistics cadmium zinc lead nickel t
deg of freedom ( u ) at 95% signfcnce level deg of freedom ( u l , u , ) at 95%
F
chromium
7.28 76
0.38 94
1.94 76
0.09 82
reject H,
reject H,
accept H,
accept H,
accept H,
5.93 65,31 reject H,
22.97 65,31 reject H,
6.56 65,31 reject H,
23.31 65,31 reject H ,
3.38 96
as blank values exceeding the 10% critical value of the lognormal distribution. The analysis of the data from methods A and B is given in Table 111. Going from methods A to B, there is a large decrease in the number of outlying values for all the metals. As the method was not changed over the experimental time scale, it is reasonable to conclude that most of the contamination for zinc, cadmium, lead, nickel, chro-
1.78 65,31 reject H,
copper 2.62 96
reject H, 3.13 65,31 reject H,
mium, and copper arises out of handling the acid preservative in the field and that this source can be virtually eliminated by using method B. In addition to reducing blank values, method B does not consume as much time in the field since the release of the acid preservative by breaking the ampule can be achieved in only a few seconds, while adding acid using the conventional me-
730
ANALYTICAL CHEMISTRY, VOL. 53, NO. 4, APRIL 1981
3 436
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Flgure 5. Plot of copper blank concentrations (in pg/L) vs. time (in days).
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Table 111. Analysis of Outlying Blank Values by Using Two Methods of Delivering Acid Preservative cadmium zinc lead nickel chromium statistics 10% critical level (one sided) for A + B a no. of points
A 11
B A+B A 0.4
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Expressed as Fg/L. thod takes between 2 and 5 min. The amount of acid added depends on the buffer capacity of the sample solution. This work was carried out on samples
collected for freshwater ambient monitoring programs, where the buf€er capacity of the samples was low. It was found that the addition of concentrated nitric acid to give a final acid
731
Anal. Chem. 1981, 53, 731-732
concentration of 1% (v/v) in the sample solution was adequate to reduce the pII to less than 2. For more highly buffered samples, such as seawater and certain types of waste waters, it may be necessary to add more acid.
ACKNOWLEDGMENT The authors wish to thank C. Khoo for carrying out the statistical analysis.
LITERATURE CITED (1) “A Guide to the Sampllng and Analysis of Water and Wastewater”; Report No. 95/79; Victorian Environment Protection Authorlty: East Melbourne, Australia, Oct 1979; p 13. (2) “Notes on Water Sampling”; Vlctorlan Environment Protection Authority: East Melbourne, Australia, May 1976; p 134.
RECEIVED for review March 6,1980. Accepted November 10, 1980.
Gas Chromatographic Determination of Polyacrylamide after Hydrolysis to Ammonia Gary G. Hawn” Alcolac Inc., 3440 Fairfield Road, Baltimore, Maryland 2 1226
Charles P. Talky GAF Corporation, 1361 Alps Road, Wayne, New Jersey 07470
Polyacrylamide is frequently used in such diverse fields as water treatment, paper making, petroleum recovery, and mineral processing. Application dosages of polyacrylamide and various cationic or anionic copolymers of polyacrylamide are usually a t the parts-per-million level. lJnfortunately, very few analytical techniques are available for measuring polyacrylamide at these low levels. Crummett and Hummel (I) have outlined two procedures, a distillation-nesslerization method and a turbidimetric method using “Hyamine 1622” (dimethyl [2- [2- [4-(1,1,3,3-tetramethylbuty1)phenoxyl ethoxyjethyl]benzenemethanaminium chloride). The polyacrylamide must be partially hydrolyzed for it to produce turbidity with “Hyamine 1622”. Attia and Rubio (2) developed a turbidimetric procedure which eliminated the need for hydrolysis, whereby polyacrylamide was precipitated directly with tannic acid and quantified by nephelometry. While the techniques described above are accurate and sensitive, colorimetric and turbidimetric procedures are subject to various matrix interferences. We have developed a procedure which has the potential to quantify low levels of polyacrylamide and to eliminate many interferences from matrix effects. The most serious limitation to this method is that other chemical compounds which can liberate ammonia will interfere with the m a y . However, this assa.y has proven useful and accurate in various applications, including the analysis for trace levels of polyacrylamides, leached from food grade paper, as required by the Food and Drug Administration. The derivatization procedure with l-fluor0-2,4-dinitrobenzene has been documented and has been shown to be simple, rapid, and quantitative with various amines (3).
EXPERIMENTAL SECTION Reagents. l-Fluoro-2,4-dinitrobenzene.Purchased from Eastman Organic Chemicals and used without further purification. (Caution: This material is reported to be an experimental carcinogen and mutagen, according to ref 4.) Borate Buffer. Prepared by dissolving 2.5 g of powdered sodium borate decahydrate (Na&3407.10H20)in 100 mL of distilled water. Polyacrylamide Stock Solutions. Prepared by dissolving 1.00 g of polyacrylamide in 1 L of distilled water followed by diluting 1,2,5, and 10 mL of this solution to 1L of distilled water. This gives final stock solutions of 1,2,5,and 10 ppm of polyacrylamide. 2,4-Dinitroaniline. Purchased from Aldrich and used without further purification. Procedure. By use of a volumetric pipet, 100 mL of the polyacrylamide stock solutions was concentrated to 1-2 mL under
a stream of nitrogen. Each solution was transferred to a 25-mL vial which has a screw cap with a Teflon rubber laminated disk. After 3 mL of 10 N NaOH was added, the vial was sealed and placed in an oven at 95 OC for 15 h. The flask was allowed to cool to room temperature and then placed in a dry ice/acetone bath for 5 min, allowing the contents to freeze. Five milliliters of 6 N HC1 was added to neutralize the contents, and the solution was quantitatively transferred to a 50-mL glass-stoppered volumetric flask by washing with distilled water. If the pH of the solution is very acidic at this point, several drops of dilute NaOH are added to bring the pH to the range of 5-6. To this flask are added 10 mL of the borate buffer and 2 drops (-0.1 mL) of l-fluoro-2,4-dinitrobenene.The flask is stoppered, shaken until a yellow color begins to form, and placed in an oven at 60 O C for 30 min. After the mixture was cooled, 5 mL of 10 N NaOH is added followed by 1.0 mL of toluene. The flask is shaken vigorously for 1min, and distilled water is added until the toluene layer is well up into the neck of the flask. The ammonia derivative which is formed, 2,4-dinitroaniline, was quantified by injecting a ~ - Maliquot L of the toluene layer on a Hewlett-Packard Model 57204 gas chromatograph equipped with a flame ionization detector. A 2 m X 2 mm glass-coiled column packed with 3% OV-225 Gas Chrom Q was used for the analysis. The column oven was held at 225 O C with the detector and injector kept at 250 OC. The Nz carrier gas flow was maintained at approximately 20 mL/min. An external standard of 2,4-dinitroaniline was used for quantification. The peak areas were determined with a Hewlett-Packard Model 3350 2B laboratory data system.
RESULTS AND DISCUSSION Four standard solutions of polyacrylamide were analyzed as described in the Experimental Section. The 100-mL sample of the 1.0 ppm polymer standard contains 0.1 mg of polyacrylamide. The amount of 2,4-dinitroaniline which can be formed, based on 100% hydrolysis and liberation of ammonia, can be calculated as follows: w t polymer X (mol wt 2,4-dinitroaniline)/(mol w t monomer unit) = w t 2,cdinitroaniline (1)
0.1 mg X 183/71 = 0.258 mg 2,4-dinitroaniline (2) The amount of 2,4-dinitroaniline formed from the other standards is calculated in a similar manner. Higuchi and Senju (5)have shown that a conversion limit of about 60% exists in the alkaline hydrolysis of polyarcylamide under moderate conditions. However, we feel that the
0003-2700/81/0353-0731$01.25/00 1981 American Chemical Society
