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Anal. Chem. 1983, 55, 793-795
Karl Fischer Titration Equation on Mass Basis Sir: In a recent paper ( 1 ) on the application of automatic Karl Fischer titration to the determination of water in solids, an equation used to calculate percent H 2 0was presented. In this equation advantage was taken of the precision and convenience of use of calibrated syringes for measuring the quantity (volume) of solvent used for extraction, the quantity of the specimen of solvent-H20 mixture, and the quantity of the solvent blank. Since publication of the paper, it has become evident that it would be desirable to also develop a similar equation in which these quantities are expressed on a mass, rather than volume, basis. It is the purpose of this correspondence to present such an equation. The equation, similar in form to that of eq 1 in ref 1, is % HzO = (MC/mg){[(A,/m,) - (Ab/mb)l/[l - (CA,/m,)ll x 100 (1)
where M is the mass of solvent used for extraction, C is the standardization factor of the titrator, mg is the mass of the sample of solid substance to be analyzed for H20, A, is the titration value (mass of H20 or volume of Karl Fischer reagent) for the specimen of solvent-extracted H 2 0 mixture, m, is the mass of the specimen, Ab is the titration value for
the solvent blank, and mb is the mass of the blank. The presence of any extraneous substance in the specimen that significantly affects the mass necessitates a correction to eq 1. The corrected equation is % HzO = RHSI X 11 i(mo/M)/[1 - (Ab/mb)/(A,/m,)l] (2) where RHSI is the right-hand side of eq 1and mo is the mass of the extraneous substance. Registry No. Water, 7732-18-5.
LITERATURE CITED (1) Jones, F. E. Anal. Chem. 1981, 53, 1957.
F r a n k E. Jones Center for Chemical Engineering National Engineering Laboratory National Bureau of Standards Washington, D.C. 20234 RECEIVED for review December 6,1982. Accepted December 27, 1982.
Comments on Sorption Capacities of Graphitized Carbon Black in Determination of Chlorinated Pesticide Traces in Water Sir: We have been using graphitized carbon black (GCB) for trace enrichment of organics from water for several years (1-3). However, we have obtained results which, in certain instances, differ markedly from those reported by others (4). The main conflict is with adsorption of n-alkanes and methyl esters, which are reportedly not adsorbed at all (4). We have performed a wide variety of experiments which demonstrate that these compounds are indeed adsorbed from water by GCB and which provide possible explanations for the inaccuracy of the previous data. We also describe alternate procedures for studying the adsorption/desorption properties of GCB which allow optimization of trace enrichment parameters. EXPERIMENTAL SECTION Procedures and equipment were similar to those reported previously (I, 2). Adsorption traps were prepared from Soxhlet extracted GCB (Carbopack B, 60/80 mesh, Supelco, Inc., Belle-
fonte, PA) and small diameter (4-5 mm) glass or stainless steel tubes (I) or large diameter (10 mm) glass tubes (2). Two general types of adsorption/elution experiments were performed. The first type involved passing a specific volume of water, which had been spiked with a solution of the test substance, through the GCB trap, and then eluting the trap with a solvent or solvent series (2). The second type involved application of a small volume of a solution of the test substance directly to the top of a trap, followed by elution ( 1 ) . Recoveries of test substances were determined by GLC, using external standards. Trap eluents were analyzed either directly or after concentration or dilution. Container surfaces and effluent water were tested by extracting with organic solvent and analyzing the extract.
RESULTS AND DISCUSSION Previous work (2) indicated that adequate preparation of aqueous solutions of nonpolar organics required the use of a water miscible cosolvent. Since Lagana et al. ( 4 ) had used hexane solutions of hydrocarbons and ether solutions of esters
Table I. Hydrocarbon Adsorption on GCB and Container Surfacesa % recovered
pentadecane hexadecane octadecane solvent trap reservoir trap reservoir trap reservoir acetone 94 0 95 0 96 0 hexane 20 72 15 76 11 79 a Percent of added amount of each hydrocarbon eluted from GCB trap or solution reservoir with 2 : l petroleum ether: toluene. A 50-pg sample of each hydrocarbon in 0.5 mL of either acetone or hexane added to 1 L of water in a reservoir and passed through a 10 X 150 mm trap. 0003-2700/83/0355-0793$01.50/00 1983 American Chemlcal Soclety
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 4, APRIL 1983
Table 11. Percent Adsorption of Esters by GCBa ester
loo
% recovery
methyl hexanoate 95 methyl heptanoate 94 methyl decanoate 95 A 50-pg sample of each ester in 0.5 mL of acetone added to 1L of water and passed through a 10 X 150 mm GCB trap. The trap was eluted with 2 : l petroleum ether: toluene.
75
z
d
i
0
c u
50
4
cx Y
-
--
L
25
w
Table 111. Hydrocarbon Adsorption on GCB and Container Surfaces % recovered
amt addeda
pentadecane hexadecane octadecane trap reservoir trap reservoir trap reservoir
800 400
13 79
200
81
88 20 18 10 0
13 65 74
86 34 26 15
13
86 40 19
59 79 100 85 81 77 20 50 94 95 0 96 0 a Amount of each n-paraffinadded (pg in acetone solution) to 1 L of water in reservoir. Percent recovered after passing the aqueous test mixture through the 10 X 160 mm trap and extracting the trap and reservoir with 2: 1 petroleum ether:toluene.
2
8
6
10
Flgure 1. Elution curve for p,p’-DDT from 10 X 150 mm GCB trap (1 mL fractions): (0) DDT applied in acetone solution directly to trap; (X) DDT dissolved In 1 L of water and then passed through the trap.
Table IV. Elution of Octadecane from GCBa fraction no. 1
2 3
4 to prepare their aqueous test solutions, it seemed likely that this could be a major reason for the anomalous results. We tested this possibility by preparing 1 L aqueous test solutions of three n-paraffins from both acetone and hexane stock solutions, passing the aqueous solutions through 10 X 150 mm GCB traps, and determining the residual amounts of hydrocarbons on both the GCB trap and in the solution reservoir (2:l petroleum ether:toluene elution). The results are shown in Table I. Additional experiments with the hexane stock solution showed the amounts of hydrocarbons retained on the walls of the reservoir to vary considerably, over a range of about 20 to 80% of the total recovered, with factors such as method of combining solutions (organic to aqueous or vice versa), mixing conditions (time, temperature, vigor), and “settling” time (time after mixing, before adsorption) all contributing to the variability. Similarly, use of an acetone stock solution of methyl esters to prepare an aqueous test solution gave the adsorption results listed in Table 11. The results reported in reference 4 show zero adsorption for both the n-paraffiis and the methyl esters. A close examination of the experimental procedures of Lagana et al. provides a possible explanation for their results. Taking the hydrocarbon case as an example, the aqueous test solution was prepared by adding a hexane solution containing 50 fig of each n-paraffin to 500 mL of water, shaking the mixture, and allowing it to stand for 10 min. The mixture was transferred to another container and then pumped through the GCB trap. Adsorption was determined by analyzing aliquots of the aqueous influent and the effluent from the trap and comparing the values. In order to reach the conclusion that zero adsorption had occurred, the amount of hydrocarbon measured in the influent and the effluent must be equal. Since, based on our experience, we would expect to find no hydrocarbon in the effluent, the amount determined in the influent would also have to have been zero. These data were not provided in ref 4 but were probably the case, as the following estimate of the expected amount indicates. On the basis of the data in Table I, the actual hydrocarbon concentration in the aqueous test solution after mixing and standing would be about 15 ng/mL, If a 20-mL aliquot of this solution
4
F R A C T I O N
%
eluent methanol methylene chloride methylene chloride methylene chloride
amt, mL 5 1.5 2
2
recovered 0 0 100 0
A 50-pg sample of octadecane in 50 pg of hexane Fraction concentrated applied to a 5 X 100 mm trap. to about 100 pL before analysis.
were extracted and the extract evaporated to 1 mL (the actual volumes were not specified in ref 41, a sample having a final hydrocarbon concentration of 300 ng/mL would be obtained. A 5-kL injection aliquot of this solution would contain only 1.5 ng of hydrocarbon, a virtually undetectable peak on most GC systems. Thus, the conclusion of zero adsorption of hydrocarbons (and esters) by GCB was quite likely based on negative findings. Even when acetone solutions are used to prepare aqueous test solutions, care must be taken not to exceed the aqueous solubilities of the test substances. The results of a series of adsorption experiments, using a range of added hydrocarbon amounts, are presented in Table 111. In order to overcome these and other difficulties inherent in using aqueous test solutions, to improve accuracy, and to simplify the process of determining adsorption/desorption characteristics of GCB,we have developed an alternate procedure. The procedure involves the direct application of an accurately known quantity of the test substances to an adsorption trap, followed by elution of the trap first with water and then with organic solvent (1). This allows the trap capacity to be quickly and accurately determined without resorting to over saturated or large volume test solutions. It also allows breakthrough volumes to be determined more accurately, since the test substance will emerge in a more concentrated, and thus more readily detectable, form. In addition, the desorption characteristics of compounds are very quickly ascertained with this method ( I ) , and the optimum recovery solvent(s) and volume can be readily determined. The direct application method was compared to the aqueous test solution method by means of elution curves (2,5). Thus, identical curves were obtained for a sample of p,p’-DDT whether it was applied directly as an acetone solution followed by a 2-mL water rinse or as a trace constituent adsorbed from 1L of water (21 petroleum ether:toluene trap elution) (Figure 1).
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Anal. Chem. 1983, 55, 795-796
Additional experiments with hydrocarbon test substances were performed by using the direct application method. Thus, when n-octadecane (50 pg in 0.5 pL of ethyl acetate) was applied to a 4 x 50 mm trap and the trap flushed with 500 mL of water at 6 mL/min, no hydrocarbon (
