793
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
