The coefficient 2 in eq. 16 is necessary because both the forward and reverse reactions of eq. 10 contribute to the observed electron transfer. Since the system is at equilibrium, the rates of these two reactions are equal. Consequently, the forward reaction of eq. 10 is only responsible for half the observed electron transfers.19 Substitution of k% = 12.1 ± 1.6 M~l sec.-1 and &e = 57.6 ± 2 M~l sec.-1 gives k% = 33.4 ± 2.6 M~l sec.-1 at 25.0°and ionic strength 3.0 .A comparison of the magnitudes of these rate constants shows that the electron exchange between Fe2+ and FeCl2 + proceeds mainly by a chlorine atom transfer mechanism (provided, as seems reasonable, the third-order term involving Fe2+, Fe3+, and chloride may be neglected).The chlorine atom transfer presumably occurs in a chloride-bridged, inner-sphere activated complex, while the electron transfer may occur in either an outersphere activated complex or in a water-bridged, innersphere activated complex. It is of interest that reaction via the chloride-bridged path is only about three (19) J. Silverman, Ph.D. Thesis, Columbia University, New York, N. Y., 1951. times as rapid as reaction via the outer-sphere or waterbridged paths. This small difference, and indeed the relatively small effect of chloride on the iron(II)-iron-(III) exchange, contrasts markedly with its effect on the iron(II)-chromium(III) and chromium(II)-chromium(III) reactions. In the latter systems, chloride bridging increases the rates of the reactions by factors of 104 and more than 2 X 106 7, respectively.18 Moreover, the chromium (I I)-catalyzed dissociation of Cr-Cl2 + is slower by a factor of more than 104 than the Cr2+-CrCl2+ exchange.20 The difference in the chloride effects cannot be ascribed to any difference in the mechanisms of the reactions since these studies show that it is very likely that the chloride-catalyzed iron (Il)-iron(III) exchange, like the chloride-catayzed chromium(II)-iron(III) and chromium(II)-chromium(III) reactions, proceeds via an inner-sphere activated complex.

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The purpose of this study was to identify the significant microenvironments that can lead to chromium exposure in Hudson County, New Jersey residential settings near or on soil contaminated with chromium waste. Measurements were made in indoor air, outdoor air, and house dust. Surface dust was found to be the best index of potential Cr exposure. The values of Cr in Hudson County household dust ranged from 3.25-320 ng/cm2 in wipe samples and 1.0-12 ng/cm2 in vacuum samples. Elevated Cr in household dust was found to be related to residential locations near large chromium waste sites, household cleaning habits, and house renovation activities. Outdoor Cr air levels were similar to those obtained in other urban areas at these seasons of the year, approximately 5-7 ng/m3. Comparisons with measurements of the Cr levels in urine found that the elevated Cr in dust was associated with elevated excretion of Cr. Site-specific Cr differences in household dust suggest different sources and routes of exposure. Within the total group of homes in the present study, Cr in household dust was the major influence on household exposure.

Exposure to chromium was assessed for 40 children living near chromium waste sites. Sampling was conducted in one Jersey City, New Jersey neighborhood during the summer and fall of 1991. Household dust samples from residences and urine samples from children living near chromium waste sites were collected and analyzed for chromium. During the summer and fall visits when the samples were collected, the children were also interviewed about lifestyle/activity patterns. Comparisons were made with similar samples collected from children and homes in other areas of New Jersey outside of Hudson County with no known chromium waste sites. Household dust masses and chromium loadings and concentrations in the dust showed a significant decline in this Hudson County neighborhood since the area was first sampled in 1990. Interim remediation of neighborhood sites and an active community education program in the interval between the first and second year of sampling may have contributed to the reduction in dust masses and chromium levels in dust. Children's urine chromium concentrations were consistent across the two sampling periods despite reported changes in activity patterns. Chromium concentrations in urine were found to be age-dependent and related to home location. In this sample of Jersey City children less than six years old, time spent playing outdoors was a weak secondary contributor to urine chromium levels. IMPLICATIONSExposure to pollutants such as chromium from hazardous waste sites can be influenced by remediation efforts and by the activities of the individuals who live near the site. Remediation of neighborhood sites removes the source; an active community education program about the need to reduce dust and implement dust reduction activities lowers residential dust masses and chromium levels on surfaces.

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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. The National Institute of Environmental Health Sciences (NIEHS) andBrogan & Partners are collaborating with JSTOR to digitize, preserve and extend access to Environmental Health Perspectives. Chromium, named for its many-colored compounds, exists in the oxidation states of -2 to +6 inclusively. The compounds exhibit a wide range of geometries including square planar, tetrahedral, octahedral, and various distorted geometries. Chromium is found in nature principally as the chromite ore FeCr2O4 in which chromium is in the +3 state. The existence of a particular oxidation state is dependent on many factors including pH, redox potentials, and kinetics. Thermodynamically, +3 and +2 are the most stable states, while the +3 and +6 oxidation states are the most common ones found in aqueous solution. Kinetically, chromium +3 is substitutionally inert: for water exchange k(sec-') = 2.5 x 104, due to the presence of the half-filled d(t2g)3'4A2 state. On the other hand, protonation/deprotonation is quite rapid. Polymerization is very slow but is promoted at higher pHs; acid cleavage of the protonated oligomers is also quite slow. Chromium +6 as the chromate ion is strongly oxidizing at low pHs and less so in basic solution. The chromate ion does form some polyacids and polyanions. These factors must be considered in analyzing samples for total chromium and for the amounts of each oxidation state.

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