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This article in SSSAJ

  1. Vol. 51 No. 3, p. 568-572
    Received: Dec 13, 1985

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Copper Activity in Soil Solution: I. Measurement by Ion-selective Electrode and Donnan Dialysis1

  1. M. M. Minnich and
  2. M. B. McBride2



Copper activity in saturation extracts (2:1 soil/solution) of sludge- and Cu-salt-treated soil was estimated by ion-selective electrode (ISE) and by Donnan dialysis. Treatments were a factorial combination of four levels of Cu additions and four levels of total sludge additions. In the Donnan system, the ionic strength of a soil saturation extract was matched with KOAc buffer on opposite sides of a cation perm-selective membrane. At membrane equilibrium, Cu concentration was determined in the KOAc buffer by voltammetry. Extracts of sludge treatments had sufficient Cu buffering capacity to maintain the same Cu2+ activity before and after dialysis; Cu-salt treatments exhibited a lower soil solution Cu2+ activity after dialysis indicating the procedure was inadequate for these treatments. The ISE results were more precise, but the technique is only suited for use in aerobic, Cu-contaminated soils. Values for Cu2+ activity obtained by ISE were approximately an order of magnitude greater than Cu2+ activity values estimated by the Donnan dialysis method. By either procedure, increased total Cu resulted in higher Cu2+ activities and conversely, increased total sludge resulted in decreased Cu2+ activities. Total soluble Cu was highest in the Cu-salt treatments; sludge rate was associated with increased soluble Cu and a decreased proportion of free Cu2+.

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