Copper Activity in Soil Solution: I. Measurement by Ion‐selective Electrode and Donnan Dialysis

Abstract

AbstractCopper 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+.