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CADDIS Volume 2: Sources, Stressors & Responses

Ionic Strength

Ways to Measure Ionic Strength

Ionic strength is a function of concentration and charge of all ions in a given solution (see Equation 1). Direct measurement of ionic strength is seldom used in ecological studies. Causal assessors are more likely to have access to measures which generally correlate with ionic strength.

Equation for calculating ionic strength.  Source: IUPAC Quantities, Units and Symbols in Physical Chemistry, 1993.
Equation 1. Ionic strength is typically calculated as the product of a given ion's concentration, ci, and its charge, zi, summed over all ions in solution, divided by two (IUPAC Quantities, Units and Symbols in Physical Chemistry, 1993), and measured either as mass per unit volume (i.e., mg/L) or in moles (i.e., mmol/L).
  • Electrical conductivity or conductance: Ability of a solution to conduct current (measured in μS/cm, mS/cm, μmho/cm, or mmho/cm), which is related to ionic strength, temperature, and the mobility of ions. One siemen (S) is equal to one mho, and these terms are used interchangeably. "Specific conductivity" indicates the measurement has been normalized to a reference temperature (usually 25°C). Conductivity increases approximately 2% for every 1°C increase in water temperature (Wetzel 2001).
  • Salinity: Salt content of water (measured in mg/L, g/L, parts per thousand, or ‰); parts per thousand can be interpreted as salt mass per 1000 units of water mass, or grams of salt per 1000 grams of water.
  • Total dissolved solids (TDS): Concentration of material dissolved in water (measured in mg/L, g/L, parts per thousand, or ‰).

Conductivity, salinity, and TDS do not fully account for variance in toxicity due to individual ions or ionic composition [refer to Mount et al. (1987) for more information on individual ions and related toxicity], especially for sub-lethal endpoints (Zalizniak et al. 2006). Different solutions with the same ionic strength, conductivity, salinity, and TDS may have different effective toxicities, specific to different organisms, if ionic compositions vary. Nevertheless, conductivity, salinity, and TDS may be useful measures of ionic strength, especially in the context of acute toxicity.

In some cases, it may be appropriate to use evidence regarding individual ions (e.g., chloride) if measurements are available and reliable for a particular study site, if the individual ion is a potential causal agent (i.e., concentrations of other ions are low relative to their toxicities), or if regional information describes potential biological effects as a function of exposure to that specific ion.

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