Correct total ammonia-N to the ANZG reference pH and temperature — correct_ammonia_ph

The freshwater ammonia default guideline values (DGVs) and the bundled ammonia SSD are expressed as total ammonia-N at the ANZG index condition of pH 7.0 and 20 °C. Ammonia toxicity is driven by the un-ionised fraction (NH3), which rises with both pH and temperature. A measured total ammonia-N must therefore be converted to the equivalent reference-condition concentration — the one that holds the same un-ionised NH3 — before it is compared against a DGV or passed to ssd_paf() as "NH3-N".

Usage

correct_ammonia_ph_temp(
  conc,
  conc_units = NULL,
  pH,
  temperature_C,
  ref_pH = 7,
  ref_temperature_C = 20
)

Arguments

conc: Measured total ammonia-N. Numeric vector or units object. Bare numeric requires conc_units.
conc_units: Character. Unit of conc when it is bare numeric, e.g. "ug/L" or "mg/L". Ignored when conc is a units object.
pH: Sample pH. Recycled against conc.
temperature_C: Sample temperature (°C). Recycled against conc.
ref_pH, ref_temperature_C: Reference index condition. Defaults to the ANZG ammonia DGV basis (pH 7.0, 20 °C); change only if your SSD/DGV uses a different reference.

Value

Total ammonia-N normalised to the reference condition (µg/L), the length of the recycled inputs. Pass this to ssd_paf("NH3-N", conc = ., conc_units = "ug/L").

Details

The conversion is C_ref = C * f_sample / f_ref, where f = 1 / (1 + 10^(pKa - pH)) is the un-ionised fraction and pKa = 0.09018 + 2729.92 / T(K) (Emerson et al. 1975), with T(K) = temperature_C + 273.15. A sample at high pH/temperature is normalised upward (more of its ammonia is in the toxic NH3 form than at the reference), and a sample at low pH/temperature downward.

Do not double-correct

add_mspaf() applies this same correction automatically from the metadata normalisation_formula (it reads the per-sample pH and temperature columns). Use this helper only for the manual ssd_paf() path. Do not pre-correct with this helper and then pass the result to add_mspaf(), or ammonia will be corrected twice.

References

Emerson K, Russo RC, Lund RE, Thurston RV (1975). Aqueous ammonia equilibrium calculations: effect of pH and temperature. Journal of the Fisheries Research Board of Canada 32(12):2379–2383.

Examples

# 900 µg/L total ammonia-N measured at pH 8.5, 20 °C is far more toxic than
# the same number at the pH 7.0 reference, so it normalises sharply upward:
correct_ammonia_ph_temp(900, conc_units = "ug/L", pH = 8.5, temperature_C = 20)
#> [1] 25394.81