NaCl: Simple NaCl-Water Solution

In CHNOSZ: Thermodynamic Calculations and Diagrams for Geochemistry

Simple NaCl-Water Solution

Description

Calculate speciation and ionic strength of aqueous solutions with a given molality of NaCl.

Usage

  NaCl(m_tot = 1, T = 25, P = "Psat", pH = NA, attenuate = FALSE)

Arguments

m_tot	numeric, total molality of NaCl (single value)
T	numeric, temperature in \degC
P	numeric, pressure in bar
pH	numeric, pH
attenuate	logical, halve changes of variables in each step?

Details

Thermodynamic models for metal solubility and speciation involving chloride complexes are commonly specified in terms of amount of NaCl rather than activity (or molality) of Cl\S- as an independent variable. This function calculates distribution of species and ionic strength in a simple aqueous solution given a total amount (m_tot, in mol/kg) of NaCl. The aqueous Cl-bearing species considered in the system are Cl\S-, NaCl, and optionally HCl. Na\S+ is present as a basis species, but the formation of Na-bearing species such as NaOH is not considered. The activity coefficients of charged species are calculated using the extended Debye-Hückel equation (see nonideal) via the IS argument of affinity. The function first sets the molality of Na\S+ and ionic strength equal to m_tot, then calculates the distribution of Cl-bearing species. Based on mass balance of Na atoms, the molality of NaCl is then used to recalculate the molality of Na\S+, followed by ionic strength. To find a solution, the function iterates until the change of molality of Na\S+ and ionic strength are both less than m_tot / 100.

At very high NaCl concentrations, which are beyond the applicability limits of the extended Debye-Hückel model and therefore not recommended for normal use, the iterations tend to oscillate without converging. Setting attenuate to TRUE, which halves the amount of change in each step, may help with convergence. If a solution is not found after 100 iterations, the function stops with an error.

If pH is NA (the default), then HCl is not included in the calculation and its molality in the output is also assigned NA. Note that only a single value is accepted for m_tot, but the other numeric arguments can have length > 1, allowing multiple combinations T, P, and pH in a single function call. However, due to limitations in affinity, only one of T and P can have length > 1.

Value

A list with components ‘⁠IS⁠’ (ionic strength calculated from molalities of Na\S+ and Cl\S-), ‘⁠m_Cl⁠’, ‘⁠m_Cl⁠’, ‘⁠m_NaCl⁠’, and ‘⁠m_HCl⁠’ (molalities of Na\S+, Cl\S-, NaCl, and HCl).

Warning

It is important to keep in mind the ionic strength limits of the Debye-Hückel equation, but this function doesn't enforce them. Furthermore, metal-ligand complexing is not calculated by this function, so metal solubility and speciation calculations will be accurate only for relatively insoluble metals in NaCl-dominated solutions.

References

Shvarov, Y. and Bastrakov, E. (1999) HCh: A software package for geochemical equilibrium modelling. User's Guide. Australian Geological Survey Organisation 1999/25. https://pid.geoscience.gov.au/dataset/ga/25473

See Also

This function is used in a few demos (demo("contour"), demo("gold"), demo("minsol"), demo("sphalerite")). demo("yttrium") uses the pH argument.

Examples

# Ionic strength calculated with HCh version 3.7 (Shvarov and Bastrakov, 1999)
# at 1000 bar, 100, 200, and 300 degress C, and 1 to 6 molal NaCl
m.HCh <- 1:6
IS.HCh <- list(`100` = c(0.992, 1.969, 2.926, 3.858, 4.758, 5.619),
               `300` = c(0.807, 1.499, 2.136, 2.739, 3.317, 3.875),
               `500` = c(0.311, 0.590, 0.861, 1.125, 1.385, 1.642))
# Total molality in the calculation with NaCl()
m_tot <- seq(1, 6, 0.5)
N <- length(m_tot)
# Where we'll put the calculated values
IS.calc <- data.frame(`100` = numeric(N), `300` = numeric(N), `500` = numeric(N))
# NaCl() is *not* vectorized over m_tot, so we use a loop here
for(i in 1:length(m_tot)) {
  NaCl.out <- NaCl(m_tot[i], c(100, 300, 500), P = 1000)
  IS.calc[i, ] <- NaCl.out$IS
}
# Plot ionic strength from HCh and NaCl() as points and lines
col <- c("black", "red", "orange")
plot(c(1,6), c(0,6), xlab = "NaCl (mol/kg)", ylab = axis.label("IS"), type = "n")
for(i in 1:3) {
  # NOTE: the differences are probably mostly due to different models
  # for the properties of NaCl(aq) (HCh: B.Ryhzenko model;
  # CHONSZ: revised HKF with parameters from Shock et al., 1997)
  points(m.HCh, IS.HCh[[i]], col = col[i])
  lines(m_tot, IS.calc[, i], col = col[i])
}
# Add legend and title
dprop <- describe.property(rep("T", 3), c(100, 300, 500))
legend("topleft", dprop, lty = 1, pch = 1, col = col)
title(main="H2O + NaCl; HCh (points) and 'NaCl()' (lines)")

CHNOSZ documentation built on May 29, 2024, 3:30 a.m.