Determination of the Ionic Association Constants of Na+ with CO32- and HCO3- Ions, in NaCl-NaHCO3-H2O Ternary Systems, at 25 °C

Abstract

The ionic association constants of sodium with carbonate ion (K1C') and acidic carbonate ions (K2C') were measured in NaCl-NaHCO₃-H₂O ternary systems to determine the distribution of sodium among the chemical species present in the growth medium of Chlorella homosphaera 424 algae. The mean activity coefficients of sodium chloride (in pure sodium chloride and in a mixture of electrolytes) were determined experimentally using two electrochemical cells, namely Ag, AgCl| KCl (3 M)|| NH₄NO₃ (1 M)| NaCl (mNaCl)| Na⁺-ISE and Ag, AgCl|KCl (3 M)|| NH₄NO₃ (1 M)| NaCl (mNaCl)| Cl^--ISE. The studies carried out show that the values of the association constants of K1C' and K2C' do not depend on the composition of the medium, but only on the effective ionic strength. The experimentally obtained γNaCl0 values in the binary system are comparable to the mean activity coefficients values for NaCl, calculated using data from the literature, with -0.9 to 0.1% relative standard deviation. The obtained results show that the experimentally determined mean activity coefficient in the ternary system, γNaCl, is smaller than γNaCl0 in the binary system over the entire field of ionic strengths studied. The ternary system NaCl-NaHCO₃-H₂O obeys Harned's rule.

Keywords: activity coefficients; algae growth media; association constant; effective ionic strength; ternary system.

Figure 1

Compliance with Harned’s rule verification for ionic strengths in the 0.1–0.5 mol/kg range.

Figure 2

Variation in Harned’s coefficient with total ionic strength for the NaCl-NaHCO₃-H₂O ternary system at 25 °C.

Figure 3

Variation in mean activity coefficients with composition in the NaCl-NaHCO₃-H₂O ternary system, at different constant total ionic strength values.

Figure 4

Variation in association constant of sodium ions with carbonate ions ($K_{1C}^{\prime }$) values with the effective ionic strength curve equation y = (26.32 ± 2.22) x² − (20.43 ± 1.28) x + (5.59 ± 0.17) with R² = 0.9804 and 0.132 standard error.

Figure 5

Variation in association constant of sodium ions with acidic carbonate ions ($K_{2C}^{\prime }$) values with the effective ionic strength curve equation y = (59.60 ± 4.98) x² − (46.44 ± 2.86) x + (9.22 ± 0.37), with R² = 0.9812 and 0.295 standard error.

Figure 6

Variation in the association constant of sodium ions with carbonate ion ($K_{1C}^{\prime }$) values with sodium chloride concentration at 0.1–0.5 mol/kg total ionic strength range.

Figure 7

Variation in the association constant of sodium ions with acidic carbonate ions values with sodium chloride concentration at 0.1–0.5 mol/kg total ionic strength range.

Figure 8

Distribution of CO₃²⁻ species in 4–12 pH range, at I_T = 0.5 mol/kg.

Figure 9

Distribution of Na⁺⁻ species in 4–12 pH range, at I_T = 0.5 mol/kg.