Article pubs.acs.org/jced
Isopiestic Measurements of Water Activities for an Arsenic Acid Aqueous Solution at 298.15 K Haitang Yang,†,‡ Qingwei Wang,§ Quanbao Zhou,*,⊥ and Jianling Yue*,†,‡ †
School of Aeronautics and Astronautics, ‡Hunan Province Key Laboratory of New Specialty Fibers and Composite Material, and Chinese National Engineering Research Centre for Control and Treatment of Heavy Metal Pollution, Central South University, Changsha 410083, P. R. China ⊥ School of Chemistry and Life Science, Guizhou Education University, Guiyang 550018, P. R. China §
ABSTRACT: The water activity of the H3AsO4−H2O system is quite important for controlling arsenic in the environment and modeling the hydrometallurgical processing related to arsenic. In this work, the water activity for this binary system, from extremely low concentrations to a molality of 18 mol·kg−1 at 298.15 K was determined by isopiestic measurements. The relative isopiestic molality deviation for parallel samples in each experimental run was better than 0.2%. When these water activity data were combined with the mean ionic activity coefficient (γm,±) calculated by the Gibbs−Duhem equation, the Pitzer model parameters for arsenic acid aqueous solution were fitted. The reliability of the data and the model parameters obtained in this work were also systematically discussed.
1. INTRODUCTION Arsenic is a common contaminant in effluents resulting from base-metal processing and related metallurgy-extracting operations, and has exerted its poisonous effect in more than 70 countries.1,2 Accurate knowledge of the thermodynamic database of As-bearing minerals and aqueous species is the primary step to resolve many hydrothermal and environmental geochemical issues of arsenic.3−6 Owing to the complexity of the thermodynamic properties of these As-bearing systems, computational simulation is an indispensable tool.7−9 Quantitative model calculations and model parametrization is not possible without reliable basic thermodynamic information on these As-bearing systems, especially for water activity and osmotic coefficient data. However, up to date, the water activity and osmotic coefficient for the fundamental binary H3AsO4−H2O system at 298.15 K are unknown. To close this gap, we carried out isopiestic measurements10−13 for the H3AsO4−H2O system at 298.15 K from 0.0005 mol·kg−1 to nearly 18 mol·kg−1. Meanwhile, the data of mean ionic activity coefficient (γm,±) was rigorously calculated by Gibbs−Duhem equation. Pitzer14−17 model parameters were comprehensively fitted by the data of water activity and mean ionic activity coefficient.
fraction purity > 0.9995, Sinopharm Chemical Reagent) were used without further purification. The trace metal content in the NaCl (aq), H2SO4 (aq), and H3AsO4 (aq) stock solutions were quantified by inductively coupled plasma (ICP atomic emission spectroscopy, Optima 5300 DV, PerkinElmer, USA). The total impurity levels on a trace metals basis were substantially less than 200 ppm. Ultrapure water (σ ≤ 1.1 × 10−4 S/m) was used in all experimental processes, which was measured by electrical conductivity meter (ECM). Silver nitrate (AgNO3, mass fraction purity > 0.999, Sinopharm Chemical Reagent), barium chloride (BaCl2, mass fraction purity > 0.999, Sinopharm Chemical Reagent), sodium hydroxide (NaOH, mass fraction purity > 0.999, Sinopharm Chemical Reagent) and potassium hydrogen phthalate (C8H5KO4, mass fraction purity > 0.999, Sinopharm Chemical Reagent) were also used in these experiments. All of the instructions of the chemical reagents used in this work were shown in Table 1. The salt content of the NaCl (aq) reference solution was determined both gravimetrically by precipitation with AgNO3 (aq),18 and by the constant-weight evaporation method.19 The content of the H2SO4 (aq) stock solution was determined both by gravimetric precipitation with BaCl2 (aq),18 and by titration with NaOH (for H+).18 The composition of H3AsO4 (aq) was determined by titration with NaOH (for H+),18 while the composition of NaOH was determined by titration with potassium hydrogen phthalate (KHP, C8H5KO4). To exclude ambient CO2 during titration, an acid−base titration apparatus
2. EXPERIMENTAL SECTION 2.1. Materials. The reference NaCl (aq) solution was prepared by purifying the GR reagent (mass fraction purity > 0.999, Sinopharm Chemical Reagent) by 3-fold recrystallization. The studied H3AsO4 (aq) solution was prepared by hydrolysis of As2O5 (High Purity Solvents, mass fraction purity > 0.9995, Sigma). The reference H2SO4 (aq) solutions (mass © 2017 American Chemical Society
Received: April 14, 2017 Accepted: August 24, 2017 Published: September 7, 2017 3306
DOI: 10.1021/acs.jced.7b00349 J. Chem. Eng. Data 2017, 62, 3306−3312
Journal of Chemical & Engineering Data
Article
Table 1. Specifications of Chemicals Used in This Work chemical name
initial mass fraction purity
source
sulfuric acid sodium chloride arsenic pentoxide arsenic acid silver nitrate barium chloride disodium EDTA potassium hydrogen phthalate sodium hydroxide water
Sinopharm Chemical Reagent Co. Sinopharm Chemical Reagent Co. Sigma Hydrolysis of arsenic pentoxide Sinopharm Chemical Reagent Co. Sinopharm Chemical Reagent Co. Sinapharm Chemical Reagent Co. Sinopharm Chemical Reagent Sinopharm Chemical Reagent
purification method
>0.9995
final mass fraction purity
analysis method
none
0.9995
ICP
recrystallized three times
0.9999
ICP
>0.9995 >0.9995 0.9995
none none none
0.9995 0.9995
0.9995
none
ICP
0.9995
none
ICP
>0.999
>0.999 >0.999
none none distilled three times
ICP ICP
0.999 0.999 σ ≤ 1.1 × 10−4 S/m
ICP ICP ECM
concentration ranged from 0.1 to 5 mol·kg−1, both NaCl−H2O and H2SO4−H2O were chosen as isopiestic reference systems. At concentrations greater than 5 mol·kg−1, H2SO4−H2O was chosen as the sole reference system. Water activity data of the NaCl−H2O20 and H2SO4−H2O21 systems are available in the literature at 298.15 K over an extensive concentration range. Water activities of these reference systems, aw*, were fitted to a polynomial in molality, m (eq 1).
was fabricated (Figure 1). Five replicates were used in each analysis run, and the mean relative deviations were ≤ 0.05%. The stock solution and studied solution concentrations are presented in Table 2.
a w* = b1 + b2m0.5 + b3m + b4m1.5 + b5m2 + b6m2.5 + b7m3 + b8m3.5
(1)
where the parameter values of b1, b2, b3, b4, b5, b6, b7, and b8 are given in Table 3. Thus, if the reference molalities are known, the water activity of the reference and test systems can be calculated. 2.4. Experimental Results. The measured water activities of the H3ASO4−H2O system at 298.15 K are shown in Table 4 and illustrated in Figure 2. For each isopiestic experimental run, the concentration (in mol·kg−1) of the reference solution is listed in the second and third line. The fourth and fifth lines
Figure 1. Acid−base titration apparatus: A, weight titration flask; B, grinding flask; C, magnetic stirrer; D, calcium oxide (CaO).
Table 2. Composition of the Stock Solutions Table 3. Fitted Parameters in eq 1 for NaCl and H2SO4 References
solution method a
−1
m /mol·kg mb/mol·kg−1
NaClc
H2SO4c
2.6709 2.6714
2.0914 2.0919
NaOHc
H3AsO4c
parameters 0.7652
c
T /K b1 b2 b3 b4 b5 b6 b7 b8 σd
1.7763
a
The concentration of the solution determined by the method of precipitation. bThe concentration of the solution determined by the method of titration or evaporation. cur(m) = 0.001.
2.2. Apparatus and Procedures. The isopiestic apparatus was presented in our previous paper. 10 Herein, the experimental processes are similar to our previous paper,10 except for some special points: (1) the equilibrium time at 298.15 K, especially for the dilute arsenic acid aqueous solution was chosen as 240 h (10 days); while for the high concentration solution, it was reduced to 168 h (1 week); (2) after the equilibrium has been reached, the sample cups were then closed in situ (at the equilibration temperature) and then the apparatus was removed from the thermostat bath directly. 2.3. Reference System. NaCl−H2O was chosen as the sole isopiestic reference system when the concentration of the measured solution was less than 0.1 mol·kg−1. When the
NaCla 298.15 1.00001 −0.00036 −0.03302 0.00247 −0.00288 0.00123 −0.00076 0.00015 1.3557 × 10−5
H2SO4b 298.15 0.99559 0.01979 −0.05471 0.02924 −0.01029 −0.00028 0.00061 −0.00007 1.26 × 10−4
a Fitting the water activity of NaCl in the literature.20 bFitting the water activity of H2SO4 in the literature when the concentration is
