Thermodynamic Analysis of Phosphorus-Sulfur-Vanadium- Water Acidic System for the Extraction of Vanadium

Abstract

To extract vanadium efficiently from the acidic leachate of vanadium slag, a thermodynamic analysis of phosphorus-sulfur-vanadium-water acidic systems was established at 298 K. Results show that in the P(V)-V(V)-H₂O acidic system, VO₂⁺ is first conver-ted to phosphovanadic heteropolyacid ions when pH=1~4 and then converted to vanadium isopolyacid ions when pH=4~7. In the S(VI)-V(V)-H₂O acidic system, VO₂⁺ and VO₂SO₄^- are the main existing chemical forms when pH=0~1 and gradually converted to vanadium isopolyacid ions when pH=2~6. In the P(V)-S(VI)-V(V)-H₂O acidic system, when pH=1~3, VO₂⁺ and VO₂SO₄^- are gradua-lly converted to phosphovanadic heteropolyacid ions. The optimum molar fraction of ΣPV₁₄ (total sum of phosphovanadic heteropolyacid ions) reaches 88.55% at pH=2. As the pH value increases to 4~6, phosphovanadic heteropolyacid ion gradually disappears and is converted to vanadium isopolyacid anions, and the optimum molar fraction of ΣV₁₀ (total sum of vanadium isopolyacid ions containing ten vanadium) is 100.00% at pH=5.

Keywords

vanadium; thermodynamic; isopolyacid; heteropolyacid; compexation

Science Press

As an important strategic metal, vanadium has many excellent physical and chemical properties, such as specific physiological functions, fatigue resistance, hardness, and tensile strength ^[1]. Although vanadium is widely distributed in the nature, it is still difficult to find vanadium in elemental form. Most vanadium coexists with other elements. Currently, vanadium-bearing converter slag and stone coal are the main raw materials for vanadium extraction worldwide, especially in China. As an intermediate product of the vanadium-titanium magnetite smelting process, vanadium-bearing converter slag is the most important raw and processed materials for vanadium extraction, and it has received increasing attention and research ^[2]. Many methods have been applied for the extraction of vanadium. Among those methods, sodium salt roasting (NaCl, Na₂CO₃, and Na₂SO₄) and water leaching were once the most widely used methods. As a pyro-hydrometallurgical process, Cl₂, HCl, SO₂, or SO₃ can be produced in the sodium salt roasting process, which can contaminate the environment and corrode equipment^[3-7]. Therefore, this method has been eliminated based on the strict requirements for environmental protection in China.

Presently, processes involving direct acidic leaching and calcified roasting with the acidic leaching of converter slag have been studied and widely adopted due to the high recovery rate of vanadium and low generation of pollution^[8-17]. In China, the majority of vanadium is extracted by the H₂SO₄ leaching process. To improve the extraction ratio of vanadium, high concentrations of H₂SO₄ are commonly used in the acidic leaching process. Accordingly, numerous impurities in converter slag are also dissolved into the leachate, which results in a complex vanadium-bearing leaching solution with a high concentration of impurities at a low pH value. Therefore, it is particularly important to separate and extract vanadium from strong acid leachates containing many impurities ^[18].

In the H₂SO₄ leachate of vanadium-bearing converter slag, phosphorus (P) and sulfur (S) have an important effect on the existing form and the effective extraction of vanadium. First, phosphorus in the vanadium-bearing converter slag originates from the conversion process of molten iron containing vanadium, which is also leached into the acidic leaching solution. In general, phosphorus mainly exists in the forms of H₃PO₄, H₂PO₄^- and HPO₄^2- in the acidic solution. However, phosphorus can form phosphovanadic heteropolyacid ions with transition metal vanadium in the actual acidic leachate. Second, sulfur existing in the acidic solution is mainly SO₄^2-. Due to the existence of vanadium in the acidic leachate, V-S complex ions can be formed through the complexation action between VO₂⁺ and SO₄^2-. Phosphovanadic heteropolyacid ions and V-S complex ions can greatly impact vanadium extraction in acidic leaching solutions and subsequent precipitation procedures^[19-24].

Therefore, this work studied the thermodynamic analysis of P(V)-S(VI)-V(V)-H₂O systems for vanadium extraction from an actual acidic leachate. Moreover, the existence of vanadium and the variation tendency of the ions (vanadium, phosphorus, and sulfur) at different pH values and different concentrations (phosphorus and sulfur) were investigated.

1 Thermodynamic Analysis

All ions in the P(V)-S(VI)-V(V)-H₂O systems are VO₄^3-, HVO₄^2-, H₂VO₄^-, V₂O₇^4-, HV₂O₇^3-, H₂V₂O₇^2-, V₄O₁₂^4-, V₅O₁₅^5-, V₁₀O₂₈^6-, HV₁₀O₂₈^5-, HV₁₀O₂₈^5-, H₂V₁₀O₂₈^4-, H₃V₁₀O₂₈^3-, VO₂⁺, H₃PO₄, H₂PO₄^-, HPO₄^2-, PO₄^3-, H₅PV₁₄O₄₂^4-, H₄PV₁₄O₄₂^5-, H₃PV₁₄O₄₂^6-, VO₂SO₄^-, VO₂HSO₄, HSO₄^-, and SO₄^{2- [}25-28]. The equilibrium reactions and corresponding equilibrium constants are listed in Table 1.

Table 1 Equilibrium reactions and corresponding equilibrium constants of P-V-H₂O (No.1~17), S-V-H₂O (No.4-20) and P-S-V-H₂O (No.1-20) systems at 298 K

No.	Equilibrium reaction	Equilibrium constants, lgK	Ref.
1	H2PO4-=PO43-+2H+	-17.650	[20]
2	H2PO4-=HPO42-+H+	-6.418	[20]
3	H++H2PO4-=H3PO4	1.772	[20]
4	H2VO4-= HVO42-+H+	-7.92	[20]
5	2H2VO4-=V2O74-+2H++H2O	-15.17	[20]
6	2H2VO4-= HV2O73-+H++H2O	-5.25	[20]
7	2H2VO4-=H2V2O72-+H2O	2.77	[20]
8	4H2VO4-= V4O124-+4H2O	10.00	[20]
9	5H2VO4-= V5O155-+5H2O	12.38	[20]
10	4H++10H2VO4-=V10O286-+12H2O	52.13	[20]
11	5H++10H2VO4-=HV10O285-+12H2O	58.13	[20]
12	6H++10H2VO4-=H2V10O284-+12H2O	61.87	[20]
13	7H++10H2VO4-=H3V10O283-+12H2O	63.47	[20]
14	2H++H2VO4-=VO2++2H2O	6.96	[20]
15	11H++14H2VO4-+H2PO4-=H5PV14O424-+18H2O	96.4	[20]
16	10H++14H2VO4-+H2PO4-=H4PV14O425-+18H2O	94.84	[20]
17	9H++14H2VO4-+H2PO4-=H3PV14O426-+18H2O	90.7	[20]
18	HSO4-= SO42-+H+	1.99	[28]
19	VO2++SO42-= VO2SO4-	0.97	[28]
20	VO2++HSO4-= VO2HSO4	-0.136	[28]

According to the data listed in Table 1, thermodynamic analysis was conducted. The concentration was used instead of the activity for the calculation. The mathematical relationships of the species can be expressed as follows ^[29-31]:

[PO43-][H+]2=10-17.650[H2PO4-]

(1)

[HPO42-][H+]=10-6.418 [H2PO4-]

(2)

[H3PO4]=101.772[H+] [H2PO4-]

(3)

[HVO42-][H+]=10-7.92[H2VO4-]

(4)

[V2O74-][H+]2=10-15.17[H2VO4-]2

(5)

[HV2O73-][H+]=10-5.25[H2VO4-]2

(6)

[H2V2O72-]=102.77[H2VO4-]2

(7)

[V4O124-]=1010.00[H2VO4-]4

(8)

[V5O155-]=1012.38[H2VO4-]5

(9)

[V10O286-]=1052.13[H+]4[H2VO4-]10

(10)

[HV10O285-]=1058.13[H+]5[H2VO4-]10

(11)

[H2V10O284-]=1061.87[H+]6[H2VO4-]10

(12)

[H3V10O283-]=1063.47[H+]7[H2VO4-]10

(13)

[VO2+]=106.96[H+]2[H2VO4-]

(14)

[H5PV14O424-]=1096.40[H+]11[H2VO4-]14[H2PO4-]

(15)

[H4PV14O425-]=1094.84[H+]10[H2VO4-]14[H2PO4-]

(16)

[H3PV14O426-]=1090.70[H+]9[H2VO4-]14[H2PO4-]

(17)

[SO42-][H+]=101.99[HSO4-]

(18)

[VO2SO4-]=100.97[VO2+][SO42-]

(19)

[VO2HSO4]=10-0.136[VO2+][HSO4-]

(20)

The total concentrations of V, P and S are expressed in Eq.(25):

In the P(V)-V(V)-H₂O acidic solution,

[V]_T(V-P-H2O)=[VO₄^3-]+[HVO₄^2-]+[H₂VO₄^-]+2[V₂O₇^4-]+

2[HV₂O₇^3-]+2[H₂V₂O₇^2-]+4[V₄O₁₂^4-]+5[V₅O₁₅^5-]+

10[V₁₀O₂₈^6-]+10[HV₁₀O₂₈^5-]+10[HV₁₀O₂₈^5-]+

10[H₂V₁₀O₂₈^4-]+10[H₃V₁₀O₂₈^3-]+[VO₂⁺]+

14[H₅PV₁₄O₄₂^4-]+14[H₄PV₁₄O₄₂^5-]+

14[H₃PV₁₄O₄₂^6-]

(21)

[P]_T(V-P-H2O)=[H₃PO₄]+[H₂PO₄^-]+[HPO₄^2-]+[PO₄^3-]+

[H5PV14O424-]+[H4PV14O425-]+[H3PV14O426-]

(22)

In the S(VI)-V(V)-H₂O acidic solution,

[V]_T(V-S-H2O)=[VO₄^3-]+[HVO₄^2-]+[H₂VO₄^-]+2[V₂O₇^4-]+

2[HV₂O₇^3-]+2[H₂V₂O₇^2-]+4[V₄O₁₂^4-]+5[V₅O₁₅^5-]+

10[V₁₀O₂₈^6-]+10[HV₁₀O₂₈^5-]+10[HV₁₀O₂₈^5-]+

10[H₂V₁₀O₂₈^4-]+10[H₃V₁₀O₂₈^3-]+[VO₂⁺]+

[VO2SO4-]+[VO2HSO4]

(23)

[S]T(V-S-H2O)=[SO42-]+[HSO4-]+[VO2SO4-]+[VO2HSO4]

(24)

In the P(V)-S(VI)-V(V)-H₂O acidic solution,

[V]_T(V-P-H2O)=[VO₄^3-]+[HVO₄^2-]+[H₂VO₄^-]+

2[V₂O₇^4-]+2[HV₂O₇^3-]+2[H₂V₂O₇^2-]+

4[V₄O₁₂^4-]+5[V₅O₁₅^5-]+10[V₁₀O₂₈^6-]+

10[HV₁₀O₂₈^5-]+10[HV₁₀O₂₈^5-]+10[H₂V₁₀O₂₈^4-]+

10[H₃V₁₀O₂₈^3-]+[VO₂⁺]+14[H₅PV₁₄O₄₂^4-]+

14[H₄PV₁₄O₄₂^5-]+14[H₃PV₁₄O₄₂^6-]+[VO₂SO₄^-]

+[VO2HSO4]

(25)

According to the aforementioned 25 equations, a thermodynamic analysis of the P(V)-V(V)-H₂O, S(VI)-V(V)-H₂O and P(V)-S(VI)-V(V)-H₂O systems was performed to determine the existing form and variation tendency of ions in the acidic leachate of vanadium-bearing converter slag^[32-35].

2 Thermodynamic Discussion

2.1 P(V)-V(V)-H₂O system

2.1.1 Molar fraction of ions containing vanadium in the P(V)-V(V)-H₂O system

The molar fraction of ions containing vanadium in the P(V)-V(V)-H₂O system and varying at different pH values is shown in Fig.1 when [P⁵⁺]=0.1 mol·L^-1 and [V⁵⁺]=0.1 mol·L^-1 (based on the concentration range of vanadium and pho-sphorus in the acid leaching solution of calcification roasting-acid leaching process and direct acid leaching process).

When the pH value is 1, VO₂⁺ is first converted to phosphovanadic heteropolyacid ions (H₅PV₁₄O₄₂^4-, H₄PV₁₄O₄₂^5-, or H₃PV₁₄O₄₂^6-). When the pH value increases to 1~4, vanadium is mainly phosphovanadic heteropolyacid ions, and the optimum molar fraction of ΣPV₁₄ (total sum of all the phosphovanadic heteropolyacid ions [ΣPV₁₄]=[H₅PV₁₄O₄₂^4-]+[H₄PV₁₄O₄₂^5-]+[H₃PV₁₄O₄₂^6-]) reaches 97.14% when the pH value is 2.5. When the pH value is 4~7, phosphovanadic heteropolyacid ions are converted to vanadium isopolyacid anions (V₁₀O₂₈^6-, HV₁₀O₂₈^5-, or H₂V₁₀O₂₈^4-), and the optimum molar fraction of ΣV₁₀([ΣV₁₀]=[V₁₀O₂₈^6-]+[HV₁₀O₂₈^5-]+ [H₂V₁₀O₂₈^4-]) reaches 98.77% when the pH value is 5.

In conclusion, VO₂⁺ is first converted to phosphovanadic heteropolyacid ions and then converted to vanadium isopolyacid ions (ΣV₁₀) in the P(V)-V(V)-H₂O system when pH=0~7.

A comparison of the molar fraction in the P(V)-V(V)-H₂O system with different concentrations of phosphorus ([P⁵⁺]=0.1 mol·L^-1, [V⁵⁺]=0.1mol·L^-1; [P⁵⁺]=0.001 mol·L^-1, [V⁵⁺]=0.1 mol·L^-1) is displayed in Fig.2.

When the pH value is 1~4, the molar fraction of ΣPV₁₄ when [P⁵⁺]=0.001 mol·L^-1 is always lower than that when [P⁵⁺]=0.1 mol·L^-1, and its optimum molar fraction is only 13.79% when the pH value is 2. As the pH value is increased, phosphovanadic heteropolyacid ions are converted to vanadium isopolyacid ions, and the molar fraction of ΣV₁₀ when [P⁵⁺]=0.001 mol·L^-1 is higher than that when [P⁵⁺]=0.1 mol·L^-1 at pH=1~5. Moreover, the optimum molar fraction of ΣV₁₀ when [P⁵⁺]=0.001 mol·L^-1 can reach 100.00% when the pH value is 4~5.

It can be concluded that a decrease in the phosphorus concentration (from 0.1 mol·L^-1 to 0.001 mol·L^-1) can significantly decrease the molar fraction of ΣPV₁₄ and increase the existing range of ΣV₁₀ in the P(V)-V(V)-H₂O system.

2.1.2 Molar fraction of ions containing phosphorus in the P(V)-V(V)-H₂O system

The molar fraction of ions containing phosphorus in the P(V)-V(V)-H₂O system and varying at different pH values is shown in Fig.3 when [P⁵⁺]=0.1 mol·L^-1 and [V⁵⁺]=0.1 mol·L^-1.

As shown in Fig. 3, the existing forms of phosphorus are mainly H₃PO₄, H₂PO₄^-, and HPO₄^2- in the acidic P(V)-V(V)-H₂O system (pH=0~7). However, phosphovanadic hetero-polyacid anions can be formed when 1<pH<4 in the P(V)- V(V)-H₂O system. When the pH value is increased, H₃PO₄ is first converted to ΣPV₁₄ and H₂PO₄^-, and then H₂PO₄^- is con-verted to HPO₄^2-. The optimum molar fraction of H₃PO₄ is 98.34% when the pH value is 0; the optimum molar fraction of ΣPV₁₄ is 6.94% when the pH value is 2.5; the optimum molar fraction of H₂PO₄^- is 98.28% when the pH value is 4.5; and the optimum molar fraction of HPO₄^2- is 79.25% when the pH value is 7.

The molar fractions of ions containing phosphorus at diffe-rent concentrations ([P⁵⁺]=0.1 mol·L^-1, [V⁵⁺]=0.1 mol·L^-1; and [P⁵⁺]=0.001 mol·L^-1, [V⁵⁺]=0.1 mol·L^-1) are displayed in Fig.4.

As shown in Fig.4, ions containing phosphorus in the P(V)-V(V)-H₂O system are still mainly in the forms of H₃PO₄, ΣPV₁₄, H₂PO₄^-, and HPO₄^2- when the phosphorus concentration is decreased from 0.1 mol·L^-1 to 0.001 mol·L^-1. The reason for this difference is that the molar fraction of ΣPV₁₄ when [P⁵⁺]=0.001 mol·L^-1 is much bigger than that when [P⁵⁺]=0.1 mol·L^-1, and it can be considerably increased to 73.59%~99.20% when pH=1~3.

This result is mainly because when [V⁵⁺]=0.1 mol·L^-1 and [P⁵⁺]=0.001 mol·L^-1, the molar ratio of phosphorus to vanadium is 1:100, which is much less than the ratio of 1:14 in ΣPV₁₄. Therefore, most of the 0.001 mol/L phosphorus is converted to ΣPV₁₄ by combining with vanadium. However, when [V⁵⁺]=0.1 mol·L^-1 and [P⁵⁺]=0.1 mol·L^-1, the molar ratio of phosphorus to vanadium is 1:1, which is bigger than the ratio of 1:14 in ΣPV₁₄. Therefore, only a small part of the 0.1 mol·L^-1 phosphorus is converted to ΣPV₁₄ by combining with vanadium. Therefore, the molar ratio of ΣPV₁₄/total P when [P⁵⁺]=0.001 mol·L^-1 is far bigger than that when [P⁵⁺]=0.1 mol·L^-1 and pH=1~3.5 in the P(V)-V(V)-H₂O system.

2.2 S(VI)-V(V)-H₂O system

2.2.1 Molar fraction of ions containing vanadium in the S(VI)- V(V)-H₂O system

The molar fraction of ions containing vanadium in the S(VI)-V(V)-H₂O system and varying at different pH values is shown in Fig.5 when [S⁶⁺]=0.05 mol·L^-1 and [V⁵⁺]=0.1 mol·L^-1.

When pH=0~1, vanadium is mainly in the forms of VO₂⁺ and VO₂SO₄^-, and the proportion is 79% and 21%, respec-tively. With continuously increasing the pH value, VO₂⁺ and VO₂SO₄^- are gradually converted to ΣV₁₀ and almost all vanadium exists in the form of ΣV₁₀ when pH value is 2~6. With continuously increasing the pH value to 6~7, vanadium exists mainly in the forms of V₄O₁₂^4- and V₅O₁₅^5-.

A comparison of the molar fraction in the S(VI)-V(V)-H₂O system with different sulfur concentrations ([S⁶⁺]=0.05 mol·L^-1, [V⁵⁺]=0.1mol·L^-1; [S⁶⁺]=0.5 mol·L^-1, [V⁵⁺]=0.1 mol·L^-1) is illustrated in Fig.6.

When the pH value is 0~1, the molar fraction of VO₂⁺ decreases from 79% to 20%, and the molar fraction of VO₂SO₄^- increases from 21% to 79% when the sulfur concentration is increased from 0.05 mol·L^-1 to 0.5 mol·L^-1. In addition, the pH value of forming ΣV₁₀ increases from 1.0 to 1.5, and the molar fraction of ΣV₁₀ when [S⁶⁺]=0.05 mol·L^-1 is always larger than that when [S⁶⁺]=0.5 mol·L^-1 at pH=1.5~2.5. Moreover, the molar fractions of ΣV₁₀ is almost the same when the pH value is 3~7 and close to 100% when the pH value is 3.0~5.5.

Based on these thermodynamic results, it can be concluded that increasing the sulfur concentration (from 0.05 mol·L^-1 to 0.5 mol·L^-1) in the S(VI)-V(V)-H₂O solution can improve the molar fraction of VO₂SO₄^- and reduce the existing range of vanadium isopolyacid ions (ΣV₁₀).

2.2.2 Molar fraction of ions containing sulfur in the S(VI)- V(V)-H₂O system

The molar fraction of ions containing sulfur in the S(VI)- V(V)-H₂O system and varying at different pH values is shown in Fig.7 when [S⁶⁺]=0.05 mol·L^-1 and [V⁵⁺]=0.1 mol·L^-1.

As observed, the anion VO₂SO₄^- is formed due to the complexation of SO₄^2- and VO₂⁺ when pH=0~1 in the acidic S(VI)-V(V)-H₂O system, and its molar fraction reaches 42%. The remaining 56% of sulfur is in the form of SO₄^2-. As the pH value continues to increase, the anion VO₂SO₄^- gradually disappears. When pH=2.5~7, almost 100% of sulfur in the solution is SO₄^2-. Moreover, HSO₄^2- and VO₂HSO₄ hardly exist in the whole pH range of 0~7.

The molar fraction of ions containing sulfur at different sulfur concentrations ([S⁶⁺]=0.05 mol·L^-1, [V⁵⁺]=0.1 mol·L^-1; [S⁶⁺]=0.5 mol·L^-1, [V⁵⁺]=0.1 mol·L^-1;) is shown in Fig.8.

Fig.8 Comparison of molar fraction (ions containing sulfur) in the S(VI)-V(V)-H₂O system at 298 K ((1): [S⁶⁺]=0.05 mol·L^-1 and [V⁵⁺]=0.1 mol·L^-1; (2): [S⁶⁺]=0.5 mol·L^-1 and [V⁵⁺]=0.1 mol·L^-1)

In Fig.8, ions containing sulfur in the S(VI)-V(V)-H₂O system are mainly SO₄^2- and VO₂SO₄^-. When the sulfur concentration is increased from 0.05 mol·L^-1 to 0.50 mol·L^-1, the molar fraction of SO₄^2- increases from 57% to 84% and the corresponding molar fraction of VO₂SO₄^- decreases from 42% to 16% when pH=0~1. Moreover, VO₂SO₄^- is gradually converted to SO₄^2-, and almost all sulfur is SO₄^2- when the pH value is 3~7 in the acidic S(VI)-V(V)-H₂O system.

2.3 P(V)-S(VI)-V(V)-H₂O system

The molar fraction of ions containing vanadium in the P(V)-S(VI)-V(V)-H₂O system and varying at different pH values is shown in Fig.9 when [P⁵⁺]=0.01 mol·L^-1, [S⁶⁺]=0.05 mol·L^-1 and [V⁵⁺]=0.1 mol·L^-1.

When pH=0~1, vanadium is mainly VO₂⁺ and VO₂SO₄^-. In addition, the molar fraction of VO₂⁺ ranges from 79.84% to 69.26%, and the molar fraction of VO₂SO₄^- ranges from 21.10% to 19.37%. As the pH value continuously increases to 1~3, VO₂⁺ and VO₂SO₄^- gradually disappear and are converted to phosphovanadic heteropolyacid ions, and the optimum molar fraction of ΣPV₁₄ is 88.55% when the pH value is 2. As the pH value continuously increases to 4~6, ΣPV₁₄ gradually disappears and is converted to vanadium isopolyacid ions, and the optimum molar fraction of ΣV₁₀ reaches 100.00% when the pH value is 5.

3 Conclusions

1) In the P(V)-V(V)-H₂O acidic system, VO₂⁺ is first converted to phosphovanadic heteropolyacid ions when pH=1~4, and the optimum molar fraction of ΣPV₁₄ (total sum of phosphovanadic heteropolyacid ions) is 97.14% when the pH value is 2.5. As the pH value increases to 4~7, the phosphovanadic heteropolyacid ions are converted to vanadium isopolyacid ions, and the optimum molar fraction of ΣV₁₀ (total sum of vanadium isopolyacid ions containing ten vanadium) is 98.77% when the pH value is 5.

2) In the S(VI)-V(V)-H₂O acidic solution, when the pH value is 0~1, the molar fractions of VO₂⁺ and VO₂SO₄^- are 79% and 21%, respectively. As the pH value continuously increases, VO₂⁺ and VO₂SO₄^- are gradually converted to vanadium isopolyacid ions ΣV₁₀, and almost 100% of the vanadium exists in the form of ΣV₁₀ when the pH value is 2~6.

3) In the P(V)-S(VI)-V(V)-H₂O acidic system, when the pH value is 0~1, the molar fraction of VO₂⁺ ranges from 79.84% to 69.26%, and the molar fraction of VO₂SO₄^- ranges from 21.10% to 19.37%. As the pH value continuously increases to 1~3, VO₂⁺ and VO₂SO₄^- gradually disappear and are converted to phosphovanadic heteropolyacid ions ΣPV₁₄, and the optimum molar fraction of ΣPV₁₄ reaches 88.55% when the pH value is 2. As the pH value continuously increases to 4~6, ΣPV₁₄ gradually disappears and is converted to vanadium isopolyacid anion ΣV₁₀, and the optimum molar fraction of ΣV₁₀ is 100.00% when the pH value is 5.