Cobalt salts

Common cobalt salts are shown in Table 1. Almost all simple cobalt compounds are divalent. Although a simple trivalent cobalt salt can be produced by a strong oxidizing agent acting on divalent cobalt, it is easily reduced to Co 2 + by water under normal conditions.

Table 1 Main salt properties of cobalt

1. Inorganic acid salts of cobalt

(A) Chloride: A cobalt metal, an oxide, hydroxide or carbonate dissolved in hydrochloric acid to thus give an aqueous solution of CoCl 2. Evaporation of this solution gave a cherry red hexahydrate salt, CoCl 2 ·6H 2 O, which was dehydrated to a blue CoCl 2 which absorbed the water in the air to a reddish color.

CoCl 2 is a light blue rhombohedral crystal with a specific gravity of 3.35, a molecular weight of 129.86, a melting point of 724 ° C, and a defect of 1049 ° C. The relationship between vapor pressure and temperature is shown in Table 2.

Table 2 Relationship between vapor pressure and temperature of CoCl 2

CoCl 2 is a stable compound at normal temperature, but when heated to 180 ° C in air, it is rapidly decomposed according to the following reaction:

CoCl 2 is soluble in water and a range of organic solvents. Supersaturated solutions are easily formed in water. CoCl 2 ·6H 2 O is a normal salt which is stable below 50 ° C and is often sold in this form. CoCl 2 is prepared by heating in a dry box at a temperature of 135-140 ° C from a normal salt, CoCl 2 ·H 2 O is obtained by steaming a CoCl 2 solution at 100 ° C, and CoCl 2 ·2H 2 O is a normal salt. It was obtained by a drier containing concentrated sulfuric acid at a temperature of 500 ° C, and CoCl 2 · 4H 2 O was obtained by crystallization in an aqueous solution of 48 to 56 ° C.

The solubility of CoCl 2 in water is shown in Table 3:

Table 3 Relationship between solubility of cobalt chloride and temperature

When increasing the chloride ion in the cobalt chloride solution, there are (CoCl) + and (CoCl 3 ) - , and the percentages of various cobalt ions in different concentrations of hydrochloric acid are shown in Table 4:

Table 4 Percentage of various cobalt ions at different hydrochloric acid concentrations

The chloride of trivalent cobalt is unstable and forms a blue solution of trivalent unstable chloride when dissolved in hydrochloric acid (the dilute solution is rose red). The CoCl 3 salt in a solid state is difficult to obtain.

(2) Nitrate: When metal cobalt and its oxides, hydroxides or carbonates are dissolved in dilute nitric acid, an aqueous solution of Co(NO 3 ) 2 is formed. After evaporation of the solution, a red crystal of Co(NO 3 ) 2 ·6H 2 O was produced, which melted to purple or green at 100 ° C. This compound is highly water-absorptive and readily soluble in many organic solvents. The nitrate of trivalent cobalt is unstable and is only present in the solution.

(III) Sulfate: When CoO, Co(OH) 2 or CoCO 3 is dissolved in dilute sulfuric acid, CoSO 4 ·7H 2 O in a pink monoclinic system is obtained. The solubility of CoSO 4 with the temperature and sulfuric acid concentration is shown in Figure 1. CoSO 4 ·7H 2 O is not easily deliquescent or dehydrated.

In order to oxidize the cobaltous cobalt in the dilute sulfuric acid solution, it is possible to form Co 2 (SO 4 ) 3 ·18H 2 O by electrolytic oxidation, ozone or fluorine oxidation. The compound is stable in sulfuric acid, but rapidly becomes a CoSO 4 crystal when water is added. It also decomposes quickly in the air.

Figure 1 CoSO 4 -H 2 SO 4 -H 2 O system solubility isotherm

When NaOH was added to the CoSO 4 solution, blue CoSO 4 ·5Co(OH) 2 ·xH 2 O was precipitated. In the presence of excess NaOH, the precipitated basic sulfate will further change or pink Co(OH) 2 .

(4) Carbonate: CoCO 3 is almost pure in nature and is called rhombop ore. The addition of Na 2 CO 3 to the divalent cobalt solution results in a precipitate which is usually a basic carbonate, but in the presence of slightly CO 2 , a purplish red CoCO 3 ·6H 2 O precipitates. It was placed in a tube and sealed to heat to 140 ° C, which turned into a reddish powdery CoCO 3 . The substance has a very low solubility in water and can dissolve CoCO 3 0.0293 g / liter at 100 ° C, which is a relatively stable compound.

(V) phosphoric acid cobalt: with H 3 PO 4 or Na 3 PO 4 3 function and CoCO 3 or Co (OH) obtained CoPO 4. The cobalt phosphate salt Co 3 (PO 4 ) 2 ·8H 2 O used in ceramic production, which hardly dissolves in dry water.

(6) Silicate: CoO is melted together with SiO 2 to form a purple water-insoluble metasilicate (CoSiO 3 ) or orthosilicate (Co 2 SiO 4 ). When the CoSO 4 and Na 2 SiO 3 solutions were mixed, blue was precipitated first and quickly became a reddish precipitate (CoSiO 3 · 2H 2 O). The blue CoSiO 3 has a strong dyeability, and 0.1% or less of CoSiO 3 can dye the glass very blue. The composition of Daqing is K 2 O·CoO·3SiO 2 , which is often used as a pigment.

Second, cobalt organic acid salts

(I) Oxalate: The oxalate of divalent cobalt is usually a compound containing two dihydrates in water, but also in the form of tetrahydrate and anhydrous. It is almost insoluble in water but soluble in concentrated oxygen. The unstable trivalent cobalt oxalate is only present in the aqueous solution. The divalent cobalt oxalate can be obtained by reacting a (NH 4 ) 2 C 2 O 4 solution with a CoCl 2 solution or a Co(NO 3 ) 2 solution.

(b) Co(CH 3 COO) 2 : Co(CH 3 COO) 2 is used as a catalyst in the petroleum and man-made fiber industries. It usually contains four molecules of water and is easily soluble in water. CoCO 3 and CH 3 COOH can be used to prepare an aqueous solution by the following reaction, and then controlled to concentrate and crystallize to obtain a Co(CH 3 COO) 2 product conforming to specifications.

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