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Alkaline solvent

Protophilic solvent

This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

Basic solvent is also called protophilic solvent. Compared with water, these solvents have a greater tendency to accept protons, and their solvation protons are higher than H three O + Its acidity is weak. Ethylenediamine Liquid ammonia It belongs to alkaline solvent.

Alkaline solvent can be used for metal leaching in metallurgical process, acid-base titration test in experimental chemistry, etc.

Chinese name: Alkaline solvent
Foreign name: basic solvent
Alias: Protophilic solvent

Classification: Caustic soda, sodium sulfide, etc
Ions: It is easy to combine with protons and ionize out negative ions
Application: Synthetic reaction, titration, etc

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Introduction to solvents

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For example, the ionic product constant of liquid ammonia is very small m NH4 + · m NH2 - =10 -33 。 Liquid ammonia is similar to water in many ways. Many reactions in liquid ammonia are similar to those in water, such as

Autoionization 2H two O←→H three O + +OH -

2NH three ←→NH four + +NH two -

Neutralization reaction in water, KOH+H three OI（H two O +HI）—→2H two O +KI

In liquid ammonia, KNH two +NH four I—→KI +2NH three

Amphoteric reaction in water, Zn 2+ +2OH - —→Zn(OH) two ↓ —→Zn(OH) four 2- (Excessive alkali)

In liquid ammonia, Zn 2+ +2NH two - —→Zn(NH two ) two ↓—→Zn(NH two ) four 2- （NH two - Excess)

However, because ammonia is more alkaline than water, the acid-base behavior of some substances in ammonia is also obviously different from that in water. For example, some substances that are weak acids in water become strong acid 。

CH three CO two H +NH three ←→NH four + +CH three CO two -

Some molecules that are not acidic in water can also be found in NH three It is weak acid.

H two NC(NH two )═O +NH three ←→NH four + +O═(H two N)CNH -

Most species considered alkaline in water ammonia Medium or insoluble, or weak base, only very strong base can be strong base.

H - +NH three ←→NH two - +H two ↑^[1]

Solvent type

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Alkaline solvents used include caustic soda (NaOH), sodium sulfide (Na two S) And sodium cyanide (NaCN), etc.

1. Caustic soda

NaOH can directly leach PbS and ZnS to generate zincate and plumbate:

PbS +4NaOH—→Na two PbO two +Na two S +4H two O

ZnS +4NaOH—→Na two ZnO two +Na two S +2H two O

The test also confirmed that the complex Pyrite FeS will be converted to Fe (OH) two However, copper nickel sulfide does not work:

FeS +2NaOH—→Fe(OH) two +Na two S

Na can also be produced in the process two S is a by-product. For leaching residue Flotation method Copper nickel sulfide concentrate is selected.^[2]

2. Sodium sulfide

Na in solution two S can be mixed with solid HgS, Sb two S three 、As two S three 、SnS two Function to generate two series of stable metal sulfur ions complex compound ：

HgS +S 2- —→HgS two 2- ； K =1.72×10 -55

Sb two S three +S 2- —→2 SbS 2- ； K =7.95×10 -44

As two S three +S 2- —→2AsS two - ； K=1.0×10 -38

SnS two +S 2- —→SnS three 2-

Dissolution product Ksp of HgS=1.6 × 10 -54 ，HgS two 2- Dissociation constant K=1.72 × 10 -55 。 Divide K value by Ksp value to obtain HgS in solution two 2- And S 2- Balance between:

[S 2- ]/[HgS two 2- ] =K/Ksp =0.107

That is to say, in order to make HgS two 2- To be stable in solution, sufficient effective [S 2- ]。 This is usually done by adding caustic soda as protective alkali, and also as [S 2- ]To achieve the purpose:^[2]

Na two S +H two O —→NaHS +NaOH

HS - +OH - —→S 2- +H two O

log[S 2- ] =-14 +pH +log[HS - ]

Na two S can be considered as completely hydrolyzed according to the above formula, while HS - The dissociation of the following formula is in equilibrium. That is, [S 2- ]Depending on pH (i.e. caustic soda concentration) and [HS - ]Concentration (i.e. Na two Concentration of S).

3. Sodium cyanide

Recently, a new method of directly leaching copper sulfide with sodium cyanide solution has been reported in the literature, which is based on the formation of a copper cyanide complex more stable than sulfide with copper and cyanide ions:

Cu two S +6CN - —→2Cu(CN) three 2- +S 2-

Cu two Solubility product of S Ksp=2.5 × 10 -50 ，2Cu(CN) three 2- K=5.4 × 10 -29 。

Similarly, the cyanide process can also directly leach low-grade copper oxide:

Cu two O +6CN - +H two O—→2Cu(CN) three 2- +2OH -

Then sulfuric acid is used to acidize the leaching solution to separate CuCN, and the regenerated HCN can be returned for use.

In addition, sulfuric acid leaching can also be used Cupric oxide Ore, passing SO into leaching solution two Gas makes Cu 2+ Restore to Cu + And then pass HCN gas to precipitate CuCN:

Cu + +HCN—→CuCN↓ +H +

The CuCN obtained by the above two ways is a white precipitate that is easy to filter. At 400 ℃, CuCN can be reduced to metal copper by passing hydrogen, and HCN can be regenerated for use:

2 CuCN +H two —→2Cu +2HCN^[2]

Solvent system

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Most of the available alkaline solvent systems are composed of lower alcohols and ammonia. Sometimes lower ketones such as acetone and butanone can be used instead of alcohols, or the aqueous solution of ethylamine can be used instead of ammonia. For example, the alkaline solvent system recorded in the literature mainly includes:

(1) Ethanol: ammonia (95:5)

(2) Ethanol: ammonia: water (90:5:5)

(3) Ethanol (95%): ammonia: water (80:10:10)

(4) Acetone: ammonia water: water (80:5:15)

(5) Ethanol: ammonia: water (80:4:16)

(6) Ethanol: aqueous solution of ethylamine (16%) (80:20)

(7) N-propanol ethanolamine (80:20)

(8) N-propanol: ammonia (70:30)

(9) N-propanol: ammonia (60:40)

(10) Butanone: water: ethylamine (100%) (921:77:2)

The ammonia water used above refers to concentrated ammonia water with a specific gravity of 0.88.

Alkaline solvent system is seldom used, because it has some disadvantages: (1) It is not easy to separate alkaloids with similar structure. For example, De Moer loose reported the R of some cinchona alkaloids in alkaline aqueous solution F Value; (2) Some alkaloids can be isomerized under alkaline conditions, and some of them can be converted into their isomers, so one alkaloid may produce more than two color spots. as Berberine (berberine), matrine, etc. We used butanol ：NH four OH (20%): water (20:5:10) uses its butanol layer as the solvent system.^[3]

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