736 POTASSIUM ACETATE

K + As → K3As

Reaction with sulfur forms three sulfides. When reactants are in molten state, the product is K2S, but in liquid ammonia K2S2 and KS2 are the main products.

Potassium reacts explosively with sulfuric acid, forming potassium sulfate with evolution of hydrogen:

K + H2SO4 → K2SO4 + H2

Potassium liberates hydrogen from ethanol forming potassium ethoxide:

2K + 2C2H5OH → 2C2H5OK + H2

Reaction with potassium nitrate yields potassium monoxide and nitrogen:

10K + 2KNO3 → 6K2O + N2

Analysis

Potassium and its salts can be identified by flame test. It imparts lilac color to the flame. Potassium ion in aqueous solution can be identified by reaction with sodium tetraphenylborate, NaB(C6H5)4. In weakly acid solution, a white precipitate of the potassium salt KB(C6H5)4 is obtained. The precipitate is filtered, dried, and weighed to measure potassium. The test is quantitative.

Potassium at trace concentrations in aqueous samples can be measured by a flame photometer at a wavelength of 766.5 nm. Either a flame photometer or an atomic absorption spectrometer operating in flame emission mode can be used for such analysis.

Potassium also can be measured by ICP/AES. The wavelengths at which it can be analyzed without interference from other metals are 766.49 and 769.90 nm. Other wavelengths may be used. Potassium ion in aqueous solution can be identified quantitatively by using a potassium ion-selective electrode attached to a pH meter having an expanded millivolt scale or to a specific ion meter having a direct readout concentration scale for potassium.

Hazard

Potassium metal can be dangerous to handle if proper precautions are not taken. Many of its reactions at ordinary temperatures can proceed to explosive violence (see Reactions). Also, it liberates flammable hydrogen gas when combined with water, acids, and alcohols.

POTASSIUM ACETATE

[127-08-2]

Formula: KC2H3O2 or CH3COOK; MW 98.14

POTASSIUM BICARBONATE 737

Uses

Potassium acetate is used in the manufacture of glass; as a softening agent for papers and textiles; as a dehydrating agent; and as a buffer. In medicine it is used as an expectorant and diuretic.

Physical Properties

White lustrous powder or colorless deliquescent crystals; density 1.57 g/cm3; melts at 292ºC; highly soluble in water, 253g/100mL at 20ºC, more soluble in hot water, 492g/100mL at 62ºC; aqueous solution alkaline, pH of 0.1M solution 9.7; soluble in methanol, ethanol and liquid ammonia; insoluble in ether and acetone.

Thermochemical Properties
∆Ηƒ° (cry)	–172.8 kcal/mol
∆Ηƒ° (amp)	–176.5 kcal/mol
∆Gƒ° (amp)	–156.0 kcal/mol
S° (amp)	45.2 cal/deg mol
Cρ (amp)	3.7 cal/deg mol

Preparation

Potassium acetate is prepared by addition of potassium carbonate in a small volume of water to acetic acid solution, followed by evaporation and crystallization:

K2CO3 + 2CH3COOH → 2CH3COOK + H2O

Analysis

Elemental composition: K 39.85%, C 24.48%, H 3.08%, O 32.60%. Potassium may be identified by flame testing. An aqueous solution can be analyzed for potassium by flame photometry, ICP/AES, or ion selective electrode (see Potassium). Acetate anion may be measured in aqueous solution by ion chromatography under appropriate conditions.

POTASSIUM BICARBONATE

[298–14–6]

Formula KHCO3; MW 100.12

Synonyms: potassium hydrogen carbonate; potassium acid carbonate

Uses

Potassium bicarbonate is used in baking powder and effervescent salts. In medicine, the salt is a gastric antacid and an electrolyte replenisher. It also is dry powder in fire extinguishers.

738 POTASSIUM BISULFIDE

Physical Properties

Colorless transparent crystal or white powder; monoclinic structure; density 2.17 g/cm3; decomposes above 100°C; soluble in water, 22.49 g/100ml at 20°C, 60 g/100ml at 60°C pH of 0.1M aqueous solution 8.2; practically insoluble in alcohol.

Preparation

Potassium bicarbonate is obtained by passing carbon dioxide through a cold, concentrated solution of potassium carbonate:

K2CO3 + CO2 + H2O → 2 KHCO3

Alternatively, KHCO3 is produced by passing excess carbon dioxide through aqueous potassium hydroxide. At first, potassium carbonate is formed which then converts to the bicarbonate as shown in the above reaction.

Potassium bicarbonate cannot be made by Solvay process because of its high solubility in water.

Reactions

Heating the bicarbonate yields normal carbonate, liberating carbon dioxide and water:

2KHCO3 → K2CO3 + CO2 ↑ + H2O↑

When the salt is added to dilute acids, carbon dioxide is liberated:

KHCO3 + HCl → K+ + Cl¯ + CO2↑ + H2O

Reaction with caustic potash in solution forms potassium carbonate:

KHCO3 + KOH → K2CO3 + CO2 + H2O

Analysis

Elemental composition: K 39.05%, C 11.99%, H 1.01%, O 47.94%. Potassium may be analyzed by AA spectroscopy in emission mode or by flame photometry (see Potassium). The aqueous solution may be treated with HCl and the CO2 evolved may be noted from effervescence and tested by GC-TCD or by GC/MS. The characteristic mass ion for CO2 is 44. Alternatively, the HCO3– anion or the CO32– anion (converted by heating the bicarbonate) may be identified by ion chromatography.

POTASSIUM BISULFIDE

[1310–61–8]

Formula KHS; MW 72.17; usually exists as a hemihydrate

Synonyms: potassium hydrosulfide; potassium hydrogen sulfide; potassium

POTASSIUM BOROHYDRIDE 739

sulfhydrate

Preparation

Potassium bisulfide is made by reacting calcium hydrogen sulfide with potassium sulfate:

Ca(HS)2 + K2SO4 → 2KHS + CaSO4

The compound can be made by reacting hydrogen sulfide with potassium sulfide:

H2S + K2S → 2KHS

Purer compound may be produced by passing dry hydrogen sulfide through a solution of potassium metal dissolved in absolute ethanol:

2H2S + 2K → 2KHS + H2

Physical Properties

Colorless crystals or white crystalline mass; rapidly deliquesces; converts to a yellow rhombohedral crystalline mass upon exposure to air, forming polysulfides and H2S; density 1.68g/cm3; the hemihydrate loses water at about 175°C; melts at 455°C to a dark red liquid; decomposes in water; soluble in alcohol.

Thermochemical Properties	62.5 kcal/mol
∆Ηƒ°
∆Ηsoln (at 17°C)	0.77 kcal/mol
∆Ηsoln (hemihyadrate at 16°C)	0.62 kcal/mol

Analysis

Elemental composition: K 54.18%, S 44.42%, H 1.40%. An aqueous solution may be analyzed for potassium by various methods (see Potassium). The compound on exposure to air evolves H2S which can be detected from its odor, as well as by various tests (see Hydrogen Sulfide).

POTASSIUM BOROHYDRIDE

[13762–51–1]

Formula KBH4; MW 53.95

Synonym: potassium tetrahydroborate

Uses

Potassium borohydride, unlike sodium borohydride, has very limited applications. The compound is a reducing agent.