760 POTASSIUM IODATE
KOH + NaBH4 → KBH4 + NaOH
Reaction with hydrofluoric acid forms potassium bifluoride:
KOH + 2HF → KHF2 + H2O
Half neutralization of a phthalic anhydride solution forms potassium hydrogen phthalate.
Analysis
The normality of KOH in its aqueous solution can be determined by acidbase titration against a standard solution of HCl, H2SO4, or HNO3 using a color indicator or by a pH meter. Potassium can be identified by flame test or by wet methods or instrumental analysis (see Potassium).
POTASSIUM IODATE
[7758–05–6]
Formula: KIO3; MW 214.00
Uses
Potassium iodate is an oxiding agent in volumetric analysis. It releases iodine in KIO3–KI solutions for iodometric titrations. It also is a topical antiseptic; and an additive to food to provide nutrient iodine.
Physical Properties
Colorless crystals or white powder; monoclinic structure; density 3.90 g/cm3; stable at ordinary temperatures; melts at 560°C with partial decomposition, releasing oxygen; moderately soluble in cold water; 4.74 g/100mL at 0°C; greater solubility in boiling water 32.3 g/100mL at 100°C; soluble in potassium iodide solution; insoluble in alcohol and liquid ammonia
Thermochemical Properties |
–119.8 kcal/mol |
∆Ηƒ° |
|
∆Gƒ° |
–100.0 kcal/mol |
S° |
36.2 cal/deg mol |
Cρ |
25.4 cal/deg mol |
Preparation
Potassium iodate can be produced by fusing potassium iodide with potassium chlorate, bromate or perchlorate:
KI + KClO3 → KIO3 + KCl
The melt is extracted with water and potassium iodate is isolated from solu-
POTASSIUM IODIDE 761
tion by crystallization.
Analysis
Elemental composition: K 18.27%, I 59.30%, and O 22.43%. An aqueous solution may be analyzed for potassium (see Potassium) and for IO3¯ by ion chromatography. The iodate, IO3¯ content can be measured by iodometric titration:
IO3¯ + 6H+ + 5I¯ (excess) → 3I2 + 3H2O
In strong acid solution IO3¯ oxidizes I¯ liberating iodine, which can be titrated against a standard solution of sodium thiosulfate. At the end point, the blue solution decolorizes.
POTASSIUM IODIDE
[7681–11–0]
Formula: KI; MW 166.00
Occurrence and Uses
Potassium iodide is found in seaweed. Some important applications of this compound involve its use in pharmaceuticals and as a source of iodine in food, especially in animal and poultry feed. Potassium iodide is added to table salt to provide iodine in human food.
Another major use is in making photographic emulsions. In analytical chemistry, potassium iodide is used in iodometric titration with starch indicator to analyze dissolved oxygen, dissolved chlorine, sulfide, and other analytes in water.
Physical Properties
Colorless or white cubic crystals or granules; becomes yellowish when exposed to bright light due to photochemical decomposition liberating traces of free iodine; density 3.13 g/cm3; melts at 681°C; vaporizes at 1,330°C; highly soluble in water, ~140 g/100mL at 20°C; aqueous solution readily dissolves iodine; sparingly soluble in ethanol (about 2 g/100mL at 25°C) and acetone; slightly soluble in ether and ammonia.
Thermochemical Properties |
–78.37 kcal/mol |
∆Ηƒ° |
|
∆Gƒ° |
–77.65 kcal/mol |
S° |
25.4 cal/deg mol |
Cρ |
12.65 cal/deg mol |
Preparation
Potassium iodide is made by absorption of iodine in potassium hydroxide:
762 POTASSIUM IODIDE
3I2 + 6KOH → 5KI + KIO3 + 3H2O
Most potassium iodate, KIO3 , is separated from the product mixture by crystallization and filtration. Remaining iodates are removed by evaporation of the solution and other processes, such as carbon reduction or thermal decompostion at 600ºC to iodide:
2KIO3 → 2KI + 3O2
Another method of preparation that does not involve the formation of iodate is by treating iron turnings with iodine solution. The product, ferrosoferric iodide, Fe3I8•16H2O, is boiled with 15 wt% potassium carbonate solution:
Fe3I8•16H2O + 4K2CO3 → 8 KI + 4CO2 + Fe3O4 + 16H2O
A similar method is used to prepare potassium bromide, discussed earlier (see Potassium Bromide.)
Potassium iodide can be prepared by reacting hydriodic acid with potassium bicarbonate:
HI + KHCO3 → KI + CO2 + H2O
It is purified by melting in dry hydrogen.
Potassium iodide also may be obtained by various electrolytic processes.
Reactions
The reactions of potassium iodide in aqueous solutions are those of iodide ion, I¯. In iodometric titration I¯ combines with iodine to form triiodide ion, I3¯. The latter adds to β–amylose fraction of the starch to form a blue complex.
Potassium iodide dissolves iodide of certain metals to form complex anions:
HgI2 + 2I¯ → [HgI4]2–
Analysis
Elemental composition: K 23.55%, I 76.45%. Potassium may be measured by various instrumental methods (see Potassium). Iodide ion in an aqueous solution can be measured by ion chromatography or leuco crystal violet colorimetric method (see Iodine).
POTASSIUM NITRATE
[7757–79–1]
Formula KNO3; MW 101.10 Synonyms: saltpeter; niter
POTASSIUM NITRATE 763
Uses
Potassium nitrate is used in explosives, blasting powders, gunpowder, matches, and fireworks. Other applications of this salt include pickling meats; tempering steel; impregnating candle wicks; freezing mixtures; preparing other potassium salts; and as a diuretic.
Physical Properties
Colorless transparent crystals or white granular or crystalline powder; rhombohedral structure; density 2.11 g/cm3 at 20°C; melts at 334°C; decomposes at 400°C evolving oxygen; soluble in cold water, 13.3 g/100mL at 0°C; highly soluble in boiling water, 247 g/100mL at 100°C; lowers the temperature of water on dissolution; very slightly soluble in ethanol; soluble in glycerol and liquid ammonia.
Thermochemical Properties
∆Ηƒ° |
–118.22 kcal/mol |
∆Gƒ° |
–94.39 kcal/mol |
Cρ |
31.80 cal/deg mol |
S° |
23.04 cal/deg mol |
Production
Potassium nitrate may be produced by several methods. It is made commercially by reacting potassium chloride with nitric acid at high temperature. Nitrosyl chloride, a product obtained in the reaction, is converted into chlorine in this manufacturing process. Also, nitric acid is partly recycled in the process. The reactions are (Dancy, W.B. 1981. Potassium Compounds. In
Kirk-Othmer Encyclopedia of Chemical Technology, 3rd. ed. Pp. 939-42. New York: Wiley Interscience):
3KCl + 4HNO3 → 3KNO3 + Cl2 + NOCl + 2H2O
2NOCl + 4HNO3 → 6NO2 + Cl2 + 2H2O
4NO2 + O2 + 2H2O → 4HNO3
Potassium nitrate also can be prepared by mixing a hot saturated solution of potassium chloride and sodium nitrate. The reaction is:
K+ + Cl¯+ Na+ + NO3¯ → NaCl↓ + K+ + NO3¯
Sodium chloride is less soluble than KCl, NaNO3 and KNO3. It separates out by crystallization. The remaining solution is cooled to ambient temperature. Potassium nitrate crystallizes out.
Analysis
The aqueous solution may be analyzed for potassium by various instru-