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Potassium Hydroxide
Physical and Chemical Properties
Physical Constants
Chemical Reactions
Dilution of Liquid Caustic Potash
Densities and Concentrations
Boiling Points
Solubility
Specific Gravities
Conversion Tables
MSDS
Material safety data for dry potassium hydroxide in Adobe .PDF format.
Physical Constants
Chemical Name
Potassium Hydroxide
Chemical Formula
KOH
Molecular Weight
56.1
Crystalline Form
Rhombic
Specific Gravity
2.044
Melting Point
380°C (716°F)
Boiling Point @ 760 mm Hg pressure
1,320°C (2,480°F)
Heat of Fusion
28.6 cal/g
Heat of Solution @ 18°C
+13.29 Kg-cal per 250 moles H2O
Solubility in Various Solvents
Cold Water @ 0°C
97 grams per 100 mL water
Hot Water @ 100°C
178 grams per 100 mL water
Organic Solvents
Very soluble in alcohol and ether
Inorganic Solvents
Insoluble in NH3
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Chemical Reactions
Moisture
KOH is deliquescent and absorbs moisture from the air until it is dissolved. Heat is
evolved during the dissolving process.
pH
KOH solutions are strongly alkaline and have a pH value greater than 12.
Carbon Dioxide
In the presence of moisture, KOH reacts with CO2 from the air to form K2CO3 and
KHCO3.
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Animal and Plant Fibers
All living tissues, especially animal fibers such as wool, leather, silk and human skin are
rapidly dissolved by KOH and thus destroyed. Plant fibers like cotton and rubber are
less readily attacked.
Heavy Organic Compounds
In order to make heavy organic compounds soluble in water, parts of the KOH
molecule are added by means of replacement and saponification reactions.
Inorganic Compounds
KOH reacts with inorganic acids and acidic anhydrides to form potassium salts. With
metals it forms insoluble hydroxides.
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Dilution of Liquid Caustic Potash
The following problems make use of the fundamental equations:
DENSITY=WEIGHT/VOLUME
Concentration 1 x Volume 1 = Concentration 2 x Volume 2
EXAMPLE 1: Dissolving Solid Forms of KOH
Problem
What amount of ASHTA flake caustic
potash (90% KOH) and what amount of
water must be added in order to obtain
20% KOH solution?
Solution
Refer to the Dilution Chart below since 90% KOH and water (0% KOH) are involved, use
line A, which connects these two points. Follow the 20% line vertically until it intersects line
A. Read horizontally left and right from the point of intersection. 22 lbs. of strong solution
(90% KOH) must e added to 78 lbs. (9.38 gallons) of dilute solution (water) in order to
obtain a 20% KOH solution. Pounds of water can be changed to gallons by dividing by
8.32 lb./gal.
EXAMPLE 2: Diluting a KOH Solution
Problem
How many gallons of water must be added
to 2,000 gallons of liquid caustic potash in
order to obtain a 29% KOH solution? The
KOH is found to have a specific gravity of
1.45 at 20°C.
Solution
Refer to the Specific Gravities Table and find that a 20°C caustic potash solution with a
specific gravity of 1.45 contains 45% KOH by weight. On the Dilution Chart, follow the
29% line vertically until it intersects line B and read horizontally left and right from the
point of intersection. 35.6 lbs. of water must be added to 64.4 lbs. of 45% KOH. Divide
each number by its density (in lbs./gal.) to change from pounds to gallons and then divide
the resulting numbers to obtain the number of gallons of water to be added to each gallon
of 45% KOH.
35.6/8.32=4.28; 64.4/12.09=5.33
4.28/5.33=0.803 gal.water/gal.45% KOH
EXAMPLE 3: Determining the Final Volume of a KOH Solution
Problem
After dilution, what will be the final volume
at 20°C of the KOH solution in example 2?
Solution
Due to the chemical nature of caustic potash, shrinkage occurs when KOH solutions are
diluted with water so that the final volume is always less tan expected. The exact final
volume may be calculated as follows: Determine the weight of the original (45% KOH)
solution and the weight of water added by multiplying the number of gallons by the density
(in lb./gal.). Add the weights together and divide by the density (in lb./gal.) of the diluted
(29% KOH) solution.
(200 x 12.09) + (1606 x 8.32) = 37542 lb.
(37542 lb.) / (10.63 lb./gal.) = 3532 gal. of 29% solution
The final volume is actually 3532 gallons compared to the expected volume of 3606 gallons.
EXAMPLE 4: Strengthening a KOH Solution
Problem
How many gallons of 45% KOH must be
added to 1,000 gallons of 25% KOH in
order to obtain a 30% KOH solution at
20°C?
Solution
Follow the 30% line on the Dilution Chart vertically until it intersects line C. Read
horizontally left and right from the point of intersection. 25 lbs. of 45% must be added to
75 lbs. of 25% in order to make a 30% KOH solution. Divide each number by its density
(in lb./gal.) to change from pounds to gallons. Densities at 20°C can be obtained from
chart 2. Divide the resulting numbers to obtain the number of gallons of 45% to be added
to each gallon of 25%.
25/12.09=2.07; 75/10.12=7.41
2.07/7.41=0.279 gal.45% KOH/gal.25% KOH
Since 1,000 gallons of 25% KOH are to be strengthened, multiply 0.279 by 1,000. 279 gallons of 45% KOH must be added to 1,000
gallons of 25% KOH in order to obtain a 30% solution.
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Densities and Concentrations
of aqueous caustic potash solutions at 20°C (68°F)
Density of KOH Solutions
Amount of KOH in Solution
% KOH
(by
weight)
Specific
Gravity
Pounds per
Gallon
Degrees
Baumé
Degrees
Twaddell
Actual
Concentrations
(100% KOH basis)
International Caustic
Potash
(90% KOH basis)
2
1.01623
8.46
2.3
3.2
0.17
0.19
4
1.03437
8.61
4.8
6.8
0.34
0.38
6
1.05262
8.76
7.3
10.5
0.53
0.59
8
1.07106
8.91
9.6
14.2
0.71
0.79
10
1.08970
9.07
11.9
17.9
0.91
1.01
12
1.10855
9.22
14.2
21.7
0.91
1.01
14
1.12763
9.38
16.4
25.5
1.31
1.46
16
1.14694
9.54
18.6
29.4
1.53
1.70
18
1.16645
9.71
20.7
33.3
1.75
1.94
20
1.18617
9.87
22.8
37.2
1.97
2.19
22
1.20600
10.04
24.8
41.2
2.21
2.46
24
1.22614
10.20
26.8
45.2
2.45
2.72
26
1.24650
10.37
28.7
49.3
2.70
3.00
28
1.26712
10.54
30.6
53.4
2.95
3.28
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1.28799
10.72
32.4
57.6
3.22
3.58
32
1.30913
10.89
34.2
61.8
3.48
3.87
34
1.33053
11.07
36.0
66.0
3.76
4.18
36
1.35219
11.25
37.8
70.4
4.05
4.50
38
1.37413
11.43
39.5
74.8
4.34
4.82
40
1.39634
11.62
41.2
79.3
4.65
5.17
42
1.4188
11.80
42.8
83.8
4.96
5.51
44
1.44164
11.99
44.4
88.3
5.28
5.88
45
1.45314
12.09
45.2
90.6
5.44
6.04
46
1.46477
12.19
46.0
92.9
5.61
6.23
48
1.48820
12.38
47.6
97.6
5.94
6.60
50
1.51211
12.58
49.1
102.4
6.29
6.99
52
1.53632
12.78
50.6
107.3
6.65
7.39
Source: Data in columns 1 and 2 are taken from Akerloff and Bender, J. Am. Chem. Soc., 63: 1088 (1941).
The remaining columns are calculated from the following formulas:
Specific Gravity = (density KOH solution) / (density H2O) = (density KOH in lbs./gal. @ 20°C) / (8.321)
°Be. = 145 - (145 / specific gravity)
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Boiling Points
of aqueous caustic potash solutions
Temp. (°C)
% KOH
(by weight)
Temp. (°C)
% KOH
(by weight)
100
0.0
230
70.67
110
25.65
240
72.46
120
36.51
250
74.07
130
43.42
260
75.76
140
48.05
270
77.52
150
51.57
280
79.05
160
54.88
290
80.32
170
57.8
300
81.63
180
60.42
310
82.99
190
62.73
320
84.03
200
64.93
330
85.11
210
66.89
340
86.16
220
68.73
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Solubility
of caustic potash in water
Temperature (°C)
% KOH (by weight)
Solid Phase
-2.2
3.6
Ice
-20.7
18.4
Ice
-65.2
30.8
Ice
-36.2
26.6
KOH · 4H2O
-32.7
43.8
KOH · 4H2O
-33
44.4
KOH · 4H2O + KOH · 2H2O
-23.2
45.9
KOH · 2H2O
0
49.2
KOH · 2H2O
10
50.7
KOH · 2H2O
15
51.7
KOH · 2H2O
20
52.8
KOH · 2H2O
30
55.76
KOH · 2H2O
32.5
57.44
KOH · 2H2O + KOH · H2O
50
58.33
KOH · 2H2O + KOH · H2O
100
64.03
KOH · 2H2O
125
68.06
KOH · 2H2O
143
75.73
KOH · 2H2O
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Specific Gravities
of aqueous potash solutions at various temperatures and concentrations
% KOH (by wt.)
32°F
0°C
50°F
10°C
68°F
20°C
86°F
30°C
104°F
40°C
122°F
50°C
140°F
60°C
158°F
70°C
0
0.99982
0.99948
0.99800
0.99553
0.99219
0.98809
0.98330
0.97789
2
1.01927
1.01822
1.01623
1.01341
1.00985
1.00562
1.00076
0.99529
4
1.03849
1.03681
1.03437
1.03124
1.02749
1.02315
1.01822
1.01269
6
1.05773
1.05548
1.05262
1.04922
1.04528
1.04083
1.03583
1.03026
8
1.07706
1.07429
1.07106
1.06739
1.06329
1.05872
1.05366
1.04804
10
1.09650
1.09327
1.08970
1.08579
1.08153
1.07686
1.07174
1.06607
12
1.11607
1.11243
1.10855
1.10443
1.10002
1.09526
1.09008
1.08437
14
1.13576
1.13177
1.12763
1.12332
1.11877
1.11392
1.10868
1.10293
16
1.15557
1.15130
1.14694
1.14245
1.13779
1.13285
1.12756
1.12176
18
1.17546
1.17098
1.16645
1.16183
1.15706
1.15205
1.14670
1.14087
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1.19542
1.19082
1.18617
1.18145
1.17659
1.17151
1.16610
1.16024
22
1.21532
1.21069
1.20600
1.20122
1.19630
1.19116
1.18570
1.17981
24
1.23575
1.23094
1.22614
1.22132
1.21635
1.21115
1.20565
1.19973
26
1.25642
1.25145
1.24650
1.24154
1.23651
1.23128
1.22576
1.21982
28
1.27730
1.27219
1.26712
1.26204
1.25691
1.25159
1.24602
1.24006
30
1.29840
1.29318
1.28799
1.28280
1.27758
1.27217
1.26653
1.26054
32
1.31973
1.31442
1.30913
1.30384
1.29852
1.29302
1.28733
1.28129
34
1.34132
1.33591
1.33053
1.32514
1.31974
1.31416
1.30840
1.30232
36
1.36314
1.35766
1.35219
1.34672
1.34124
1.33558
1.32976
1.32364
38
1.38524
1.37969
1.37413
1.36857
1.36303
1.35729
1.35141
1.34524
40
1.40765
1.40199
1.39634
1.39071
1.38510
1.37930
1.37335
1.36713
42
1.43037
1.42459
1.41884
1.41313
1.40747
1.40160
1.39560
1.38932
44
1.45347
1.44751
1.44164
1.43585
1.43015
1.42422
1.41817
1.41182
46
1.47700
1.47079
1.46477
1.45889
1.45315
1.44718
1.44108
1.43465
48
1.50097
1.49442
1.48820
1.48222
1.47645
1.47045
1.46431
1.45779
50
1.52565
1.51861
1.51211
1.50600
1.50022
1.49419
1.48802
1.48139
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Conversion Tables
The conversion factors below are calculated on a basis of 100% purity. To convert compound A into an equivalent amount of compound
B, multiply by the A to B factor. To convert compound B into compound A, multiply by the B to A factor.
Compound A
Compound B
A to B
B to A
K
KOH
K2 CO3
KCl
K2 SO4
K2O
1.205
0.840
0.682
0.632
0.541
0.831
1.191
1.467
1.583
1.850
K2 CO3
KHCO3
CO2
0.318
0.220
3.141
4.545
K2 CO3
KOH
0.812
1.232
K2 SO4
SO4 =
0.551
1.814
KCl
2.103
0.856
0.737
0.479
1.169
1.356
ClK2 SO4
KNO3
Conversion Factor A to B = (Equivalent weight B) / (Equivalent weight A)
Conversion Factor B to A = 1 / (Conversion Factor A to B)
Equivalent Conductance
of caustic potash solutions
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°C
°F
Reference
18
64.4
238.7
(40)
18
64.4
239.0
(3)
25
77.0
274.9
(24)
25
77.0
274.9
(16)(17)
25
77.0
283.9
(33)
25
77.0
285.46
(32)
90
194.0
599.0
(8)
Heat of Neutralization
of caustic potash and caustic soda solutions at 20°C
Mol Acid / 100 Mol H2O
Mol KOH / 100 Mols H2O
(Calories)
Mol NaOH / 100 Mols H2O
(Calories)
HCl
14014
13895
HBr
13988
13843
HI
13915
13779
HNO3
14086
13836
Heat of Solution
°C
°F
Mol of Water
per Mol KOH
Heat of Solution
kcal/mol
B.T.U. per lb. KOH
Reference
11.4
52.5
260
12.46
399.9
(5)
18.0
64.4
250
13.29
426.5
(62)
100
212
260
16.8
539.2
(6)
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