In a mixture of methyl and ethyl alcohols which is known to be free from other substances except water, the proportions of the two alcohols can be readily ascertained by determining the refractive index of the mixture.1 Or, what comes to the same thing, instead of the refractive index we may use the refractive value, expressed in terms of the scale degrees of the particular instrument employed. The Zeiss immersion refractometer is the most suitable apparatus. Any other form, however, may be used, so long as the index of refraction can be determined, though the immersion instrument gives the most accurate results. Leach and Lythgoe's tables give the refraction values at 20° corresponding with percentages of alcohol by weight: their data are used in the following description.

The refraction value of methyl alcohol is less than that of ethyl alcohol of the same strength. The greater the strength of the

1 Leach and Lythgoe, J. Amer, Chem. Soc., 1905, 27, 964, alcohols, the greater is the difference of refraction value. For example, at 10 per cent. strength the difference is 11.2° (Zeiss scale); at 50 per cent., 50.5°; and for the absolute alcohols, 89°.

As the strength continuously increases, the refraction.value of each alcohol first rises to a maximum, and then falls. With methyl alcohol, the maximum is reached at about 50 per cent. strength (by weight); with ethyl at about 75 per cent.

To make a determination, the specific gravity of the mixture is taken, and then the refractometer reading at 20°. From the specific gravity, the percentage of total alcohol is ascertained by means of the ordinary tables (p. 239). No appreciable error is caused by using these tables for the mixture, since the densities of ethyl and methyl alcohols are substantially the same. The refractometer reading of the mixture will lie between the values for ethyl alcohol and methyl alcohol in the refraction tables, whence the proportions of the two in the mixture are found by a simple calculation, thus: -

Suppose the total alcohol to be 30 per cent., and the refractometer reading 42.0°. From the table, the readings corresponding with 30 per cent. alcohol are 328 and 690. Then 690 . 328 = 36.2, and 690 . 420 = 270; so that (27/36.2) x 100 = 74.58 per cent. of the total alcohol is methyl alcohol; and as the total alcohol is 30 per cent., the methyl alcohol is 30 X 0.7458 = 22.37 per cent, of the mixture, by weight.

Apart from errors of reading the instrument, the chief source of inaccuracy to guard against is the presence of any appreciable quantity of acetone, essential oils, or other volatile substances, which may notably affect the refraction. If no such impurities are present, fairly accurate results are obtainable. An idea of the degree of accuracy may be gathered from the following results: -

Methyl alcohol, per cent.

Present.

Found.

Difference.

5.98

6.48

+ 0.50

8.50

8.92

+ 0.42

12.21

11.77

- 0.44

22.84

23.75

+ 0.91

45.68

47.41

+ 1.73

68.52

69.88

+ 1.36

Scale readings of Zeiss immersion refractometer at 20°

(Leach and Lythgoe).

Alcohol.

Per cent.

by weight.

Readings.

Methyl alcohol.

Ethyl alcohol.

0

14.5

14.5

1

14.8

16.0

2

15.4

17.6

3

160

19.1

4

16.6

20.7

5

17.2

22.3

6

17.8

24.1

7

18.4

25.9

8

19.0

27.8

9

19.6

29.6

10

20.2

31.4

11

20.8

33.2

12

21.4

35.0

13

22.0

36.9

14

22.6

38.7

15

23.2

40.5

16

23.9

42.5

17

24.5

44.5

18

25.2

46.5

19

25.8

48.5

20

26.5

50.5

21

27.1

52.4

22

27.8

54.3

23

28.4

56.3

24

29.1

58.2

25

29.7

60.1

26

30.3

61.9

27

30.9

63.7

28

31.6

65.5

29

32.2

67.2

30

32.8

69.0

31

33.5

70.4

32

34.1

71.7

33

34.7

73.1

34

35.2

74.4

35

35.8

75.8

36

36.3

76.9

37

36.8

78.0

38

37.3

79.1

39

37.7

80.2

40

38.1

81.3

41

38.4

82.3

42

38.8

83.3

43

39.2

84.2

44

39.3

85.2

45

39.4

86.2

46

39.5

87.0

47

39.6

87.8

48

39.7

88.7

49

39.8

89.5

Alcohol.

Per cent.

by weight.

Readings.

Methyl alcohol.

Ethyl alcohol.

50

39.8

90.3

51

39.7

91.1

52

39.6

91.8

53

39.6

92.4

54

39.5

93.0

55

39.4

93.6

56

39.2

94.1

57

39.0

94.7

58

38.6

95.2

59

38.3

95.7

60

37.9

96.2

61

37.5

96.7

62

37.0

97.1

63

36.5

97.5

64

36.0

98.0

65

35.5

98.3

66

35.0

98.7

67

34.5

99.1

68

34.0

99.4

69

33.5

99.7

70

33.0

100.0

71

32.5

100.2

72

31.7

100.4

73

31.1

100.6

74

30.4

100.8

75

29.7

101.0

76

29.0

101.0

77

28.3

100.9

78

27.6

100.9

79

26.8

100.8

80

26.0

100.7

81

25.1

100.6

82

24.3

100.5

83

23.6

100.4

84

22.8

100.3

85

21.8

100.1

86

20.8

99.8

87

19.7

99.5

88

18.6

99.2

89

17.3

98.9

90

16.1

98.6

91

14.9

98.3

92

13.7

97.8

93

12.4

97.2

94

11.0

96.4

95

9.6

95.7

96

8.2

94.9

97

6.7

94.0

98

5.1

93.0

99

3.5

92.0

100

2.0

91.0

In this country it is more usual to take the refractometer readings at 15.6° C. (= 60° F.), and the results are often wanted by volume, or at proof strength. The following tables, calculated by J. Holmes, give the required values correlated with specific gravities: -

Specific gravities, alcohol percentages, and refraction.values of aqueous solutions of methyl alcohol and ethyl alcohol.

Specific gravity.

Percentage of

Refractometer reading on

Zeiss immersion instrument at 15.6°.

At 15.6°

15.6°.

At 15.6°

4°.

Methyl alcohol.

Ethyl alcohol.

Proof spirit.

By volume.

By weight.

By volume.

By weight.

Methyl alcohol.

Ethyl alcohol.

1.0000

0.9990

___ -

___0

- ______

-

-

15.4

-

0.9982

0.9972

1.25

1

1.20

0.96

2.09

16.0

16.9

0.9965

0.9955

2.50

2

2.36

1.89

4.12

16.6

18.3

0.9948

0.9938

3.75

3

3.56

2.85

6.23

17.2

19.9

0.9931

0.9921

4.99

4

4.81

3.84

8.38

17.7

21.5

0.9914

0.9905

6.23

5

6.09

4.88

10.62

18.3

23.3

0.9898

0.9889

7.46

6

7.34

5.89

12.81

19.0

25.1

0.9882

0.9873

8.69

7

8.64

6.94

15.09

19.7

27.0

0.9866

0.9857

9.91

8

9.99

8.04

17.44

20.3

29.1

0.9851

0.9842

11.13

9

11.31

9.11

19.72

20.9

31.0

0.9836

0.9827

12.35

10

12.65

10.21

22.07

21.6

33.3

0.9821

0.9812

13.57

11

14.04

11.34

24.50

22.3

35.5

0.9806

0.9797

14.78

12

15.46

12.51

26.98

23.1

37.9

0.9791

0.9782

15.99

13

16.91

13.72

29.53

23.9

40.2

0.9776

0.9767

17.19

14

18.40

14.94

32.11

24.7

42.7

0.9761

0.9752

18.39

15

19.88

16.17

34.70

25.5

45.2

0.9746

0.9737

19.58

16

21.37

17.41

37.32

26.3

47.8

0.9732

0.9723

20.77

17

22.74

18.56

39.73

27.1

50.1

0.9717

0.9708

21.96

18

24.20

19.78

42.28

27.9

52.6

0.9703

0.9694

23.15

19

25.55

20.90

44.64

28.7

54.8

0.9689

0.9680

24.33

20

26.86

22.01

46.93

29.6

570

0.9675

0.9666

25.51

21

28.15

23.09

49.18

30.6

59.2

0.9661

0.9652

26.69

22

29.39

24.15

51.37

31.6

61.4

0.9647

0.9638

27.86

23

30.59

25.18

53.48

32.5

63.4

0.9633

0.9624

29.03

24

31.75

26.17

55.51

33.4

65.2

0.9619

0.9610

30.20

25

32.87

27.13

57.47

34.2

66.9

0.9605

0.9596

31.36

26

33.95

28.06

59.37

34.9

68.5

0.9591

0.9582

32.52

27

35.00

28.97

61.20

35.6

70.1

0.9576

0.9567

33.67

28

36.08

29.91

6310

36.2

71.6

0.9561

0.9552

34.82

29

37.13

30.84

64.97

36.7

73.1

0.9546

0.9537

35.96

30

38.16

31.74

66.77

37.2

74.4

0.9531

0.9522

37.10

31

39.16

32.62

68.52

37.7

75.7

0.9515

0.9506

38.23

32

40.19

33.53

70.32

38.2

76.9

0.9499

0.9490

39.36

33

41.19

34.43

72.09

38.6

78.2

0.9482

0.9473

40.48

34

42.23

35.35

73.91

39.0

79.3

0.9466

0.9457

41.60

35

43.19

36.22

75.59

39.4

80.5

0.9449

0.9440

42.71

36

44.19

37.12

77.34

39.7

81.5

0.9432

0.9423

43.82

37

45.17

38.02

79.07

40.0

82.6

0.9415

0.9406

44.92

38

46.13

38.89

80.74

40.3

83.6

0.9398

0.9389

46.02

39

47.06

39.75

82.38

40.6

84.5

0.9380

0.9371

47.11

40

48.04

40.65

84.10

40.9

85.6

0.9362

0.9353

48.19

41

49.00

41.54

85.78

41.1

86.6

0.9343

0.9334

49.27

42

49.99

42.48

87.53

41.4

87.6

0.9325

0.9316

50.35

43

50.93

43.35

89.17

41.6

88.6

0.9307

0.9298

51.42

44

51.83

44.21

90.76

41.8

89.4

0.9288

0.9279

52.48

45

52.77

45.10

92.40

42.0

90.2

0.9270

0.9261

53.54

46

53.65

45.94

93.95

42.1

91.0

Specific gravities alcohol percentages, and refraction-values of aqueous solutions) of methyl alcohol and ethyl alcohol . cont.

Specific gravity.

Percentage of

Refractometer reading on

Zeiss immersion instrument at 15.6°.

At 15.6°

15.6°.

At 15.6°

4°.

Methyl alcohol.

Ethyl alcohol.

Proof spirit.

By volume.

By weight.

By volume.

By weight.

Methyl alcohol.

Ethyl alcohol.

0.9251

0.9242

54.59

47

54.57

46.82

95.56

42.2

91.7

0.9232

0.9223

55.64

48

55.47

47.70

9716

42.3

92.4

0.9213

0.9204

56.69

49

56.38

48.57

98.73

42.4

93.1

0.9194

0.9185

57.73

50

57.27

49.44

100.29

42.5

93.8

0.9175

0.9166

58.76

51

58.15

50.31

101.84

42.5

94.4

0.9155

0.9146

59.79

52

59.06

51.21

103.44

42.5

95.1

0.9136

0.9127

60.81

53

59.92

52.06

104.94

42.5

95.7

0.9116

0.9107

61.82

54

60.81

52.95

106.50

42.5

96.3

0.9096

0.9086

62.81

55

61.69

53.83

108.05

42.4

96.9

0.9075

0.9066

63.81

56

62.61

54.76

109.67

42.3

97.6

0.9054

0.9045

64.80

57

63.52

55.68

111.28

42.2

98.2

0.9033

0.9024

65.80

58

64.43

56.62

112.87

42.1

98.8

0.9011

0.9002

66.76

59

65.37

57.58

114.52

41.9

99.2

0.8990

0.8980

67.72

60

66.25

58.50

116.07

41.7

99.7

0.8968

0.8959

68.69

61

67.17

59.46

117.68

41.5

100.2

0.8945

0.8937

69.64

62

68.13

60.45

119.34

41 2

100.7

0.8922

0.8914

70.58

63

69.06

61.44

121.00

40.9

101.2

0.8899

0.8891

71.53

64

69.99

62.42

122.63

40.5

101.7

0.8876

0.8868

72.44

65

70.91

63.42

124.25

40.1

102.1

0.8862

0.8844

73.36

66

71.87

64.45

125.93

39.6

102.6

0.8828

0.8820

74.28

67

72.82

65.47

127.59

39.2

103.0

0.8804

0.8796

7519

68

73.76

66.49

129.23

38.8

103.4

0.8781

0.8773

76.09

69

74.64

67.47

130.79

38.3

103.7

0.8757

0.8749

76.98

70

75.55

68.48

132.39

37.8

104.0

0.8733

0.8725

77.86

71

76.46

69.48

133.99

37.3

104.2

0.8709

0.8701

78.74

72

77.36

70.50

135.57

30.7

104.4

0.8685

0.8677

79.61

73

78.25

71.51

137.13

361

104.6

0.8661

0.8653

80.48

74

79.12

72.51

138.68

35.5

104.9

0.8637

0.8629

81.34

75

80.00

73.51

140.21

34.9

105.0

0.8612

0.8604

82.19

76

80.90

74.56

141.78

34.2

105.2

0.8587

0.8579

83.03

77

81.79

75.60

143.35

33.5

105.3

0.8562

0.8554

83.86

78

82.66

76.63

144.89

32.8

105.4

0.8537

0.8529

84.69

79 .

83.53

77.66

140.41

32.1

105.5

0.8512

0.8504

85.51

80

84.38

78.68

147.91

31.3

105.6

0.8486

0.8478

86.32

81

85.26

79.74

149.45

30.4

105.0

0.8460

0.8452

87.11

82

86.12

80.79

150.97

29.5

105.5

0.8434

0.8426

87.90

83

86.97

81.84

152.46

28.5

105.4

0.8408

0.8400

88.69

84

87.80

82.88

153.92

27.6

105.4

0.8382

0.8374

89.47

85

88.62

83.91

155.36

26.6

105.3

0.8355

0.8347

90.23

86

89.46

84.98

156.83

25.6

105.1

0.8329

0.8321

90.99

87

90.25

85.99

158.21

24.6

104.8

0.8302

0.8294

91.74

88

91.05

87.03

159.63

23.5

104.5

0.8275

0.8267

92.48

89

91.84

88.08

161.01

22.4

104.2

0.8248

0.8240

93.21

90

92.61

89.11

162.36

21.3

103.9

0.8221

0.8213

93.94

91

93.35

90.12

163.07

20.1

103.5

0.8193

0.8185

94.65

92

94.11

91.16

165.00

18.9

103.1

0.8165

0.8157

95.35

93

94.84

92.18

166.28

17 6

102.6

0.8137

0.8129

96.04

94

95.55

93.19

167.54

16.3

102.0

0.8109

0.8101

96.73

95

96.23

94.19

168.74

15.0

101.3

0.8081

0.8073

97.41

96

96.91

95.17

169.92

13.7

100.5

0.8052

0.8044

98.07

97

97.57

96.16

171.08

12.3

99.7

0.8023

0.8015

98.73

98

98.21

97.15

172.22

11.0

98.8

0.7994

0.7986

99.37

99

98.83

98.11

173.30

9.6

97.8

0.7964

0.7956

100.00

100

99.44

99.09

174.38

8.2

90.8

For use in the tropics, the following table of the refraction.values of ethyl alcohol solutions at 30° C. (=86° F.) may be found serviceable. It is due to Browning and Symons.1 The alcoholic strengths were determined by reference to the United States' official tables.

Readings at 30°. Zeiss immersion refractometer.

Alcohol per cent. by weight.

Reading.

1

13.4

2

15.1

3

16.7

4

18.3

5

19.9

6

21.7

7

23.4

8

25.0

9

26.6

10

28.5

11

30.4

12

32.1

13

34.0

14

35.8

15

37.6

16

39.4

17

41.2

Alcohol per cent.

by weight.

Reading.

18

42.9

19

44.6

20

46.5

21

48.0

22

50.0

23

51.5

24

53.1

25

54.7

26

56.2

27

57.7

28

59.1

29

60.6

30

61.9

31

63.2

32

64.6

33

65.9

34

67.1

Alcohol per cent.

by weight.

Reading.

35

68.4

36

69.4

37

70.4

38

71.4

39

72.4

40

73.3

41

74.2

42

75.1

43

76.0

44

76.8

45

77.5

46

78.3

47

79.0

48

79.8

49

80.5

50

81.3

51

82.0

Refraction values of aqueous solutions of higher alcohols Readings at 15.6°. Zeiss immersion instrument.

Per cent. by volume.

Normal propyl.

Isopropyl.

1

17.1

17.2

2

19.0

18.8

3

20.9

20.5

4

22.9

22.3

5

24.9

24.1

6

26.9

26.0

7

28.9

27.9

8

31.0

29.8

9

33.1

31.8

10

35.3

33.8

11

37.5

35.8

12

39.7

37.8

13

41.9

39.8

14

44.1

41.9

15

46.3

43.9

16

48.5

46.0

17

50.7

48.1

18

52.9

50.2

19

55.1

52.4

20

57.2

54.6

N.Primary butyl.

Isobutyl.

Isoamyl.

17.6

17.4

17.7

19.9

19.7

20.1

22.2

22.0

22.7

24.5

24.3

25.5

26.8

26.6

29.1

29.0

31.6

31.3

34.0

33.6

36.4

35.8

38.0

40.0

1 J. Soc. Chem. Ind., 1914, 33, 821.

Refractive indices corresponding with scaie readings of immersion refractometer (wagner: "tabellen").

Scale reading.

Refractive Index. ND 1.3.

15

3320

Diff. 39.

16

3358

17

3397

1

3.9

18

3435

2

7.8

19

3474

3

11.7

4

15.6

20

3513

5

19.5

21

3551

6

23.4

22

3590

7

27.3

23

3628

8

31.2

24

3667

9

35.1

25

3705

26

3743

27

3781

28

3820

29

3858

Diff. 38.

30

3896

1

3.8

31

3934

2

7.6

32

397.2

3

11.4

33

4010

4

15.2

34

4048

5

19.0

6

22.8

35

4086

7

26.6

36

4124

8

30.4

37

4162

9

34.2

38

4199

39

4237

40

4275

41

4313

42

4350

43

4388

44

4426

Diff. 37.

45

4463

1

3.7

46

4500

2

7.4

47

4537

3

11.1

48

4575

4

14.8

49

4612

5

18.5

6

22.2

50

4650

7

25.9

51

4687

8

29.6

52

4724

9

33.3

53

4761

54

4798

55

4836

56

4873

57

4910

58

4947

59

4984

Scale reading.

Refractive Index. ND 1.3.

60

5021

61

5058

62

5095

63

5132

64

5169

65

5205

66

5242

67

5279

68

5316

69

5352

1

3.6

2

7.2

70

5388

3

10.8

71

6425

4

14.4

72

5461

5

18.0

73

5497

6

21.6

74

5533

7

25.2

8

28.8

75

5569

9

32. 4

76

5606

77

5642

78

5678

79

5714

80

5750

81

5786

82

5822

83

5858

84

5894

85

5930

86

5966

87

6002

88

6038

89

0074

90

6109

91

6145

Diff.35.

92

6181

93

6217

1

3.5

94

6252

2

7.0

3

10.5

95

6287

4

14.0

96

6323

5

17.5

97

6359

6

21.0

98

6394

7

24.5

99

6429

8

28.0

9

31.5

100

6464

101

6500

102

6535

103

570

104

6605

105

6640

Example: Scale reading 23.1 = ND 1.3 + 0.03628 + 0.000039

= 1.336319, or 1.33632 to the nearest fifth decimal.