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 onZeiss immersion instrument at 15.6°. At 15.6°15.6°. At 15.6°4°. Methyl alcohol. Ethyl alcohol. 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 onZeiss 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.