December. 1933
INDUSTRIAL AND EXGINEERING CHEMISTRY
(14) International Critical Tables, Vol. I11 a n d V, McGraw-Hill, 1927. (15) Kirkbride a n d hfoCabe, IKD.ESG. CHEM.,23, 625 (1931). (16) Kraussold, Forsch. Gebiefe I n o e n i e u r w . , d3, 21 (1932). (17) Landolt-Biirnstein, Phys. Chem. Tabellen, Springer, 1912. (18) Lawrence and Sherwood, ISD. ESG. CHEM.,23, 301 (1931). (19) RlcAdams a n d Frost, I h i d . , 14, 1 3 (1922). (20) hlerkel, "Die Grundlagen der ~ ~ ~ i r m e ~ ~ b e r t r a g l ip. n g 140, ," Theodor Steinkoyff, Dresden and Leipzig, 1927. (21) h l o n r a d , IXD. ESG. CHEX, 24, 505 (1932). (22) Monrad and Badger, I b i d . , 22, 1103 (1930).
(23) (24) (25) (26) (27) (28)
1331
Morris a n d Whitman, Ibid., 20, 234 (1928). Kusselt, 2. Ver. deut. Ing., 54, 1154 (1910). I b i d . , 60, 541 (1916). Othmer, IKD.ESQ. CHEY.,21, 576 (1929). Sherwood, Kiley, a n d Mangsen, Ihid., 24, 273 (1932). Webster, Trans. I n s t . Engr. ShipSuilders, Scot., 57, 58 (1913).
REC&VEDM a y 13,1933. Presented before the Division of Petroleum Chemiatry at the 85th Meeting of the American Chemical Society, Warrhington, D. C., March 26 t o 31, 1933.
Studies in Distillation 11.
Liquid-Vapor Equilibria in t h e Systems Ethanol-Water, MethanolWat,er, and Acetic Acid-Water'
L. VALL LACE CORNELLAND RALPH.E.
fiIONTONNA,
University of Minnesota, h'hneapolis, Minn.
The nieihod of Rosanoff, Bacon, and White the i n d i c a t e d level in a suithas been used for the determination, at atmosable l i g h t oil, held a t a temmental study of the plate perature high enough t o efficiencies Of a pheric pressure, of the liquid-vapor equilibrium p r e v e n t condensation of the column for different binary mixOf systems ethanol-water, mefkanol- v a p o r s . The oil b a t h was tures, it was found that values considerably Over 100 per writ water, and acetic acid-water. This method is heated electrically, the temwere obtained for ethanol-water shown to be consistent and reliable. The equilibperature being controlled to rium data obtained by it are when the most reliable equigraphi- *0.5 " c. b y t h e m e r c u r y bulb, M, through a relay. The librium data were used* The cally with all other data found in the literature. t e m p e r a t u r e w a s varied b y method of calculation was that raising or lowering the contact proposed hy McCabe and Thiele (12) and gave the plate requirements for theoretically perfect wire in the capillary tube, C. The heater, H , in the still, S, was of KO,24 nichrome wire operation; hence, it was indicated that the equilibrium data were not accurate. It was decided, therefore, to determine in the work on the alcohols, but for acetic acid-water it was the eauilibrium curve for ethanol-n-ater by a reLable method, necessary to change to KO.30 platinum wire. This heater was a small coil wound in the form and later also those for methanolof a spiral as indicated in Figure 1, .AY water and acetic acid-mater. and the l e n g t h s of wire were as uT0 AC. EXPCRIMEKTAL PROCEDURE f o l l o w s : KO.24 n i c h r o m e , 36 31L LEV inches (91.4 cm.); No. 30 platinum, There are a number of different _ _ - - - 11-- - -38 inches (96.5 cm.). Contact was methods for the determination of made with the mercury in the leadequilibriuni curves of binary liquid in tubes by means of loops of No. m i x t u r e s . d careful review of 24 platinum wire sealed through the literature indicated that the the glass. methods of Rosanoff and his coI n the work with the alcohols, the workers are r e l i a b l e . They are corks which were in contact with r e c o m m e n d e d by Y o u n g (23). hot vapors were covered with lead Rosanoff, Lamb, and Breithut (19) foil, but t h i s was r e m o v e d f o r a n d R o s a n o f f arid Easley (18) acetic acid-water. have developed an a c c u r a t e but The oil bath was heated to a temi n v o l v e d m e t h o d . Rosanoff, p e r a t u r e 2" to 5" C. above the Bacon, and White (17)have worked initial boiling point of the liquid out a less involved method based mixture to be tested. The apparaon an entirely different principle. tus was dried before each run by Young (22) states that these two drawing air through it for 10 to 15 methods have been found to give m i n u t e s . Then the temperature M results in good a g r e e m e n t , a n d inside the still was allowed to rise for this reason the simpler method to 4" to 5" C. below that of the oil of Rosanoff, Bacon, and White was bath, a n d a b o u t 130 ml. of the selected for this work. binary mixture were run into the A diagram of the apparatus is ALL DIMENSIONS inner boiling vessel, S, through A . shown in Figure 1. The still, of IN CENTIMETERS GLASS WORM The composition of this liquid had Pyrex g l a s s , w a s i m m e r s e d t o been determined previously. The CONDENSER * The first paper in this s e r i e s was addition of the liquid lowered the p u b l i e h e d by L. H. Shirk and R. E. FIGURE 1. APPARATUS FOR DETERMINATION OF Montonna, IND. ENQ.CHEM.,19, 907-11 COMPOSITION OF VAPORS FROM BOILING BINARY temperature shown by the still ther(1927). mometer, and, as soon as this had SOLUTIONS
I
S THE course of an experi-
A
1
1332
I N D U S T R I A L A N D E I\; G I I\; E E R I N G C H E M I S T R Y
risen to a value 2" to 3" C. below the initial boiling point of the binary mixture, the electric heater, H , was turned on, the distillation commencing in about 5 minutes. Enough heat was supplied to distill over 60 to 70 grams in 30 t o 45 minutes. During the distillation the oil b i t h temperature w a s maintained 2" t o 4" C. above that in the s t i l l . T h i s difference was so small that there could hardly have been any superheating of the inner chamber, and yet, as will be shown later, there was evidently no condensation. For a c e t i c acid-water and for the runs at t h e higher concentrations of the alcohols, four fracFIGURE2. DATA FROM APPARATUS OF ROSANOFF, BACON,AND WHITE t i o n s Of l 3 to l 5 ml* were collected, but for ON ETHANOL-WATER the o t h e r alcohol runs five were taken. The fractions were collected in small vials (20 t o 25 ml.) connected to the bottom of the condenser by a cork with only a small opening to the air. Since the condensed distillate was quite cold (about 15" C.), no other precaution was needed t o prevent loss by evaporation. At the end of the run the waste liquor was siphoned from the still, and a sample was bottled and analyzed if desired. The fractions of distillate were weighed and their compositions determined. From these data were calculated the percentage compositions for different total weights of distillate, assuming that the fractions were combined successively. By plotting these values, it was possible t o extrapolate back t o zero weight of distillate and obtain the composition of the first infinitesimal fraction of vapor evolved from the liquid, which was the desired vapor composition (curve A , Figure 2 ) . By similar calculation the composition of the last infinitesimal fraction of vapor evolved from the residual Iiquid in the still could be found (curve B, Figure 2). For a more detailed description of the method of calculation, the reader is referred to the original article of Rosanoff, Bacon, and White (17).
FIGURE3. ETHANOL-WATER EQUILIBRIUM DIAGRAM
Vol. 25, No. 12
MATERIALS.Redistilled water was used in all runs. Absolute alcohol was prepared from c. P. 95 per cent undenatured ethanol by treatment with lime followed by careful distillation. The finished product was a fraction distilling within 0.1" C., and contained 99.8 to 99.9 per cent ethanol by weight (specific gravity, 0.7898 a t 20" C.). General Chemical Company c. P. methanol was fractionated in a 12-inch column sled with short glass tubes, the middle fraction (distilling over within 0.1" C.) being used in this work. This fraction gave negative tests for acetone, ethanol, aldehydes, and reducing substances when tested by the methods given by Murray (f3), and contained about 99.8 per cent methanol by weight (specific gravity, 0.7923 a t 20" C.), Grasselli's reagent grade acetic acid was distilled through a short column from chromic oxide to remove any formic acid, the first and last fractions being discarded. The middle fraction analyzed 99.5 per cent acetic acid according to the freezing point (data of Worden, 8 f ) and by titration with sodium hydroxide (freezing point, 15.64" C.). METHODS OF ANALYSIS.For both ethanol and methanol the compositions of the samples were determined at 20' C. by means of a 10-ml. pycnometer, using the specific gravity data of the U.S. Bureau of Standards as given in the Handbook of Chemistry and Physics ( 4 ) for ethanol, and those from International Critical Tables (IO) for methanol. The thermometer in the pycnometer stopper was checked against a Bureau of Standards thermometer. The compositions of the original and residual liquids were determined in duplicate, while those of the various fractions were obtained from one carefully made specific gravity determination, a check being run only when there was indication that this was necessary. In several runs with ethanol and with methanol, determinations were made with the AbbB refractometer, for reasons which will be given later. Immersion refractometer scale readings were taken from the Handbook of Chemistry and Physics (5),and these readings were converted into refractive indices by means of a table published by Zeiss ( 2 4 ) . The compositions of the acetic acid samples were determined by titration with 0.5 N sodium hydroxide solution with phenolphthalein as indicator, the solution being boiled to remove carbon dioxide. Duplicate determinations were made on the original liquid and the first two fractions, the others being checked only when it seemed necessary to do so. The samples for analysis were weighed out in small stoppered vials.
PRELIMINARY TESTSON THE APPARBTCS. Eight preliminary runs were made with ethanol-water to test the reliability of the apparatus. The data are given in Table I. With the exception of runs E and F, in which the conditions were the most abnormal, the apparatus gave consistent results over a wide range of operating conditions.
FIGURE4. METHANOL-WATER EQUILIBRIUM DIAGRAM
I N D U S T R I A L A N D E N G I N E E R I NG C H E M I ST R Y
December, 1933 TABLEI.
PRELIMINARY TESTSON APPARATUSO F B WON, AXD WHITE (17 )
ROSANOFF, TABLE 111. EXPERIMENTAL LIQUID-VAPOREQUILIBRIUM DATA FOR ETHbNOL-WATER, BASEDON RESIDUAL LIQCID1 3 STILL
(Ethanol-water) TEMP.OF VAPOR OF or FRICOIL VERUN RUN CHARQE"T I O N S BATH LOCITVb Mzn. M1. C. Cm./soc. 92-101 2.3 B 1120 130 100-101 2.8 130 E 90 92-101 4.6 130 60 C 100-101 7.0 100 30 G 92-101 7.5 130 35 A 100-101 8.0 130 35 D 94-101 8.7 130 35 H 100-101 9.5 21 100 F 5 Composition of liquid charged to still = 10.75 per vent weight; the initial boiling point was about 90' C. b Average velocity, for the entire run, u p the inside of the (Figure 1). LENQTH VOL.
EtOH
RUN
IN VAPOR
Wt. % 54.0 53.7 54.3 54.2 54.5 54.3 54.5 54.9 ethanol by 3-cm. tube
24 25 26 27 34 28 29 30 31 32 35 36 37 39
LIQUID-VAPOR EQUILIBRICX DATA TABLE TABLEII. EXPERIMENTAL IV. IFOR ETHANOL-WATER RCN
53 50 54 55 48 3 46 4 44 5 20 6 7 8 11 21 12 43 14 15 42 17 33 18 52 19 22 23 24 25 26 27 34 28 29 30 31 32 35 36 37 38 39 40 41
1333
(730 to 750 mm. of mercury) EtOH I N VAPOR EtOH I N LIQUID Wt. Mole Wt. Mole % fraction % fractzon 6.2 0.025 0.6 0.003 0.046 1.1 0.005 10.9 0.089 2.0 0,008 20.0 0.088 2.1 0,009 19.9 26.2 0.122 3.0 0.012 0.138 0.014 29.0 3.44 0.190 0,020 37.4 4.9 42.3 0.223 0.025 6.15 44.8 0.241 0.028 6.8 49.3 0.275 0.035 8.5 0.307 0,042 53.1 10.14 0.359 0,058 58.9 13.6 0.380 0.065 61.1 15.14 0.400 0.075 63.0 17.14 0.417 0.084 64.7 19.04 65.1 0.422 0.086 19.4 66.3 0.435 0.094 20.9 0.451 0.103 67.8 22.7 69.1 0,466 0.117 25.35 0.473 0.126 69.7 26.85 0.493 71.3 0.143 29.9 72.3 0,505 32.95 0.161 72.9 0.513 0.163 33.3 73.7 0.523 0.184 36.6 0.527 74.0 0.194 38.0 0.537 74.8 41.24 0.215 0.562 75.9 45.1 0.243 0.564 76.8 48.9 0.272 0.576 77.6 0.305 53.0 0,590 78.7 0.341 57.0 79.9 0.608 0.382 61.3 80.9 0.623 0.427 65.6 81.7 0.636 0.460 68.6 0.650 0.488 82.6 70.9 0.671 0.537 83.9 74.8 85.0 0.690 0.581 78.0 86.5 0.715 0.633 81.5 87.7 0.735 0.672 83.9 88.8 0.756 0.706 86.0 90.0 0.778 88.0 0.741 0.803 0.780 91.3 90.1 93.3 0.844 0.834 92.8 94.31 0.866 0.864 94.20 96.16 0.907 0.910 96.28 97.88 0,948 0.952 98.07
When the weight distilled-per cent composition data for any of the runs in Table I were plotted, the curve produced had an S-shape, as shown by curve C in Figure 2. For liquids of lower alcohol content the curves were similar to curve D in Figure 2 . As may be seen, it was difficult to determine just where the curve should cross the y axis with only five points as a guide. I n order to overcome this difficulty, for ethanolwater and methanol-water, the Abbe refractometer was used to determine the compositions of the distillates from liquids below 10 per cent by weight. Since the refractometer was not as accurate as the pycnometer, two runs were made on liquids of identical composition, one in the usual way using the pycnometer, and the other with fractions of 3 t o 5 grams for the first 20 to 25 grams distilled, using the refractometer. The refractometer points gave a reasonably accurate indication of the exact shape of the curve near the y axis, and the pycnometer curve was drawn with as nearly the same shape as possible, as shown by curves D and E in Figure 2. The y intercept of the pycnometer curve was taken as the vapor
RUN 22 20 18 16 15 24 14 13 25 12 11 26 10 2 30 9 27 8 7 3 6 28 5 4 29
(730 to 750 mm. of mercury) EtOH IN LIQUID EtOH Wt. Mole W1. % fractaon % 0.118 69.8 25.5 72.2 0.166 33.7 75.2 0,218 41.7 77.1 0,286 50.6 77.7 0.306 53.1 79.0 0.356 58.6 80.6 0.417 G4.7 82.3 0.477 (0.0 84.2 0.553 76.0 85.7 79.3 0.600 87.1 82.6 0.651 89.2 86.7 0.718 90.7 89.3 0.765 94.22 94.05 0.861
IN
VAPOR Mole fractzon 0.475 0.504 0.543 0.568 0.577 0.595 0.619 0.646 0.677 0.701 0.725 0.763 0,792 0.865
EXPERIMENTAL LIQUID-VAPOR EQUILIBRIUM DATA FOR METHANOL-WATER (730 t o 750 mm. of mercury) MeOH I N LIQEID MeOH Wt. Mole Wt. % fractaon % 9.5 1.3 0.007 18.5 3.0 0.017 27.8 5.06 0.029 35.0 7.0 0.041 0.060 44.0 10.2 0,090 53.5 15.0 0.091 53.1 15.1 0.124 61.5 20.2 0.140 63.8 22.4 25.1 0.159 66.8 70.5 30.2 0.196 0.233 73.8 35.1 76.3 39.6 0,269 0.354 81.0 49.3 49.55 0.356 81.0 0.357 81.1 49.7 84.6 59.5 0,452 84.7 60.0 0,457 88.4 69.9 0.566 91.6 78.9 0.677 92.5 81.3 0.710 0.825 95.7 89.4 0.841 96.4 90.4 0.913 98.1 94.9 95.1 0.916 98.1
I N \-.APOR
Mole fractzon 0.056 0.113 0.178 0.233 0.307 0.393 0.389 0.473 0.498 0.530 0.573 0.613 0.644 0.705 0.705 0.706 0.755 0.756 0.810 0.860 0.874 0.925 0.938 0.966 0.966
LIQUID-VAPOR EQUILIBRIUM DATA TABLEV. EXPERIMENTAL FOR ACETICACID-m'ATER
14 7 15 6
(730 t o 750 mm. of mercury) Hz0 HzO IN LIQUID wt. Mole Wt. % fraction % 16.0 9.9 0,268 16.9 10.4 0.278 23.1 15.0 0,370 29.1 19.6 0.449 24.2 0.515 34.8 0.592 42.1 30.3 0.636 46.8 34.3 0.686 52.4 39.6 56.6 44.1 0.724 61.6 49.5 0.765
20 18 3 19 5 21
74.9 75.4 80.0 84.7 89.9 90.3
RUN 11 10 12 9 13
8
0.909 0,911 0.930 0.949 0.967 0.969
81.2 81.4 85.0 88.5 93.6 92.8
I N VAPOR
Mole fraction 0.388 0.404 0.500 0.578 0.640 0.708 0.745 0.785 0.813 0.842
0.935 0.936 0,950 0.962 0.976 0.977
composition in equilibrium with liquid of the composition of the initial charge. EXPERTMEUTAL DATA. The experimental results given in Tables 11, IV, and V are based on the liquid originally charged to the still, while those in Table I11 are based on the residual liquid. The results in Table 111,with the exception of runs 24 and 25, are in excellent agreement with those in Table 11. The writers feel that this is a further indication of the reliability of the method. The system ethanol-water was investigated more thoroughly than the other two, for two reasons-the wide disagreement between the previously published results for this system,
1334
I hTD U S T R I A L A N D E N G I N E E R I D; G C H E %I I S T R Y
Vol. 2 5 , No. 12
The experimental results were carefully plotted, both in mole fraction and in weight per cent, and the coordinates of points read from these curves are given in Table T’I. TABLEVI.
COORDINATES OF CCRVE DRAWN THROUGH PERIMESTAL POISTS FOR ETHANOL-KATER
EX-
(730 t o 750 mm. of mercury) IN
IN
LIQUID VAPOR
IN
IN
LIQCID VAPOR
IN
IN
LIQUIDVAPOR
IN
IN
0.740 0.760 0.780 0.800 0.820 0.840 0.860 0.880 0.900 0.920 0.940 0.960 0.980
0.777 0.790 0.804 0.818 0.833 0.848 0.864 0.881 0,898 0.917 0.938 0.956 0.978
14.0
83.6 84.3 85.0 85.8 86.8 87.7 88.8 90.0 91.2 92.6 94.2 95.9 97.8
LIQUIDVAPOR
MOLE FRACTION OF ETHANOL
0.010 0.020 0.030 0.040 0.050 0.060 0.080 0.100 0.120 0.140 0.160 0.180 0.200
0.104 0,190 0.250 0.297 0.332 0.364 0.410 0.442 0.468 0.488 0.505 0.51Q 0.531
0.220 0.240 0.260 0.280 0.300 0.320 0.340 0.360 0.380 0.400 0.420 0.440 0.460
1.0 2.0 3.0 4.0 5.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0
10.3 19.2 26.3 32.5 37.7 41.9 48.1 52.7 56.5 59.5 61.9 63.9 65.6
22.0 24.0 26.0 28.0 30.0 32.0 34.0 36.0 38.0 40.0 42.0 44.0 46.0
0.541 0.551 0,560 0.568 0.576 0,584 0.591 0.599 0.607 0.614 0.621 0.629 0.637
0.480 0.500 0.520 0.540
0,560 0.580 0.600 0.620
0.640 0.660 0.680
0.700 0.720
0.646 0.654 0.663 0.672 0.681 0.690 0.699 0.709 0.719 0.730 0.741 0.753 0.765
WEIQHT PER CENT ETHANOL
FIGURE 5. ACETIC ACID-WATEREQUILIBRIUM DIAGRAM
and a desire to test the consistency of the apparatus. No special attempt was made to determine the composition of the constant-boiling mixture of ethanol and water.
DISCUSSION OF RESULTS ETEL~soL-WATER.Equilibrium curves for this system a t atmospheric pressure have been determined by Evans (S), Blacher, as given by Hausbrand (9), Lewis and Carey ( I I ) , Rayleigh ( I @ , Bergstrom as quoted by Hausbrand ( 6 ) , and Sorel (60). Blacher’s data were obtained by interpolation from a graph reported by Hausbrand, and are not therefore extremely accurate. The data of Sorel were taken from Elliott (6). A comparison of the various sets of equilibrium data is given in Figure 3. The curve is that drawn through the points determined experimentally in the present work, only the data from Table I1 being plotted. The data of Len+ and Carey did not appear in the literature until after the completion of the present work, and their results are in excellent agreement with the latter, being only a little low in the middle portion of the curve. It is obvious that the data of Evans and of Sorel are seriously in error, while those of Rayleigh are evidently low. Blacher and Bergstrom agree fairly well with the results of the present work.
67.2 68.4 69.4 70.4 71.3 72.0 72.8 73.4 74.0 74.6 75.1 75.6 76.1
48.0 50.0 52.0 54.0 56.0 58.0 60.0 62.0 64.0 66.0 68.0 70.0 72.0
76.6 77.1 77.5 78.0 78.4 78.9 79.4 79.9 80.5 81.1 81.7 82.2 82.9
(6.0
78.0 80.0
82.0 84.0 86.0 88.0 90.0 9’2.0 94.0 96.0 98.0
METHANOL-WATER. Only three equilibrium curves for this system a t atmospheric pressure could be found in the literature. These were determined by Bergstrom, as quoted by Hausbrand ( 7 ) , Bredig and Bayer ( I ) , and Blacher as given by Hausbrand (9). A graphic comparison of the various sets of data is given in Figure 4. Bergstrom’s curve is quite a little higher than that found in the present work. The data of Bredig and Bayer are high also, but their data are badly scattered. The data of Blacher were obtained in the same way as for ethanol-water, and, u hile the reading of the points from the graph may not have been extremely accurate, every one of them checks the present curve almost exactly. The coordinates of points read from smooth plots of the experimental data are given in Table VII. TABLEVII.
CURVEDRANNTHROUGH ExPOINTS FOR METHANOL-WATER
COORDINATES OF
PERIYESTAL
(730 t o 750 mm. of mercury) IN
IN
LIQUID VAPOR
IN
IN
LIQUID VAPOR
IN
IN
IN
IN
LIQUID VAPOR LIQUID VAPOR
MOLE FRACTION O F METHANOL
0.140 0.160 0.180 0.200
0.420 0.739 0.440 0.749 0.460 0.759
0.480 0.500 0.520 0.540 0.560 0.580 0.600 0.620 0.640 0.660 0.680 0.700 0.720
0.768 0.778 0.787 0.797 0.806 0.815 0.825 0.834 0.843 0.852 0.862 0.871 0.880
WEIQHT PER CENT METHANOL
1.0 2.0 3.0 4.0 5.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0
WEIGHT PERCENT OF MORE VOLATILE C O U P O M h T N LIQUID
FIGURE 6 . EQUILIBRIUM DIAGRAMS
22.0 24.0 26.0 28.0 30.0 32.0 34.0 36.0 38.0 40.0 42.0 44.0 46.0
48.0 50.0
52.0
54.0 56.0 58.0 60.0 62.0 64.0 66.0 68.0 70.0 72.0
87.6 88.3 89.1
74.0 76.0 78.0 80.0 82.0 84.0 86.0 88.0 90.0 92.0 94.0 96.0 98.0
ACETICACID-WATER. Equilibrium curves for this system a t atmospheric pressure have been determined by Rayleigh (If?), Blacher as quoted by Hausbrand (8), Bergstrom as
Effect of Addition Agents upon the Corrosion Rate of Aluminum by Alkalies Effect of Various Substances F. H. RHODESAND F. W. BERKER,Cornel1 University, Ithaca, N. Y. N THE pickling of steel with acid it is well known that the action of the acid upon the metal may be minimized b y adding t o the acid certain inhibitors that retard the corrosion of the metal without at the same time seriously interfering with the solution of the scale. I n the present investigation the effects of various inorganic salts and organic compounds upon the rate of corrosion of aluminum by alkalies has been determined, with a view to the possibility of finding some substance that would minimize or prevent the corrosion of the metal by alkaline solutions. Some work on the effects of addition agents on the rate of corrosion of aluminum by alkalies has already been published, Rohrig (5)found that the addition of sodium silicate to a solution of sodium carbonate decreases the rate of action of the solution on aluminum, although the silicate increases the rate of attack of the metal by sodium hydroxide in moderately
Into the reaction tube, M , was placed sufficient of the soluton to be tested to cover the test strip of aluminum. Gaseous hydrogen was passed through the solution for about 3 minutes in order to saturate it with hydrogen and thus eliminate any error due t o the solubility of the gas in the solution. The reaction tube wvas then immersed in a bath of water maintained at a constant temperature of 30" C. and allowed t o stand until the solution had attained the temperature of the bath. A cleaned and weighed strip of aluminum, 5 cm. long, 1 cm. wide, and 0.0635 cm. thick was introduced, and the reaction vessel was connected immediately with a Hempel buret in which the volume of evolved hydrogen was measured. Readings were taken every minute for 20 minutes. At the end of this period, the aluminum was removed, rinsed, and again weighed. The loss in weight of the metal served as a check upon the amount of hydrogen evolved as a measure of the total extent of the corrosion. The aluminum used was commercial sheet aluminum, 99.3 per cent pure.
EFFECTS OF ADDITIONAGENTSON CORROSION ALKALIESALONE. I n the first series of experiments the rate of corrosion of aluminum in sodium hydroxide solutions of various concentrations was measured. The p H of each solution was determined, using a standard hydrogen electrode balanced against a saturated calomel electrode. The results are shown in Figure 2, in which the values for p H and for rates of corrosion are plotted against the concentrations, and in Figure 3, in which is shown the variation in the rate of corrosion with the change in pH. The rates of action of molar and 0.5 molar solutions of potassium hydroxide were also measured :
m
ALKALI Sodium hydroxide Potassium hydroxide Sodium hydroxide Potassium hydroxide
F
FIGURE 1. APPARATUS FOR DETERMINATION OF CORROSION RATE E, F. M,
Control stopcocks Reaction bulb
S. Capillary spiral 2'. Inlet tube for hydrogen X. Hydrogen inlet control
Y . Inlet for alkaline solution
CONCENTRATION RATIO Molar Cc. Hn/min. 1 2.50 1 2.13 0.5 1.60 0.5 1.42
Although potassium hydroxide is more nearly completely dissociated than is sodium hydroxide in solutions of these concentrat,ions, there is a distinct and consistent difference in the rate a t which they attack aluminum. SALTS. I n the next series of experiments the effects of salts were investigated. I n each experiment the salt, in the concentration indicated, was dissolved in a 0.5 molar solution of sodium hydroxide. SALT
None NaCl NaCl NaCl
CONCN. RATE Molar Cc. Hl/min. 1.6 0.5 1.96 1.0 1.93 1.5 1.89
...
SALT KCl NazSOo KnSO, NrtsPO4
COXCN. RATE Molar Cc. Hdmin 0.5 1.66 0.5 1.98 0.5 1.83 0.5 3.98
concentrated solutions. Seligman and Williams (6) also investigated the effects of sodium silicate as a n addition The addition of sodium chloride increases the activity of agent in the cleaning of aluminum by alkalies. The use of silicate as a retarder in this process was patented by Lea ( 3 ) . the hydroxyl ions from the sodium hydroxide and markedly Rohrig (5) found that certain colloidal substances, such as accelerates the corrosion. The acceleration is most marked glue, agar agar, and gelatin, exhibited inhibiting action. in solutions of 0.5 molar concentration and decreases as the Other substances for which similar action has been claimed concentration rises above this value. -4 similar effect is include ammonium compounds, dichromates, sodium molyb- observed with solutions of sodium chloride in molar alkali. date, permanganates, and salts of the noble met.als (1, 2, 4). Sodium sulfate shows a n effect similar to that of sodium chloIn the present work the rate of action of the alkaline solu- ride but even more pronounced. The addition of sodium phostions on the aluminum was determined by measuring the phate increases the rate of corrosion very greatly, the increase rate of evolution of the hydrogen formed by the reaction. being due in part at least t o the hydrolysis of the salt and the The construction of the apparatus used is shown in Figure 1. resulting increase in concentration of hydroxyl ions. 1336
