T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
778
with 0.1 N sodium iodide solution. The temperature was kept constant t o within 5' by means of a low flame under the beaker. Table 111 gives the amounts of sodium iodide added, the corresponding readings of the millivoltmeter, and also a comparison of the amounts of As206 found in 500 cc. of the solution. The data are shown graphically in Fig. 3, where the ordinates represent millivolts, and the abscissas cc. of 0.1 N sodium iodide. cc. Millivolts 0.1 N NaI Millivolts 380 16.6 485 16.82 450 17.0 415 410 17.2 15.5 16.0 17.4 405 18.0 16.2 395 16.4 19.0 390 Found: Electrometric method 2.345 g. AszOa per 500 cc. 2.351 g. Asnos per 500 cc. Official method 2.323 g. As206 per 500 cc. Theoretical 1 Factor 0.9669. End-point.
12, No.
8
remain constant, or a t most rise only a few millivolts even when allowed t o stand several hours. After passing the end-point, however, they may rise considerably. Table V shows the results of a titration Cc. NaI
0.0 1 .o
2.0 3.0 3 ~. 4.
.
TABLEV (.l) Millivolts 3601 505 480 465 455
1
(?)
Millivolts 510 500 480 460 450 ~. .
440
3.6 3.8 4.0 4.2 4.4
TABLBI11 cc. 0.1 N NaI1 5.0 10.0 15.0
Vol.
400
37043604360+ 5.0 355 Solution stood in contact with electrode over night.
made by allowing the indicated quantities of solution t o flow in slowly, taking one reading a t once (Column I ) and another after the voltage had become constant (Column z ) , and Fig. 4 gives the data in graphical
*
The graphic method of determining the end-point for this titration is, however, as unnecessary as in the case of trivalent arsenic. The voltage read, using a solution wherein the arsenic is just reduced, approximated very closely t o 350 millivolts, under the conditions of our work. Hence by setting the sliding contact so t h a t the millivoltmeter gave this reading and titrating till the galvanometer gave no movement or just reversed its direction, the end-point was obtained with accuracy and convenience. As in the preceding instance, it was found well t o get this end-point approximately with about 0.5 cc. less sample than the final volume desired. This amount may then be added and used for the accurate estimation of the end-point by the addition of small increments of the titrating medium. I n Table IV are given some results obtained in this way and the corresponding figures obtained by the official method in which the pentavalent arsenic was determined by reduction t o the trivalent form by means of hydriodic acid and titrated with iodine. TABLBIV -Per cent AszOsElectrometric Official
MATERIAL Lead arsenate: Sample No. 1 . . Sample No. 372.. SamDle No. 382.. Calcium arsenate.. London purple A , . ........ London purple B..........
......... ....... ....... ........
32.85 32.86 26.54 26.69 31.87 32.22 56.79 56.49 27.56 27.77 22.83 22.69 G . AszOs per 100 Cc. 0.0929 Sodium arsenate.. ........ 0.0931 99.86 Arsenic acid'.. 100.14 1 Prepared from pure AszOa and results calculated to per cent purity of this material.
............
I n making the titration for AsV, care must be taken not t o add the iodide solution too rapidly as the voltmeter reading changes somewhat after the addition of the solution. If, for instance, the titration is made by allowing large increments t o flow in as rapidly as the buret will deliver the solution, and the reading taken a t once, it will be found t h a t this reading is too low. If such a solution is allowed t o stand i t will be observed t h a t the voltmeter reading rises rather rapidly at first and then slowly for some time. If, on the other hand, the titrating solution is allowed t o run in quite slowly the voltages before reaching the end-point
FIG. 4
form. This change is sufficiently slow, however, t o permit the operation t o be carried out as outlined in a preceding paragraph with sufficient accuracy for all ordinary purposes. SUMMARY
An electrometric method of titration had been shown to be applicable: ( I ) To the determination of trivalent arsenic even in colored solutions. ( 2 ) To the direct titration of pentavalent arsenic, with the consequent elimination of such intermediate steps as the removal of organic matter when such is present and the reduction of Asv t o As"'. RAPID EXAMINATION OF SHELLAC, SHELLAC VARNISH, AND LACQUER By T. Linsey Crossley 43 SCOTTST.,TORONTO, CANADA Received March 15, 1920
The writer was called upon in the spring and summer of 1918 t o examine varnish, specified t o have the composition of 7 5 parts of shellac and 2 5 parts of rosin, for
Aug., 1920
T H E J O C R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
use in shells, t o be cut with denatured alcohol, only. The specification stated t h a t the dried gum should have a n iodine number not higher than 71, according t o Dr. Langmuir’s modification of Wijs’ method.’ For lack of reagents, and pressure of time in most cases, i t would have been impracticable t o make t h e above test and, therefore, some other means for approxi-mate confirmation of adherence t o specifications had t o be devised. The question of solubility of shellac in various solvents was looked into. Sulfuric ether seemed the most promising, though we found no reference t o i t in the literature a t hand. The method referred t o by Dr. Langmuir2 calling for the use of petroleum ether has the disadvantage t h a t pure shellac is soluble only t o a slight extent in petroleum ether, and the soluble portion is not resinous but waxy. P. C. McIlhiney3 advocates a combined alcohol and petroleum ether determination. This method did not suit our convenience a t t h e time (1918, JulyAugust) because we could then obtain petroleum ether only by redistillation from gasoline in small amounts, and absolute alcohol with difficulty. We believe the sulfuric ether method, while perhaps not so exact, gave us the necessary information in less time. Among samples of rosin, grades about G t o H, which we examined for shrapnel use and which we dissolved in petroleum ether, we found one t h a t contained 2 0 per cent of matter insoluble in t h a t medium, which was readily soluble in sulfuric ether. If such a rosin were used in varnish, t h e insoluble matter would be considered shellac by the petroleum ether method. Upon treating a shellac rosin varnish with sulfuric ether, a flocculent precipitate comes down in a very short time, leaving a n almost clear supernatant liquid which is decanted away with ease. The precipitate may be washed by repeated additions of ether and decantation. I t appeared t h a t if the solubility of pure shellac in sulfuric ether were constant or nearly so, this property could be applied for a n approximate determination of the proportions of rosin and shellac in such varnishes. Samples of shellac of different grades, which they had every reason t o believe were genuine shellac gum, were obtained b y t h e courtesy of three varnish manufacturers. One of these was said t o be of the highest quality obtainable. These gums were treated with sulfuric ether, and t h e following results were obtained, the ether solution being also evaporated and weighed: 1 2 3
Soluble in Ether Per cent 30.0 28.7 31.8 30.1
....................... ....................... ....................... AVERAGE ..........
Insoluble in Ether Per cent 70.0 70.2 70.1 70.1
No. 3, it will be noted, totals over IOO per cent. A repetition of this determination gave 31.9 per cent soluble in ether. The presumption is t h a t oxidation takes place. T h e soluble matter was found after drying t o be not entirely soluble in ether. The writer is indebted t o Messrs. Rogers and Pyatt, 1
9
8
J . A m . Chem. Soc., 29 (1907), 1221. J . Soc. Chem. I n d . , 24 (1905), 12. J . A m . Chem. Soc., 30 (1908), 867.
7 79
of New York, for a sample of raw stick-lac which was examined after grinding t o pass go-mesh sieve, with t h e following results: Per cent
..................
W o o d y matter, fiber, etc.. Gums not soluble in ether.. Gums soluble in ether..
8.5 ................ 70.7 25.0 .................... -
104.2
A second sampling of the gum gave a total insoluble in ether of 77.2 per cent, including the woody matter, as compared with 79.2 per cent above. I n view of these results it was felt justifiable t o assume 70,per cent as a constant for gums insoluble in sulfuric ether for technical purposes. Throughout t h e work commercial sulfuric ether was used, as the object was t o evolve a quick test t o be made in the ordinary technical laboratory. Four samples of sheliac rosin varnish were prepared by manufacturers t o meet the specifications and submitted for certification of compliance. The results obtained, presuming pure shellac t o be 7 0 per cent insoluble in ether, were: 1
2 3 4
......................... ......................... ......................... .........................
Rosin Per cent 27 60 30 23
Shellac Per cent 66 40 70
77
The maker of No. z stated t h a t the right proportions had been used and t h a t the shellac gum previously examined was one of the constituents. This appeared t o destroy our hopes. I t was tried again with similar results. Careful inquiry in the factory elicited t h e fact t h a t by mistake only half the required amount of shellac had been used, confirming our results. I n t h e case of No. I some of the insoluble matter was lost by accident, but the weighing of the soluble matter indicated a reasonable compliance with specifications. I n view of factory conditions these results confirmed our view t h a t a technical method for the analysis of shellac rosin varnish could be based on the solubility of lac gum in sulfuric ether, and t h a t examination in this way would detect any marked or deliberate deviation from the specifications. METHOD
I-Weigh in a small beaker 5 t o 7 g. of varnish (have beaker weighed), place 5 g. more on other pan, pour varnish quickly until pan falls, weigh without adjusting. a-Allow varnish t o stand in warm place or, if pressed for time, place on water bath until varnish is pasty but not dry. 3-Add t o beaker 50 t o 6 0 cc. sulfuric ether (a large excess is needed as there is some alcohol in t h e ether and this, with what is present in the varnish, holds some of the shellac in a colloidal condition). Stir t h e mixture well; t h e insoluble matter usually clots readily and can be decanted with ease. 4-Weigh another beaker for t h e soluble portion which is now decanted into i t through a fairly open filter paper, wash the clotted insoluble matter in t h e beaker with successive lots of 20 cc. ether until t h e washings are colorless or only faintly; colored. Before pouring in the first washing i t is advisable t o wet t h e filter with a little ether from a wash bottle. T h e
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T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
Vol.
12,
No. 8
APPARATUS upper edges of the paper will need washing finally, as well as the funnel above the paper. Three beakers, 125 cc. capacity; one 2 . 5 in. funnel, filter 5-Allow the washed insoluble matter t o dry spon- stand, stirring rods, wedgewood mortar and pestle, 30taneously in the beaker. It will be well divided and mesh sieve, balance sensitive t o I mg., commercial granular and almost entirely rid of ether, I t can sulfuric ether, and denatured alcohol. These can then be finished over the water bath and weighed, be found in any paint laboratory. The small amount of insoluble matter on the filter DISCUSSION may usually be ignored. 6-The soluble portion is evaporated rapidly in, The results are positive, the actual substances being not on, a hot water bath without flame. This keeps weighed, soluble and insoluble. Their properties and the upper portion of beaker hot and prevents creeping. appearance are sufficient demonstration of their differIf the soluble portion is quite clear after passage ence in character. The results obtained by iodine abthrough t h e filter and does not cloud up on standing, sorption are indirect and deductive, the iodine absorpit can be finished on the water bath, and not further tion of rosin being given by different writers as from treated. If not, proceed as in Paragraph 7. 176 per cent t o 235 per cent and t h a t of shellac from 9 7-The soluble portion, after drying, is washed into per cent t o 31 per cent. We believe, therefore, t h a t another beaker. Usually i t is not necessary t o pass the iodine absorption method has no advantage over through a filter as the insoluble matter is precipitated t h a t of separation outlined above, and if oil were t o as a thin film on the lower sides of the beaker while be added t o the varnish its absorption would introduce evaporating. This film adheres t o the glass and the a third unknown quantity. rosin can be washed off. If the filtrate is very muddy, I n our work we have obtained fairly good results it would be well t o take it down a t once on the water where linseed oil has been used, but in such cases care bath t o a pasty condition and treat i t with about 50 must be taken t o avoid oxidation of the oil by heating cc. ether, filtering into a third beaker. the varnish when the alcohol is being eliminated. I n 8-The second beaker containing the additional these cases very little evaporation t o clot the insoluble insoluble matter is then dried and weighed. matter should be attempted. 9-The third beaker containing the soluble portion Varnishes containing very much rosin and very is dried and weighed. little shellac give difficulty owing t o the finely divided Io-Evaporate 5 t o I O g. of the original varnish in ether precipitate, but on standing over night t h e a weighed dish with rod t o determine the proportion soluble matter may generally be decanted. If not, of gums and vehicle. i t was always found practicable t o filter and weigh t h e I n testing a sample of shellac gum i t is ground t o a t soluble portion. least 30 mesh before treatment with ether, and will We did not find the use of tared filters advisable as not need as much ether in the first place as the pre- the insoluble matter a t times sticks t o the sides of the pared varnish. Weigh one gram of t h e ground gum beaker. and moisten i t in a small beaker with 5 cc. of alcohol Both the insoluble matter and the rosin should be (wood spirit will do); this softens i t and permits of the subjected t o temperatures up t o 110' C. or 1 2 0 ' C. ether coming into contact with all of the gum. t o eliminate the last traces of alcohol and water, t h e These results were found on one sample of varnish: latter introduced t o some extent by condensation from Varnish taken, 5 2 g. the air. Compositiou of varnish: Gum, 35 per cent We have also found the method useful for deterSpirit, 65 per cent Gum taken = 35 per cent of 5.2 g. = 1.82 g. mining the percentage of shellac in shellac-turmeric Gum should contain: Rosin, 25 per cent = 0.455 g. lacquers. We found t h a t of the 2 0 per cegt or so Shellac, 75 per cent = 1.365 g. of the turmeric t h a t was suspended in the spirit prac1.820 g. tically all was soluble in sulfuric ether. 0.8825 Found insoluble in ether, 0.8825 g. = Incidental t o the above work, the solubility of 70 loo> = 1.265 g. shellac shellac in petroleum ether and commercial wood alcohol (The insoluble matter was inclined to be slimy and there was a small was determined. The results may be of interest: loss on the filter paper.) = 0.455 g. Rosin required 25 per cent of gum The extraction with petroleum ether was made by Ether soluble of shellac = 30 per cent of 1.365 = 0 410 g. soaking t h e gum, ground t o 30 mesh, a t room temperature for 2 4 hrs. a t a time for 5 days. The petroleum 0.865 g. Required soluble in ether 0.860 $. Found soluble in ether ether had a boiling point of 85' C.
(
Shellac found = 1.265 g. = 69.5 per cent of 1.82 g. Rosin found = 0.860 (30 per cent of 1.265) = 0.480 g. = 26.4 per cent of 1.82 p.
-
We reported: Shellac 7 0 per cent, rosin 2 7 per cent, considering this a reasonable adherence t o specification. A small amount of oil had been added t o this varnish, and this we believe caused the trouble in separation. Where only shellac and rosin, in proportion not over 30 per cent rosin, are cut in denatured alcohol, we have been able to get results close enough for the purpose in 2 or 3 hrs.
SOLUBILITY IN PETROLSUM ETHER
Commercial Shellac Per cent 0.40 0.45 0.25 nil
Stick-lac Per cent 3.01 0.61 0.31 nil
1.10
3.93
...............
1st and 2nd extractions (48 hrs.) 3rd extraction (24 hrs.) ....................... 4th extraction (24 hrs.). 5th extraction (24 hrs.).
....................... ......................
-
-
T w o other samples of commercial shellac gave solubilities of 1.83 per cent'and 1.4 per cent in petroleum ether., I n view of these results care should be
Aug.,
1920
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
taken t o ensure complete extraction before reporting solubilities. SOLUBILITY IN WOODALCOHOL
.... ., .. .. ...... .. .. .. .. .. .. .. .. ..,. .... ..
Commercial shellac.. ,,. Stick-lac..
..... . .
91.66 per cent 8 1 . 4 per cent
The stick-lac was t h a t referred t o , containing 8.5 per cent of woody matter. I wish t o acknowledge indebtedness t o my colleague, Mr. J. D. Stuart, who did much of t h e work incidental t o the above investigation.
78 r
tures as low as -72' C. were obtained. Where possible, a standardized mercury thermometer was used, but a t extremely low temperatures a toluene-filled thermometer which had previously been compared with the mercury thermometer, was used. Extreme
THE FREEZING POINTS OF MIXTURES OF SULFURIC AND NITRIC ACIDS1 By Walter C. Holmes EASTERN LABORATORY, E. I. DU
P O N T D3
NEMOURS & CO.,CHESTER, PA.
Mixtures of sulfuric and nitric acids, commonly designated as mixed acids, are used in enormous amounts in t h e chemical industry in nitration processes, as carried out in the explosive and dye industries. Owing t o a case where a mixed acid had become frozen during storage, under conditions where freezing seemed improbable, % HMO, %H,O determinations of t h e freezing points of a large number of mixed acids were carried out in this laboratory. supercooling was met with, one case occurring where The original object of t h e work was simply t o obtain t h e acid did not freeze at a temperature over 6 0 " C. empirical data regarding t h e temperatures of freezing below t h e true freezing point, as subsequently deterfor different mixtures. The results, however, were of .mined. As a rule, however, solidification could be exceptional interest, as indicating t h e formation of a induced by inoculation of the sample with a small definite chemical compound between sulfuric and nitric crystal of frozen acid, care being taken t h a t this.crysta1 acids. While thethorough explanationof thephenomena should be from a n acid of similar composition. Preobserved was beyond the scope of the work, i t is cautions were also taken that, in t h e final determinabelieved t h a t t h e presentation of these results will be tions on an acid, the surrounding bath should not be of value as an addition t o t h e knowledge of mixed more than a few degrees lower than t h e freezing point acids. of the acid. The only chemical compound of sulfuric and nitric acids which could be found in the literature is t h a t mentioned by Weber12 who gives i t the composition 4S0~.N~Os.3H20, though this composition does not seem proved. The most thorough study of the properties of mixed acids appears t o have been carried out by Saposchnikow,s who investigated particularly the vapor pressures, specific gravities, and conductivities of such mixtures in all proportions. EXPE R I M E " IAI.
Three sets of mixed acids were made up of 100,gj, and 103 per cent total acidities, respectively, SO t h a t in each set of acids the only variables were t h e sulfuric acid and t h e nitric acid contents. The nitric content of t h e mixtures was varied from zero t o approximately 50 per cent in the I O O and g j per cent mixtures, and up t o about 30 per cent in t h e 103 per cent mixtures. I n all cases the compositions of the mixtures were determined by careful analysis. The freezing was carried out in test tubes of about one inch diameter, the thermometer being used as stirrer. The test tubes were immersed in an insulated bath. The freezing mixture consisted of ether and carbon dioxide snow, by means of which tempera1 Read at the 59th Meeting of the American Chemical Society, St. Louis, Mo., April 12 to 16, 1920. 2 Ann. Phys. Chem. (Pogg.), 142 (1871), 602. 8 2 physik. Chem., 49, 697; 51, 609; 63, 225 (1904-1905).
The exact procedure was as follows: A sample was frozen by cooling below t h e true freezing point and by subsequent inoculation or vigorous stirring, as the case demanded. The temperature of freezing was noted, this temperature being taken a t t h e highest point reached, as in all cases a rise in temperature took place a t the time of solidification. The bath was then, regulated t o not more t h a n 4' or 5 O below t h e temperature recorded, and a second sample was frozen. This. procedure was repeated until t h e highest point a t which an acid would freeze had been found and checke& several times.
