ANALYTICAL CHEMISTRY
326 of 0 to 200 p.p.m., 10 ml. of 2% aqueous solution of amine should be used. This provides a plentiful excess of amine if the p H of the copper solution is not less than 3.5 to 4.0. Regulation of the p H t o this range can be satisfactorily accomplished by dropwise addition of dilute sodium hydroxide solution and testing with an indicator paper. If too much sodium hydroxide is added and cupric hydroxide precipitate;, it is all right to add the amine but the mixture must be allowed to stand several’minutes to permit complete solution of the cupric hydroxide. Observations over a period of 48 hours reveal that the color is perfectly stable for a t least this time. The ions R hich interfere with this amine are the ones which interfere with ammonia ( 3 ) .
ACKNOWLEDGME3TS
Two students, David A. Warriner and Isidore Cohn, provided valuable assistance in making some of the preliminary observations. The Carbon and Carbide Chemicals Corporation generously provided research samples of three amines. LITERATURE CITED
(1) Hoffpsuir and O’Connor, Am. DgestzcflRReptr., 31, 395 (1942). (2) Jonassen, Crumpler, and O’Brien. J : Am Chem. SOC.,67, 1709 (1945). (3) Mehlig, I N D . ENG.CHEDI., A N 4 L . ED.,13, %3 (1941). (4) Y o e and Barton, Ibid., 12, 456 (1940). (5) Poe and Crurnpler, Ihzd.. 7, 281 (1936).
Determination of Water in Caustic Soda and Other Alkaline Materials A Distillation- Tit rirnet ric Met hod H . R. SCTER, Wyandotte Chemicals Corporution, Wyandotte, *With. A method has been developed by which 20 to 200 mg. of water may he determined in caustic soda or other alkaline inorganic material with an accuracy of * 5 % . It consists of a preliminary separation of the water by distillation with xylene and titration with Fischer reagent. It is generally applicable to inorganic materials, where numerous interferences with the dirert use of Fischer reagent exist.
R
ELI.1ULE niethods for the determination of water in caustic.
soda are lacking in the literature. The primary interest of producers and consumers is in the content of sodium hydroxide, salts, and metallic contaniinants, all of which are readily determinable by well-established chemical and spectrographic methods. Water is usually estimated by difference, if at all. Investigations carried out in this laboratory of reactions in molten caustic soda have necessitated direct determinations of water as a means of studying reaction mechanisms. The method here described permits direct determination of viater in solid caiistic soda or other alkaline inorganic materials. Conventional oven-drying methods are, of course, inapplicable because of carbonation of the caustic by the atmosphere. Attempts a t gravimetric determinations by fusing the samples, cont,ained in silver crucibles, in currents of dry, carbon dioxidefree air have yielded erratic results. Weighing difficulties due t o the extreme hygroscopic character of caustic soda, and the tendency of the fused caustic t o climb over the walls of the crucible, in addition t o the very slow rate of expulsion of water, are the principal objections to this method. These considerations indicate the use of distillation methods, which have the obvious advantage of providing a continuously renewed anhydrous atmosphere in int’imate contact with the particles of the solid being dried, so that there is practically no partial pressure of water vapor over the sample. This condition is probably never realized in a n air stream, since the bulk of the air passes around the container, not actually through the sample. Thus the vapor pressure equilibrium is displaced very slowly, being dependent on diffusion of the vapor out of the container. I n distillation methods, ordinarily the solvent used is one which is immiscible with water, so that the condensat,e separates into two phases, the water being retained in a graduated trap where its volume is measured. Separation in the trap is dependent on the difference in density between water and the entraining liquid, and
the excess of the latter passes through the trap and is recycled t o the still pot. Liquids commonly used are toluene, xylene, perchloroethylene, tetrachloroethane, and o-dichlorobenzene. Xone of the common types of stills and traps are well adapted to the collection and measurement of small amounts of water. I n the Bidwell-Sterling (4)type and modifications thereof, there is a tendency for the water vapor to condense a t a higher zone in the condenser than the entraining liquid, because of the lower boiling point of the former. Some of the water inevitably hangs on the walls of the condenser, and must be scrubbed doim a t the end of the distillation. Sharp separation of the phases is not readily obtained in the measuring tube, since the liquid there is static in most of these stills. I n the Thielepape and Lundin stills (12, 17) the vapor is introduced a t the top of the condenser, so that the separated droplets of water are presumably continuously flushed downxnrd to the trap. Actually, experiments made in this laboratory show that as much as 0.3 ml. of xater may be held up in the condenser. Replacement of the spiral tube condenser with a bulb type resulted in no great improvement. I n these stills, the entire stream of condensed entraining liquid passes through the column of water in the measuring tube, which results in clean separation of the phases but necessitates a rather large bore to prevent the water from being pushed back into the distilling flask. There is no apparent way t o eliminate these errors in collection and measurement of the separated water. Lindsay (11) has designed a trap for small amounts, but he reports only a single recovery experiment, in which the error was 470. Usually the collection and measurement errors are minimized by collecting a fairly large volume-i.e., several milliliters. For materials containing low percentages, this requires inordinately large samples. In the method here presented, the entire distillate is collected in a flask which also serves as the condenser, and the water is determined by means of Fischer reagent. This permits the deter-
V O L U M E 19, NO. 5, M A Y 1 9 4 7 mination of 20 to 200 mg. of water, a range ivhich requires caustic soda samples of the order of 20 grams in size. Fischer reagent has been widely applied in the determination of waterinorganicsubstances(l,2,3,5,7,8,9,10,14,19). Although the reagent is far from being specific for water, as is often stated, interfering substances are relatively few in the organic field. One of the principal limitations is the lack of solubility of the material in the reagent, which is commonly circumvented by using a common solvent. or extracting the material n-ith methanol or pyridine (3,11,16). In general, the interfering substances are: 1. Oxidants or reduct'ants which affect the iodine-iodide couple. 2. Rletallic hydroxides or oxides which react with hydriodic acid releasing water (15). 3. Substances which esterify with methanol, releasing water, under the conditions prevailing in the reagent. These include the strong mineral acids, boric acid, sulfonyl chlorides, etc. Thus the direct use of the reagent is very sharply limited in the inorganic field. I t has been shon-n by Mitchell et nl. ( I S ) that many of the interfering reactions are stoichiometric and are therefore suhject to correction. This scheme is not practical for the determination of small quantities of water, since the n-ater \vould be represented by a small difference hetween tivo analytical values of relatively large magnitude. The same investigators report t,hat ivater in some 25 inorganic hydrates may be determined by suspending the finely ground solid in methanol or glycol, which apparently extracts the water from the hydrates, even those of high thermal stability ( 6 ) . This procedure is not applicable to alkali hydroxides because their solubility lvould result in a determination of the total hydroxide plus water. Highly alkaline salts such as tribasic phosphates and orthosilicates could be expected to hydrolyze appreciably Ivith the traces of water present in the methanol. By the combination of the distillation and Fischer methods, as described below, the above-mentioned liniitations of each are practically eliminated. Although the present study has been confined largely to caustic soda, it is believed that the method is applicable to any inorganic material Tvhich does nut release a volatile oxidant or reductant on heating-e.g., hypochlorites and sulfides. I t is best suited to the determination of small percentages of water because of accuracl- limitations as described below. It has been successfully used in this laboratory over a period of three years. DEVELOPXIENT OF METHOD
The ideal entraining liquid for use in the distillation n-ould fulfill the following requirements : 1. The boiling point must be high enough t,o ensure rapid vaporization of water from the sample. hleasurement,s of the vapor pressure of water over solid caustic soda made by C. AI. Allen of this laboratory indicate that about 150" C. is suitable. 2 . The solvent must not react with the sample in such a way as either t o take up or release water. 3. The sample (caustic soda) should dissolve completely in the liquid to facilitate expulsion of the water. 4. The solvent should be conipletely miscible nit11 Fischer reagent to facilitate the titration. 5. Dehydration of t'he solvent prior to use should not be unduly difficult.
To the author's knowledge no solvent exists which fulfills requirement 3 Tvithout violating requirement 2. It has been necessary to compromise here by grinding the sample and alloiving mfficient distilling time for diffusion of water to the surface of the particles. Chlorinated hydrocarbons are ruled out by requirement 2. Xylene has been chosen as being the most suitable. I t boils between 138" and 144" C., is inert to alkalies and water, and is readily dehydrated. It is not miscible with Fischer reagent but may be made so by addition of methanol prior to titration. Fundamentally the distilling apparatus need only con'sist of a hot zone where the sample is heated in the entraining liquid under
327 partial or total reflux, and the vapor pressure of water is high; and a cold zone n-here the vapor pressure is low. If such astill is operated long enough, all the water must eventually collect in the cold zone. There should be no condenser between the still pot and receiver. The receiver should serve as the titration vessel, so that no transfer is necessary. The amount of glass surface of the still should be a t a minimum because of the appreciable amounts of vater which is adsorbed by glass. The design shon-n in Figure 1 has been the only one tried. Preliminary experiments showed that, operation was satisfactory and that dehydration of the caustic soda was complete, within experimental error, after 4 hours' distillation. Originally the heat was applied a t such a rat,e that distillate dropped continuously into the receiver and the delivery tube kept hot all the way to the receiver. This required unduly large amounts of xylene. In later runs, the distillation was started rapidly, so that the surface Ivater from the solid was swept into the receiver Lvith the first few milliliters of distillate. The heater vias then regulated so that the xylene refluxed a t a zone in the vertical leg of the delivery tuhe, the water condensing just past this zone as it )vas released from the sample. At the end of the distillation period the condensed Tvater is swept into the receiver v i t h the bulk of the xylene. In this v a y 100 ml. of xylene suffice. Fifty milliliters of methanol Tvere required to render this amount of xylene miscihle n i t h the quantities of Fischer reagent usually used. I t was found that the amounts of water contained in the above quantities of solvents, together n-ith that adsorbed on the inner 1 ~ 1 1of s the .;till, were appreciable. The success of the method is dependent on reducing the blank correction to a small and reproducible value. In the first work the xylene was dehydrated by distilling off the first portion containing the bulk of the water, collecting the remainder of the batch, and storing it over Drierite. The methanol \vas stored for several days over Drierite, decanted, and distilled. The blank correction amounted to about 20 mg. of mater and was riot satisfactorily reproducible. It became evident that much of the xater was contributed by the methanol, the "absolute" grade containing about 0.0570 of water as purchased. The treatment described reduced this to 0.03%. Fifty milliliters of such methanol contain about 15 mg. of xvater, an unsatisfactorily large amount. The scheme ultimately arrived a t consists of adding an excess of Fischer reagent to the portion of methanol and tit,rating it to the equivalence point with standard,water in methanol solution just prior to use. This, in effect, eliminates the water content of the methanol. Using this adjusted methanol and xylene dehydrated by distilling and storing over sodium, the blank amounts to less than 5 mg. of water and is reproducible, as shown by consecutively obtained values of 0.85, 0.80, and 0.83 ml. of standard water in methanol solution, corresponding to 3.75, 3.53, and 3.66 mg. of water. REAGENTS
Xylene. Technical grade xylene was dehydrated by distilling, discarding the first portion, and storing over sodium. The storage bottle was fitted with a drying tube filled with Drierite and with a siphon. Methanol. Merck's absolute methanol was treated with an excess of Fischer reagent and titrated to the equivalence point with standard viater in methanol solution just prior to use. Fischer Reagent and Standard Water in Methanol Solution. Both reagents mere prepared and standardized according to the directions of Wernimont and Hopkinson (18). The addition of a few milliliters of butyl Cellosolve to each batch of Fischer reagent greatly facilitated wett'ing and drainage of the buret. The precision of the standardization reported by Wernimont and Hopkinson of 2 parts per thousand was confirmed. APPARATUS
Buret Systems. The first, dispensing system for Fischer reagent used consisted of a Machlet,t Auto-Buret with a 25-ml. buret and a 2-liter reservoir. The following difficulties arose during its use: The outlet being a t the bottom of the reservoir, crystals deposited
328
ANALYTICAL CHEMISTRY
from the reagent on aging found their way into the buret, plugging the small orifices and necessitating emptying the entire system for cleaning. Traces of stopcock grease which inevitably get into the buret and interfere with wetting and drainage are exceedingly difficult to remove, this being a one-piece all-glass assembly with no direct access to the buret. The experience of previous investigators ( I S , 18) regarding deterioration of the reagent and the necessity for several titrations to establish the equivalency ratio of the reagents has been confirmed, as many as five or six titrations sometimes being made before the ratio became constant. It was evident that the deterioration 'was more rapid in the buret and delivery tube than in the reservoir. This may be due to an accelerating effect of light on the degradation reactions.
strunient furnishing the small polarizing current. The performance of the instrument was very satisfactory, being unaffected by minor fluctuations in line voltage and seldom requiring any adjustment. of the sector control. One drop (0.01 ml.) of the standard water solution of the usual strength produces a full-scale deflection of the shadow pattern. Almy, Griffin, and Wilcox (3) have reported breaks of only 20 to 25 millivolts when a plat'inum-tungsten elect'rode system is used. Probably the tungst,en reference electrode changed potential at t.he equivalence point as well as t.he platinum, since the present work shows that considerably larger breaks are observed with a polarized platinum-platinum system. Distilling Apparatus. The distilling apparatus is shown in Figure 1. A represents an electric heater wit'h a rheost,at control, R a 500-ml. 2-necked flask with 24/40 standard-taper openings. The delivery t,ube, C, has two male 24/40 joints, the one leading to the receiver being fitted with a drip-tip, so that the distillateiiariuii f r u i i i \ l e i r i , \ia t e r , 7 0 246 0 226 0.255 0 250 0 217 0 226 0 136
26.0 17 7 15.1 15.7 16.4 17.9 14 9
329
0.0614 0.0413 0.0352 0,0370 0,0387 0.0420 0.0352
0.1969 0 1585 0.2308 0.1715 0,0486 0.1094 0 1875
0,010 0.010 0.019 0.014 0,019 0.010 0.000
0.~58:i 0 1998 0.2660 0.2085 0.0873 0.1514 0 I297
o
MO 2075 2784 2065 0867 o 1565 0 236.5 0 0 0 0
(I
1 12 1 74 1 31 0 ;7X 0.84 1 18
i.n!i
1.18 1 82 1 30 0 52'
n
1
,?i
106.;
103.n
104.6 99.0 99.2
103 4 106.2'
initial \vater c~intciit. Watel, \viih : i t i t l ( ~ l t o ~ : i n i ~ ) Ic~nit:iiiied rs iii \\-eighing bottles from a Xvriglit 1)ipc.t. .4fter :I f w v niinutrs' itanding, the samples had ronipletely t:tkeri up tlie \vatel, :w f:ir as the eye could perceive, mid the aiialyscs w r e niade as usual, using a11 of tile saniple i n tile \veigliing kiottlc for w c l i test. S o difficulty \vu> exy)c~~~ic~iicwI \\-it11t h r samples sticking t o tlir weighing bottles. Presunial)ly thr, \vatet. \v:is c*oniplrtelyconvrrtrd to sodium hydi,oxide nionoliytlrtitr. Tatilt~11 giver t lit) rcrults of these experiineiits, \yhirli intiic~:itr~ th:it the ortlrr ( i t ' a(~r.uixcyis the same as tlir precision, :il)out jSr, ( i f tlie itniolnit ( i f \v:itci. tleterniineti.
sample 1 being a rubber compounding grade containing 0.(i570 calcium hydroxide and having a mean particle size of 0.03 tiiicron :E determined by the electron microscope, and sample 2 i ) ~ i n g a paper coating grade free from calcium hydroxide m t l having ,z mean particle size of about 1.5 microns as'ertiniateti t)!- ientrifngal sedimentation. Determinations of nioi-ture i n tiotti of these materials by tlierinal drying is sonicivha? t n~uhlrronic. Pt,obahlg because of the large surface area, h l t l i ai'i' c,sti,cinely hygroscopic \i-hen dry, and will rapidly p i l i \vrligiit i i i :I ticsicrator over a drying agent. T h r samplri ninrt I I P (xpo;.ed to the oven atmospheres in verp thin lnyei~i. .1 ~ t m i p l edried i11 this laboratory a t 180" C. for 26 houw still i~etaiiirtlnvci' 0.1 % nioisture in the loner levels of a layer 1.0 cni. (0.75 i n r l i : i i ~depth, Determinations ivei'e niactr tiy drying at 180' C. for 16 lionrs, Iiy direct application of Fiscliei, rragetit , hy iinniei~~ioii in metha1101for 12 hours follon-ed by titration \\-it11 Fi-cher rragent, and l)y t'he di.ti1latioii-titratioii prcicodurt~. Tlic re-ulth given in 'Table I I1 sIio\v that tlie direct application of tiit reagent. with or without *uspending tlic sample in niettianol, gives lniv results. In the w 3 e of sample 1 , liigit wsults \vei,e expected iiccauic of the calcium Iiydroxide present, \vhic*h i ~ ~ i c~vitli t s Fiwher reagent, 1 inole of the former being equivalrnt to 2 n i o l e ~of water 16). It nus st he concluded that the, renioval of water ir incompltttr, for rrasons not entii.ely clear, :ind that t h t tlistillatinii method, or a iweful tliei.nial drying, is pi,t,fei,:itilcs. ~~
T a b l e 111.
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