(6) Glover, C. A., Hill, C. P., Ibid., 25, 1379 (1953). (7) Glover, C. A , Stanley, R. R., lbid., 33,477 (1961). (1957). (8) Harris, I., J . Polymer S C ~ 8, . 353 ( 2 ) Beckman, E., 2. Physak. Chenz. 4, 532 (1952). 11889). (9) Lehrle, R. S., Majury, T. G., Ibid., ( 3 j Billmeyer, F. W.,Jr., J . Am. Chem. 29. 219 11958). SOC.75, 6118 (1953). (10) ’Rleneies, A. IT. C., Wright, P.L., Jr., (4) Bonnar, R. U., Dimbat, U.,Stross, J . Am. Chem. SOC.43. 2314 (1921). F. H., “Sumher-Average Molecular (11) Ray, N. H., Trans: Faraday SOC.48, Weights,” Interscience, Sew York, 1958. 809 (1952). (5) Dimbat. 11.. Stross. F. H.. A I ~ A L . . ,CHEJI. (12) Smith, H., Ibid., 5 2 , 406 (1956). 29, 1517 (195‘7). LITERATURE CITED
(1) Ashley, C. E., Reitenour, J. S., Hammer, C. F., J . ilm. Chem. SOC. 79, 5086
(13) Tremblay. R., Sirianni, A. F., Puddington, J. E., Can. J . Chem. 36, 725 (1958). (14) Trementozzi, Q. *4., J . Polymer Sci. 23,887 (1957). (15) Tung, L. H., Ibid., 24, 333 (1957). (16) Uberreiter, K., Orthman, H. J., Sorge, G., Xzkrornol. Chem. 8,21(1952). RECEIVED for wview August 3, 1959. Resubmitted December 5, 1960. hccepted December 5, 1960. Division of Analytical Chemistry, 135th Meeting, iZCS, Boston, Mass., April 1959.
Spectrophotometric Determination of Microgram Quantities of Vaporizable Water from Solids Using Karl Fischer Reagent DUMAS A. OTTERSON lewis Research Center, Nafional Aeronautics and Space Adminisfrafion, Cleveland, Ohio
b Karl Fischer reagent is used as the basis of a colorimetric method capable of 0.5-pg. sensitivity a t 475 mp for the determinaiion of water. Water in a solid is determined after its isolation by heating in a stream of dry nitrogen, collecting in a cold trap, and transferring to the reagent. Results of water determination in BaClz. 2Hz0 samples containing 1 1 to 26 pg. of water show that the average error is about 2 pg.
D
a n investigation a t the Lewis Research Center of the Xational Aeronautics and Space Administration, it became necessary t o determine the amount of water adsorbed on salt crystals. Recent literature contains methods for the determination of water that appear to be very sensitive, S e s h (9) used methylene blue as a color reagent to determine as little as 30 pg. of nater in mineral oil. An attempt to adopt this reagent for our purposes produced insensitive results due to a necessary modification-namely, the suspension of methylene blue in dry acetone. The sensitivity of Karl Fischer titrations has been increased by Meyer and Boyd ( 7 ) to about 5 p g . for liquid samples and by Bastin. Siegel. and Bullock ( 2 ) to 3 p g . Bruckenstein ( 3 ) measured the n-ater in acetic acid by adding and measuring the amount of acetic anhydride that reacted spectrophotometrically. H e determined about 90 Mg. of water. Keidel (5) developed a n extremely sensitive method based on the electrolysis of water that is especially suited to steady-state conditions and that can be used to determine as little water entering the system as 7 X IO-* 450
URING
0
ANALYTICAL CHEMISTRY
gram per minute. Armstrong, Gardiner, and Adanis ( 1 ) used this method to determine microgram quantities of water in paper. However, their calibration curve shovied a number of points that are in error as much as 9 pg. Sivadjian (10) presented a unique method for the determination of water called hygrophotography. As yet, we have not been able to appraise its value for the purpose a t hand. This paper gives a method in n hich Karl Fischer reagent is used as a color reagent by measuring the change in absorbance that accompanies its reaction with water. The sensitivity is 0.5 pg, of water. I n addition, a method is presented in which water can be transferred from a solid sample to a collcction apparatus from nliich it can be accurately transferred for measurement. The method is trsted using BaCl? 2H20 as a standard. The main difficulty encountered in this work is the exclusion of c.;tranrous v,-ater while the sample is added to the system, but this would be encountered in virtually every method of similar sensitivity. Some of the difficulties n-ill be described in detail nith the hope that the reader may gain sufficient insight to solve the unique problems presented by his work. SPECTROPHOTOMETRIC METHOD FOR WATER
The color change of Karl Fischer reagent from reddish brown to yellon. has been used to indicate the end point of its water titration (8). This color change might also be used as the basis of a colorimetric method in which the change in color intensity that accompanies a minute nater addition is meas-
ured. Preliminary tests with diluted reagent readily indicated that this color change is eytreniely sensitive. The need to protect the reagent JTas indicated by the fact that the reagent in an unprotected absorption cell was completely converted to the yellow color (in about 5 minutes) by the water absorbed from the air. Sleeved rubber stoppers for serum bottles have been used to protect Karl Fischer microtitrations (6). Experience a t this laboratory shows that these stoppers can adequately protect the contents of an absorption cell containing Karl Fischer color reagent. The use of hypodermic syringes and needles permits addition of the reagent and the water to be determined to such stoppered cells. It mas very difficult to add color reagent to the color cell in such a way as to repeat the absorbance reading. TYater on the vialis of the color cell, the stopper, needles, and syringe rarely appeared to be the same for subsequent color reagent additions. This difficulty suggested the techniques used in this work. A known volume of color reagent was added to a stoppered color crll. K h e n the w t e r in the cell had reacted completely, the absorbance was recorded. The water to be studied was then added. The cell was shaken and the absorbance measured. The change in absorbance depends on the amount of water added. Apparatus. A Beckman Model B spectrophotometer is used t o make the absorbance measurements a t 475 mp. Color cells with the usual square opening and a 1-em. path are used. The contents are protected from airborne water (Figure 1) with a sleeved rubber stopper for a serum bottle
molded from sulfur-free stock and v i t h a n outside diameter of the top of about 12 mm. The stopper is placed inside out over the opening of the cell with the sleeve pulled over its walls. The plug end of the stopper is cut off as close t o the top as possible. A way to seal the sleeve tightly to the cell was with a special rubber band which was made by cutting a 1/8-inch lmgth of 3/8-inch outside diameter amber gum rubber tubing with 1/18-inchthick walls. (Also tried was a color cell equipped with a ground-glass stopper using a sleeved serum bottle stopper in the normal manner. The rate of water pickup was less. However, the difficulty of finding the correct spot to inbert the covered hypodermic needle of the water transfer apparatus precluded its use.) ,4 5-ml. hypodermic syringe with a ll/p-inch Sumber 20 needle is used for measuring and transferring the color reagent to and from the color cell. A Gilmont ultramicroburet made by the Emil Greiner Co.. n-hich delivers 0.00001 ml. per division, measured the standard water solution for the data of the calibration curve. Reagents. T h e Karl Fischer reagent is a stabilized form sold by the Fisher Scientific Co. (catalog S u m ber So-K-3) \\hose original titer was 5 mg. of water pel milliliter of reagent. Presumably any Karl Fischer reagent free from extraneous dyes may be used. The standard water solution is prepared by pipetting 10 ml. of water into a 100-ml. volumetric flask that has been rinsed and partially filled \vith methanol from a freshly opened bottle and finally diluting to the mark n i t h the methanol. The methanol usuallv contains about 0.15yo water. The color reagent is Karl Fischer reagent diluted with methanol. A 2our% serum bottle is filled about half full with methanol and the bottle is stoppered with a serum bottle stopper. Karl Fischer reagent (about 5 ml.) is added b j 7 hypodermic syringe and needle until the absorbance is in a convenient range to work-namely. 0.5 to 1.5 when measured against a colored cornparison solution. The comparison solution was a dilute sodium chromate solution n ith an absorbance of about 1.0 when measured against water. The use of colored comparison solutions has been shon n by Hiskey (4) to increase the sensitivity of colorimetric measurements of intensely colored solutions. Procedure. T h e color reagent is used as follows. T h e color reagent is drawn from its stoppered container into a 5-nil. hypodermic syringe. T h e volume is adjusted t o 3.0 nil. T h e hypodermic needle is n ithdrawn and inserted through the stopper of the color cell. The reagent is transferred t o the cell. After withdraning the, needle, the liquid on the outside of the stopper is niped off mith clean absorbent cotton. The cell is shaken and the absorbance measured a t 475 mp using a colored solution in the comparison cell. The shaking and subsequent measurenwnt of the absorbance are repeated until virtually
CUT
U Figure 1. Protected color cell
no difference is noted in several consecutive measurements X a t e r is then added to the cell. The cell is shaken and the absorbance is measured and recorded when it becomes constant. The difference between the two constant readings depends on the water added. All additions of water to the cell, including those for 0 pg., for the calibration curve are made as follows. The ultramicroburet is rinsed and filled n-ith 10% water in methanol. The tip of the buret is Tviped with absorbent cotton and allowrd to stand several minutes before use. The tip of the buret with the methanol solution not quite reaching to the tip is pushed through the stopper of the color cell. (This stopper should have a number of needle holes. Those that did not created a suction when the buret was withdrawn that pulled out too much liquid from the buret.) The buret is adjusted so that the 1Oy0mater solution just reaches the tip. The reading is made. The buret is manipulated so that the desired amount is forced out of the buret. The tip is immersed in the color reagent to the top of the ground glass for a moment. The buret is removed from the color cell carefully so that the stopper closes as perfectly as possible. [If a ridge is formed on the stopper upon removal of the buret, water nil1 leak into the cell from the air and the absorbance n-ill not reach a constant value. Such data are discarded (see Table I).] The cell is shaken and the absorbance measured. The tip of the buret is niped n i t h absorbent cotton. About 10 divisions of solution are run out of the buret and the tip is again niped. The buret was allowed to stand until transfer of the next water sample in about 5 or 10 minutes. Discussion. Table I shows t h a t the calibration curve is linear and therefore Beer's law is obeyed. One microgram of water causes a change in absorbance of 0.011. T h e method has a sensitirity of 0.5 ,ug. of water and a potential sensitivity of 0.2 pg. The absorbance of the color reagent is the result of contributions from both the unreacted and reacted forms of the reagent. This and the fact that the absorbance of the reacted form increases rapidly with decreasing wave length near
475 nip frustrate the accurate mcasurement of the spectral curve of the unrcacted form. Its absorbance appears to increase slowly as the wave length decreases from 475 mp, The wave length chosen for this work is a compromise between that Tvhich has good sensitivity and still keeps the contribution of the reacted form sufficiently small. This portion of the work was carried on in a period of lon. relative humidity. Under conditions of high humidity, one might expect greater difficulties. APPLICATION
TO WATER IN SOLIDS
The 7-irtual omnipresence of water in the laboratory and the use of a stoppered color cell as thc reaction verse1 limit the ways in which water from a solid may be added to the reagent for its determination. dttempts were made to determine water adsorbed on sodium chloride by adding the solid directly to the color cell. Finely ground sodium chloride produced a cloudy solution. This is one reason why the water should be separated from the sample before mixing n-ith the reagent. Such a separation procedure could then be applied to a large number of solid materials with only minor changes. I n the procedure given here, water was scparated from the solid by heating the solid in a stream of dry nitrogen, collecting the water, and finally transferring it to the absorption cell. The problem of drying the nitrogen and that of trapping the water from the sample are interdependent. During a collection the amount of water in the gas leaving the collector must be the same as that leaving the dryer. Furthermore, it must be possible to add the sample nithout adding extraneous water and without losing any from the sample. Techniques and apparatus are described that meet these requirements
Table
I.
Water Added, &. 0
5 10
20
Absorbance Change Due to Water Addition
Change in Absorbance
Average Change in Absorbance
(475 M p ) (475 RIP) 0 . 0 0 2 , 0.005, 0.006 0.041,5 0.019,a 0.014, 0 006 0.055, 0 , 0 5 5 , 0.054 0.052 0,120. 0 . 1 ~ 3 7 . ~ 0.113 0,102, 0 , l j 5 , "
0.114 0.215, 0.237
0.226
Absorbance reading did not become constant within several readings but drifted. These data are ignored.
VOL. 33, NO. 3, MARCH 1961
451
and that permit the collected water to be transferred readily to the color cell. The materials used in constructing the successful apparatus were chosen after considerable trial and error. Glass and quartz, for example, can only be used where there is no change in temperature during the operation. On the other hand, gold may be subjected to large changes in temperature. Other metals can be used in place of gold a t least for a short time. Inconel and nickel after several weeks' use, for instance, developed a tendency to pick up water and did not lose it until they were heated. Sitrogen instead of air is used to minimize the oxidation rate of the base metals in the system. A clear crystal of BaC12.2H20 constituted each sample used to test the method Such a sample contains negligible amounts of water of occlusion and absorption. The dihydrate is stable except under conditions of unusually low humidity. The samples mere kept in a hydrostat containing a saturated solution of sodium chloride to prevent any loss of water of crystallization. The error in weighing is less than 1 7 fig. of BaClz. 2H20 which corresponds to + 1 fig. of water. Apparatus. The water transfer apparatus (Figure 2) consists primarily of an oven and a collector, in series with two dryers. The dryers are essentially elongated side arm test tubes sitting in liter Dewar flasks packed n i t h pulverized dry ice. The test tubes are packed with copper turnings and have 'j4-inch copper tubing extending from near the bottom through the rubber stopper used to seal the test tube. The copper tubing from one dryer is connected to the oven while the other dryer is connected to the exit tube from the collector by gum rubber tubing coated TI ith paraffin. The gas inlet tube of the oven holds an iron slug attached to a push-rod of nonmagnetic stainless steel. The boat and its handle are made from 0.005inch thick fine gold sheet and are long enough so that other parts of the rod never enter the heated part of the oven. The boat makes a snug fit inside the oven. The side arm for admitting samples is closed with a rubber stopper and has a cup fashioned from Styrofoam surrounding it. The inner and outer joints used to make the oven are sealed together with Apiezon W wax. About 1 inch of the inner joint is cut off to eliminate some of the constriction due to the joint. The electric heater is made by winding 0.187-inch diameter Nichrome wire around a 5-cm. length of the indicated portion of the oven (Figure 2) and imbedding it in asbestos. Heat-resistant tape is used to hold the heater in position. The oven was precalibrated and the temperature was controlled a t about 400' C. by a variable voltage transformer. The stopcock and the transfer syringe are lubricated with clean petroleum jelly. The transfer syringe is a 2-ml. hypodermic syringe
452
ANALYTICAL CHEMISTRY
Figure 2. Water transfer apparatus
cemented into the glass tube with red sealing wax. The collector is made from glass tubing cemented into the hub of a 20-gage 2-inch hypodermic needle, a brass weatherhead 90' elbow with pressure fittings for ljrinch copper tubing, 3/s-inch latex surgical drainage tubing, and a sleeved rubber stopper. The needle is soldered into a hole drilled into the back of the elbow. The sleeve covering the needle is made by using rubber cement to seal the surgical tubing to the stopper. The sleeve is cemented to the elbow with a rubber-tometal cement. The glass tubing is cemented into the hub of the needle with molten sulfur and sealed with paraffin. The inner portions of the collector are coated with paraffin by rinsing with 3% paraffin in chloroform and purging for a t least an hour. A short length of 'j4-inch copper tubing extends toward the dryer from the elbow. Amber gum rubber tubing is used to join the collector to the exit tube of the oven and the dryers to the inlet and exit tubes. Molten paraffin is used to seal all places where water might enter the system, such as metal-to-metal pressure fittings, all rubber joints including their outer surfaces, and metal-to-glass joints. The stopper on the side arm is not sealed because the side arm is used to add samples. Filling the Styrofoam cup with dry ice prevents water of the air from entering the system by happing it on the walls of the side arm. A cylinder open a t both ends that fits inside the side arm is made from 0.005-inch sheet gold and is used to line the side arm during sample additions to prevent any of the condensed water from contaminating the sample holder. The sample holders for the small
Table II.
Water Determinations
HzO, fig.
Added as BaClz .2H20 25 11 14 20 22 30 23 21 15 23 25
Found after Subtracting Correction for Blank 25 8 15 18 19
27 23 23 15 21 23
BaClz 2H20 samples (75 to 200 pg.) are platinum cups 3 mm. in diameter and 6 to 8 mm. deep with a loop to facilitate removal from the apparatus. Procedure. The water transfer apparatus with the oven a t temperature is dried by passing 40 t o 50 ml. of nitrogen per minute through the dryers in series with the system for 12 t o 16 hours. The Styrofoam cup is then filled with dry ice, and purging of the system is continued for another hour. Blank runs are then made t o ascertain the dryness of the system. These differ from sample runs only in that the sample holder does not contain any sample. Vhen blanks agree to 1 or 2 pg. and are at the most 6 or 7 pg. in magnitude, the samples are run. The blank and sample runs are as follows. The sample holder and the gold liner are heated to redness over a flame and allowed to cool several minutes. The sample is placed in the sample holder. %-hen the collector is dry, the collector cup is filled with dry ice and the stopper from the side arm is removed. The gold liner is inserted, and the sample holder is slid through it into the gold boat. The liner is removed and the stopper replaced. The boat is then moved into the hot section using a magnet t o push the iron slug. Water is collected for 5 minutes. The stopcock is closed and dry ice 1s removed from the collector. The collected water is transferred to the absorption cell in the following manner. The cell containing 3.0 ml. of color reagent is shaken and the absorbance a t 475 mp is measured. T h e n these two operations produce repeated constant absorbance readings, the reading is recorded The stopper on the color cell and that a t the end of the collector sleeve are dusted with flowers of sulfur. The two stoppers are held face to face while the collector needle is plunged through them both into the color cell. The plunger of the 2-ml. hypodermic syringe is used to draw the color reagent into the glass tube to the approximate level indicated in Figure 2 and to return it to the color cell. After rinsing three or four times, the color cell is removed. Any liquid on the stopper is wiped 1%-ithabsorbent cotton, the cell is shaken, and the absorbance is again read. The difference in the t n o constant readings is a measure of the water collected. The collector is rinsed with four or five portions of dry methanol, and the system is purged for 5 minutrs to dry it for the next run.
It is necessary to keep all dry ice levels high in their respective containers. The 1-liter Dewar flasks each containing a 6/s-inch diameter trap held sufficient dry ice to last overnight but during runs were filled every hour or two. The Styrofoam cup was filled every half hour and the collector cup held sufficient dry ice to last for the 5-minute collection. It was discovered by accident that dusting the stoppers with flowers of sulfur reduced the water pickup t o virtually nothing during the water transfer step. Results and Discussion. Table I1 gives the results obtained when water was determined on the first eleven consecutive samples of barium chloride dihydrate t h a t were run on the apparatus described. T h e effectiveness of the water separation and the water transfer technique are
indicated by the average error of 2 pg. The greatest error appeared to be connected with a rapid change in humidity such as occurs with a n abrupt weather change. The blank a t the beginning of the afternoon was 5.7 pg. of water and at the end was 3.8 p g . During that time the weather cleared up and presumably the relative humidity dropped. If the relative humidity of atmosphere surrounding the water transfer aPParatus were controlled, the precision might well be improved. I n this method no purging of the is required to any serious interference by extraneous water from the air. The removal and collection of water from a sample can be carried out almost immediately after removing from an atmosphere of controlled humidity* Thus an accurate determination of the water of a solid in
equilibrium a t a known relative humidity is possible. LITERATURE CITED
(1) Armstrong, R. G., Gardiner, K. W., Adams, F. A m L . C m H . 32, 752
w.,
(1960). (2) Bastin, E. L., Siegel, H., Bullock, A. B., Zbid,, 31, 467 (1959). (3) Bruckenstein, S., Ibid., 28, 1920 (1956); 31, 1757 (1959). (4) Hiske 3 C. F., Zbid.1 21, 1440 (1949). (5) Keidef F. A., Ibid., 31, 2043 (1959). (6) Levy, G. B., Murtaugh, J. J., Rosenblatt, M., IKD.ENG.CHEY.,ASAL. ED. 17, 193 (1945). (7) &feYer, A. S.7 Jr., Boyd, C. hf., A N A L . CHEW 31, 215 (1959). (8) Mitchel,
