1927
V O L U M E 27, NO, 12, D E C E M B E R 1 9 5 5
This type of equilibria, known t o exist a t low temperature, would decrease the concentration of the base component and, effectively, increase the acidity of this system. T h e solubility relations for the system under investigation, uranium trioxide-sulfuric acid-water, fa11 within the most sensitive range of p H variation nTith mole ratio. For the application of this method to other systems the sensitivity of p H in the particular solubility range must be considered. There are, however, possible modifications of the analytical method. For example, an acid-base system, in which solubilities do not fall in the pHsensitive range, might be investigated using p H control curves by adding a known quantity of standard acid or base t o the liquid phase sample t o make the sample p H fall within the sensitive range for analysis and then correcting for the addition. ACKNOWLEDGMENT
PHiZ5-C.i
Figure 6. pH at 25’ C. of sulfuric acid-water solutions saturated at various temperatures with uranium trioxide
tionship should approach ideality as the sulfate concentration approaches infinite dilution. The deviations expected a t finite concentrations tvould be attributed t o the changes in activity coefficients of the solution components and to the extent of complexing a t the elevated temperature. The “reverse S”-shaped curvesof pH data a t 26” C. for the saturated solutions a t elevated temperature (Figure 6) might well not exist if p H data were measured a t the elevated temperature. If, however, this type of curve does exist a t the high temperature, the isotherms shown in Figure 6 could represent the general character of curves of hydrogen ion activitv measured at elevated temperature us. log molality of sulfuric acid, but would not represent the actual hydrogen ion activity a t high temperature. The assumption is made that the hydrogen ion activity of various solution concentrations changes proportionally to each other as a function of temperature. In this event the reversal in curvature as shown in region d of Figure 6 is characteristic of any acidbase system in which the ratio, base/acid, increases as a particular function of increase of acid or base concentration. A minimuni in pH as a function of concentration must necessarily appear in the system in which complexing does not occur or is a minor influence on acidity relative to change of ratio. The second reversal in curvature shoivn in region B is indicative of complexing in the particular system and, in this case, might imply and substantiate polymerization of uranvl ion by equilibria of the following type: UOa’T UO2(OH)z e U*Os’+ HzO
+
+
The author wishes t o acknowledge the support of C. H. Secoy, in whose group this work was accomplished, and the suggestions of R. W. Holmberg and K. -4.Kraus on determination of p H using a vibrating reed electrometer. LITERATURE CITED
(1) Bates, R. G., Chem. Reus., 42, 1 (1948). (2) Booth, H. S., and Bidwell, R. M., Ibid., 44, 491 (1949). (3) Booth, H. S., and Bidwell, R. AI., J . Am. Chem. Soc., 72, 2567 (1950). (4) Day, H. O., Gill, J. S., Jones, E. V., and Marshall, W. L,, ASAL. CHEM., 26,611 (1954). (5) du Pont de Kemours & Co., E. I., published reports on properties of Teflon (reported transition temperature = 327O C.). (6) Helmholz, L., and Friedlander, G., “Physical Properties of Uranyl Sulfate Solutions,” -4tomic Energy Commission Declassified Report MDDC-808 (1943). (7) Kraus, K. d.,Holmberg, R. W., and Borkowski, C. J., ANAL. CHEX,22, 341 (1950). (8) Lietske, 11. H., Wright, H. W., and Marshall, 11‘. L., unpublished work. (9) NacInnes, D. A., and Longsworth, L. G., “Measurement and Interpretation of pH and Conductance Values of Aqueous Solutions of Uranyl Salts,” Atomic Energy Commission Declassified Report MDDC-911 (1942). (10) Marshall, W.L., Wright, H. W ,and Secoy, C. H., J . Chenz. Educ., 31, 34 (1954). (11) Orban, E., “Measurements of pH of UOa-UOzSOJ-H20 Solutions,” AECD-3583, AECD-3580, U. S. Atomic Energy Commission Declassified Reports (1952). (12) Secov, C. H.. J . Am. Chem. Soc.. 72. 3343 (1950). (13) Zachariasen, W. H., -4rgonne Sakonal Laboratory Report CP-3774 (1947). RECEIVED f o r rel-iem April 14, 1935. Accepted September 12, 1955. Di. vision of Physical and Inorganic Chemistry, 127th Meeting. ACS, Cincinnati, Ohio, March 2 9 t o April 7, 1955.
Direct Determination of Moisture in Sodium Hydroxide CLAYTON L. DUNNING and CHARLES D. HILDEBRECHT Research Center, Diamond Alkali Co,, Painesville, O h i o
A simple, direct method for the determination of the moisture content of anhydrous caustic soda has been developed. The method involves heating the caustic to about 350’ C. in a closed nickel container. The moisture is purged from the caustic into tared anhydrous magnesium perchlorate filled U-tubes by means of a stream of dry nitrogen. The elapsed time for analysis is about 45 minutes. Individual determinations fall within &0.012$7”of the true moisture content 95% of the time.
T
HE established technique for the determination of the moisture content of anhydrous caustic soda (sodium hvdroxide) has been the indirect method. This consists of subtracting the sum of all components analyzed in the caustic sample from 100% and calling the difference moisture. It is evident that this method gives, at best, only an approximation of the true content. The possibility of using an oven drying method is ruled out in this case, because of the carbon dioxide present in the air. Several distillation-extraction methods using an immiscible solvent
ANALYTICAL CHEMISTRY
1928 have been tried. Various stills and traps (1-9, 6,) have been used, but the difficulty seems t o lie in the fact that significant amounts of water remain in the condenser. Collecting a large volume-i.e., several milliliters-is one way of m i n i d i n g this error. However, if the water content of the ssmple is low, this requires extremely large samples. Suter ( 4 ) devised a distillation-titrimetric method whereby be used xylene to distill the water from the sample and then titrated the xylene-water mixture with Karl Fisher reagent. The author claimed an accuracy of ahout 5% of the water present. However, the procedure takes over 4 hours per sample. The apparatus and procedure described have been used in this laboratory for over a year, having been used for the analysis of caustic soda samples which have ranged from 0.02% to over 1% water.
in weight of the U-tubes is equal to the water content of the caustic. While the caustic is still molten disconnect the container and pour the caustic into a dry metal container, Later cool the container in water and wash out the remaining caustic. After drying,
gloves ;hen
cleaning out the container. EXPERIMENTAL
It was believed that the temperature necessary to drive off the moisture from the caustic in B reasonable time would probably be above its fusion temperature of approximately 318" C.; however, too high a temperature may cause decomposition of the caustic resulting in a release of water.
APPARATUS
Complete assembly of the apparatus is shorn in Figures 1 and 2. Detail of the head assembly is given in Figure 3. PROCEDURE
Tharouehlv drv the nickel container and assemble the a .. m&ratus a8 s5oGn. Adjust a flow of nitrogen to 2000 cc. per minute. After several minutes of purging, run a blank on the system. Disconnect the U-tubes and weieh them on an analvtical balance. then d a c e them back in p o s h n and allow the nkrogen t o flow'at the same rete for 45 minutes. Again disconnect the tubes and weigh them. If the tubes have gained weight, a larger dryer for the nitrogen gas is needed. If there is no blank on the nitrogen, check the system for leaks bv introducing a weighed sample of water into the container.
weigh the U-tubes. If 'the recovew of water is hot quantitative, ~~~~. ~~~~~~
Figure 1. Complete assembly
~
tained on the b t u b e s , place &emuin position and a d j h the nitrogen flow to 2000 cc. per minute. Open the container and quickly pour in the weighed sample (50 to 100 grams) of caustic. Replace the cover on the container immediately and tighten. Begin heating the container and raise the temperature to 350" C. Maintain this temperature for 45 minutes. At the end of this time, disconnect the U-tubes and reweigh. The gain
Table I. Precision of Determination of Water in Caustic Soda with High Moisture Content (500' C. for 60 minutes)
Run No.
% HnO
1
0.97
2 3 4
0.95
Deviation from Mean 0.02 0.00 0.03
0.92 0.96 0.95
w Figure 2.
0.01
Mean Av. dev. from mean (1.6% of water present) 0.015
A. B.
c,
I
Table 11. Preoision of Determination of Water in Caustic Soda with Low Moisture Content
1
2 3 4 5 6 7 8
9 10
60 60
500 500
0.067 0,070
60 0.068 500 0.068 60 500 60 500 0.066 60 500 0.071 30 350 0.077 30 350 0.064 30 350 0.073 30 350 0.062 Mean 0.069 Av. de". from mean (4.3% Of vster Present1
0.002 0.001 0.001
0.001 0.003
0,002 0.008 0.005 0.004 0.007
0.003
Sohematic diagram of apparatus
Asoarite column, 1b y 10 inoh tubing Anhydrone column. 2 b y 2 0 i n o h tubing Q1"---+--
E . Ca F . Th 0. 1 5
A sample of high moisture content caustic (ca. 1%) was run a t 500" C. for GO minutes. These results are shown in Table I. More runs were made a t lower temperatures and shorter run times. At 350" C., all the moisture was recovered in 30 minutes. When making the runs shown in Table 11, two different conditions were used. The first six runs were made a t 500' C. for GO minutes, and the last four runs were made a t 350" C. for 30 minutes. As there was no significant variation in the results, it was decided to use 350' C. and 45 minutes in the method.
1929
V O L U M E 2 7 , NO. 1 2 , D E C E M B E R 1 9 5 5 K ,
,
rJ
Table 111.
(350' C. for 45 minutes)
I Run No. 1 2 3 4
5 6 7 8 9 10
U H. I. J. R. I,.
1 2
I
I
GRAMS
Figure 4.
I
Sample, Grams 54.9 54.7 112.4 79.1 36.8 126.0 62.8 86.9 143.0 52.6
In sample 0.0377 0.0376 0.0772 0.0843 0.0253 0.0866 0.0432 0.0596 0.0982 0.0362
I
IV V H20, Grams Total Added present 0.1558 0.3770 0.0743 0.9852 O.Ofi65 0.2379 0.3802 0.1812 0.7454 0.3127
0.1935 0.4146 0,1515 1 0395 0.0918 0.3245 0.4234 0.2408 0.8436 0.3489
VI11
VI
VI1
Recovered
Calcd.
Found
0.18136 0.4025 0.1530 0.9753 0,1130 0.2601 0.3848 0.2690 0.8665 0.3511
0.36
0.34 n 74 0 14 1 27 0 32 0 21 0 61 0 31 0 60 0.66
nlo
0.75 0.1-1 1.30 0.23 0.26 0.67 0.28 0.59 0.66
% -
Comparison of Duplicate Analyses (3.50' C. for 45 minutes)
Thermocuple well Inlet tube for gas stream Cover plate, 3/8-inch nickel plate Exit tube for gas stream Recessed circular well cut t o snugly fit lip of nickel container. T h e well is lined mith a n asbestos gasket
I
111
I1
Table IV.
Detail of head assembly
Figure 3.
Recovery of Water Added to Caustic Soda
Difference between Duplicates
Sample XO.
1
PRESENl
Recovery of water
Any carbon dioxide in the nitrogen purge gas reacting with the caustic would release additional water. To eliminate this possibility an Ascarite column was placed ahead of the magnesium perchlorate drying train. PRECISION
Several determinations were made on one sample of caustic soda which was kept in a tightly stoppered bottle. The results are shown in Table I. Another sample of caustic which had a low moisture content was used to evaluate the method. A series of determinations were made on this sample. The results are shown in Table 11. ACCURACY
N o samples of caustic soda were available that were known to be completely anhydrous. Therefore, it was necessary to use the sample of caustic which had been analyzed by this method (Table 11) in order to ascertain the accuracy of the method. Known quantities of water were added to varying nreights of sample with the average of the 10 determinations shown on Table I1 used as the original water content. The sample was weighed and placed in the nickel container to which the varying weights of water were immediately added from a weight pipet. The addition was made by distributing the water over a large area to minimize local overheating and to prevent any significant loss of water. The container was then immediately closed and the determination completed in the usual manner. The results of these runs are shown in Table 111.
1
0.004
2
0.014
3
0.002
4
0.012
5
0,002
6
0.008
7
0.001
R
0.003
9
0,006
10
0.008
11
0.011
The data from columns V and F? of Table I11 are shown on Figure 4. Since circumstances required that the water content of the original caustic sample be determined by the analytical method being evaluated, it is necessary to assume that there is no constant error or bias-Le., line passes through the origin. The straight line, restricted to pass through the origin, that best fits the data is shown on Figure 4. The slope of this line does not differ significantly from the theoretical. This gives rise to the conclusion that there is no inherent error in the method of analysis. Thus, there is no correction to be applied to the results. If this is true, the deviations represent only experimental error. Examination of data obtained from duplicate determinations, for regular caustic samples tested under routine laboratory conditions, indicates that individual determinations fall within ~k0.012%of the true moisture content 95% of the time (Table IV). This estimate of error is less than would be predicted from the data in Table I11 for synthetic samples. There is more chance for error when performing the additional manipulationv involved in the preparation of synthetic samples, and this no doubt explains the larger error associated with the analysis of such samples. ACKNOWLEDGMENT
I t is a pleasure to acknowledge the assistance of Ralph E. Swackhamer in statistically evaluating the data given. LITERATURE CITED (1) Bidxell, G. L., and Sterling, W. F., Ind. Eng. Chem., 17, 147 (1928). (2) Lindsay, W. S . ,IND. ENG.CHEM., ANAL.ED.,18,69 (1946). (3) Lundin, Harry, Chem.-Ztg., 55, 762 (1931). (41 Suter. H. R.. IND.ESG. CHEX.. ANAL.ED. 19. 326 (1947). i5j Thielpape, E m s t , and Fulde, Alfred, 2. Ver: deut. Zucker-Ind., 81, 567 (1931). RECEIVEDfor review June 18, 1955.
Accepted August 17, 1955.
