Drastic reduction of the pH of water samples to prevent the adsorption of the micro amounts of silver of interest in this study did not appear promising. For potable water samples collected for the determination of silver, it is recommended that the sample be collected in the presence of sufficient scdium thiosulfate to produce a 10-15% Na2S203solution, thus eliminating the losses of silver to the container surfaces. Storage of samples for 30 days results in serious losses of silver to container walls
unless stabilization with thiosulfate is employed. An analytical procedure essentially free from evaporation, pipett h g , and other manipulative steps involving contact with various material surfaces is essential for accurate analyses of dilute silver solutions.
“The Bacteriological and Chemical Behavior of Silver in Low ConcentraR~~~~~ w 60-4,Health 1960. Service Technical tions,” Public (3) Dagnall, R. M., West, T. S., A m i .
Chim. Acta 27, 9 (1962). (4) Eichholst G. G.p A. R. B., ANAL.&EM. 37, 863 (1965). ( 5 ) Hensley, J. W., Long, A. O., Willard, J. E., Znd. Eng. Chem. 41, 1415 (1949).
LITERATURE CITED
(1) Chambers, C. W., Proctor, C. M., Kabler, P. W., J . A m . Water Works, A ~54, 208 ~ (1962). ~ ~ . (2) Chambers, C. W., Proctor, C. M.,
RECEIVEDfor review June 24, 1966. Accepted July 22, 1966. This investigation was supported by United States Public Health Service Research Grant Number WP 00788.
A Comparative Condensation Pressure Analyzer for Uranium Hexafluoride Purity W. S. PAPPAS, S. A. MaclNTYRE, and C. W. WEBER Technical Division, Oak Ridge Gaseous Diffusion Plant, Union Carbide Corp. Nuclear Division, Oak Ridge, Tenn.
b
A simple pneumatic analyzer has been developed for continuous purity determination of vapor streams. The condensation pressure of the impure sample stream is automatically compared to the vapor pressure of a pure sample at the same temperature. The comparative instrument provides a more sensitive and accurate measurement than can be obtained by direct condensation pressure methods. A prototype model analyzer has given several years of trouble-free service where it is used to monitor the purity of a uranium hexafluoride system. A standard deviation of 10.014 mole % uranium hexafluoride was attained. The pneumatic response can b e fed to a control valve for process control.
P
URITY OF URASIUM HEXAFLUORIDE
is normally determined by gravimetric (6) or freeze point (10) methods aud, by difference, using mass spectrographic ( 2 ) and infrared techniques (7) to analyze for impurities. The acoustic gas analyzer ( I ) , constriction response analyzer (9), and condensation pressure analyzer ( 6 ) have been used to monitor uranium hexafluoride systems. At this laboratory it was important to monitor continuously the purity of a uranium hexafluoride system to within 10.1 mole %. The above analyzers were inadequate due to poor precision or lack of specificity. Since the condensation method is simple and specific, it was selected for further development to fill this need. The condensation pressure analyzer (5) provides a recording of the sample condensation equilibrium pressure a t a constant temperature. For ideal mix1570
ANALYTICAL CHEMISTRY
TRANSMITTER
T-$ ’ VACUUM
PRESSURE
SIGNAL
MOLE FRACTION UFg = Po/Pz OR MOLE FRACTION I M P U R I T I E S * ( P ~ - P o ) / P ~ OR MOLE FRACTION IMPURITIES =API/(Po+API)
Figure 1.
Comparative condensation pressure analyzer
ture the condensation pressure varies inversely with the concentration of that constituent which is first to condense a t the selected temperature and prevailing molar concentrations. The reliability of the instrument is dependent upon the thermal stability of the condensation detector and the accuracy of the condensation pressure measurement. For the desired analytical accuracy of *0.1% uranium hexafluoride, using the direct condensation pressure approach, the detector temperature could vary no more than j=0.006’ C. and the equilibrium pressure would need to be measured t o within *0.05% of the value, assuming the errors were additive. The temperature requirements are calculable from the known vapor pressuretemperature relationship for uranium hexafluoride (3). This paper describes a method (4) in which the sample condensation pressure is compared to the vapor pressure of the pure condensable component a t the same temperature; thus, the effects of temperature fluctuations are minimized. For relatively pure samples, accuracy of
the comparative pressure measurement is also less critical than in the direct method, as errors apply to minor constituents only, PRINCIPLES
I n the new analyzer (Figure 1) a sensing bulb containing pure uranium hexafluoride vapor and solid is placed in the cool bath with the analyzer capillary (5). The sample condensation pressure, P2, is measured relative to the reference pressure, PO. Because temperature variations affect both pressures (Pa and P,) identically, thermal errors are eliminated. I n the following discussions it is assumed that the dynamically measured vapor pressure (P2for a pure sample) is equal to the static vapor pressure, PO. I n practice the dynamic measurement is slightly lower due to pressure loss, as it is made downstream of the condensate. This results in a small shift of the calibration curve which is easily determined with a pure sample. Purity Ratio. At condensation equilibrium,
DIRECT CONDENSATION PRESSURE, P2
E
c
roo-
I
I
PURITY OR IMPURITY R A T I O , Po/Pp OR APiIPpI
I
4 6 MOLE P E R c E w IMPURITIES
I
\ I 8
IN
IO
ur,
Figure 2. Analytical errors caused by bath temperature variation of 0.1 ' c.
Mole fraction uranium hexafluoride = vapor pressure P O sample pressure P Z (1) An inexpensive force bridge (8) will continuously perform this division, giving a purity determination to better than 1% relative accuracy. Impurity Ratio. The difference in PO, provides a the pressure, P Z measure of the partial pressure of the impurities. Therefore, Mole fraction impurities =
For high purity systems, accuracy is less critical in measuring this impurity ratio, as relative errors of measurement apply to the minor constituent impurities. For esample, a t 1% impurity, a 5y0error in the impurity ratio measurement influences the analysis by only o.O5y0absolute. Pressure Difference Method. If the bath temperature is maintained constant to within k 0 . l o C. (at 41' f 0.1' C., Po is 300 =k 2.4 mm. Hg abs.), a direct measurement of the pressure difference (Ps - PO) provides acceptable absolute accuracy. Component variables are reduced, the system is simpler, and temperature compensation is almost complete in the range of interest. Equation 2 can be rewritten, Mole fraction impurities
=
where AP1 = P2 - PO. If AP1 is monitored as the analytical response, the calibration curve is not linear. However for relatively pure samples 4P1 is very small compared to Po and the denominator of Equation 3 is essentially constant. The calibration curve is almost linear between 0 and 2% impurity. At 1% impurity nonlinearity amounts to only 0.01% absolute. Where only APl is measured, temperature compensation is not complete. Figure 2 shows: the purer the sample, the better the compensation; a t 0% impurity, compensation is complete. Figure 2 also presents the thermal ef-
Figure 3.
Continuous process analyzer
PCV -Pressure control valve S -Solenoid valve A -Regulated air supply TRANS.-Transmitter
fects on the ratio methods and the conventional condensation pressure analyzer. DESCRIPTION
OF PROCESS MODEL ANALYZER
Several analyzers using the pressure difference measurement are now in routine use for analysis of uranium hexafluoride in the range 95100%. A schematic drawing of this instrument is presented in Figure 3. All components exposed to the condensable sample gas are housed in a n insulated cabinet maintained at about 60" C. The analyzer is self-contained, requiring only a n instrument air supply and electrical power. Sample Flow Pattern. Normal sample flow path is designated by the darkened line with arrows. Prior to the analytical system, the sample passes through a manifold, where a standard sample of pure uranium hesafluoride can be automatically introduced when desired. Sample gas enters the analytical system through the inlet control valve, PCV 1, passes through the cool capillary and the flow element needle valve, and leaves at a controlled rate through the outlet control valve, PCV 2. Flow Control System. A differential pressure t y p e flow controller limifs the flow and also isolates the controlled pressure zone from any variations in lowside process pressure. The flow control system consists of the needle valve, the AP transmitter (range 6-12 mm. Hg) and the outlet control valve, PCV 2. The transmitter output pressure is fed to the top of the control valve through solenoid valve S 2. Flow settings are made by adjusting the fixed pressure below the diaphragm of the flow control valve. A ballast is connected to the output system with a long tube to dampen out cycling and pneumatic 'hoise" of the control. This device is analogous to a resistor-capacitor combination in a n electrical circuit. The solenoid valve,
S 5, locks the output control pressure in the btlllast during periods when the instrument is bypassed, so. that the system is readily put under control again when the analyzer is placed on stream. The Bath. The bath consists of a 1200-ml. stainless steel can, filled with water. This bath, of course, is the coolest zoiie through which the uranium hesafluoride sample gas flows. It is insulated with 2 inches of Styrofoam. A magnetic stirrer minimizes thermal gradients in the bath. Cool Capillary. A 5-inoh loop of 0.125-inch o.d., 0.061-inch i.d. copper tubing serves as the cool capillary. It enters and leaves through the lower wall of the can where i t is sealed with silver solder. It is connected to the warmer lines with nickel tubing to avoid excessive heat transfer to the bath. The range of the capillary AP control transmitter is 3-9 mm. Hg differential pressure. UF6 Bulb. The reference bulb is actually a 4-fOOt length of '/a-inch standard copper tubing coiled within the bath. It also is solder-sealed through the lower wall of the can and connected with nickel tubing. It is filled with pure uranium hexafluoride vapor and contains only a slight excess to maintain a solid phase. Temperature Control. By a vapor pressure thermometer-controller, the absolute pressure developed by the purified uranium hexafluoride reference is monitored and used as the control signal to regulate the bath temperature. The bath is placed adjacent to the warm section of the cabinet. Heat conducted to the bath from the warm cabinet plus the mechanical heat from stirring was sufficient to maintain the bath slightly above the desired temperature of 41' C. Therefore air-cooling was satisfactory for control. The reference uranium hexafluoride pressure is sensed by a suppressed VOL 38, NO. 1 1 , OCTOBER 1966
1571
Figure 4. analyzer
Rear view of comparative condenrotion pressure
range pressure transmitter (Figure 3). The transmitter output pressure is recorded and is fed back to control the cooling air which is passed through a ’/rinch a d . cooling coil to maintain a constant UFs vapor pressure-Le., constant bath temperature. Using a pressure transmitter with a suppressed range of 297 to 303 mm. Hg ahs., the full range temperature span is only 0.3” C. A Taylor proportional-and-reset pressure controller (Model No. 360 RF 1) is useful in dampening out control oscillations. The inexpensive pneumatic temperature regulator controls the bath temperature to *0.003° C. Not shown in the preceding diagrams is a refrigeration unit which provided a lower sir temperature for cooling, permitting operation at high ambient temperatures.
Table I.
Comparison of Impurity Analyses
Com-
Darative conden-
Other methods
sation
pressure analyzer,
% (0,
%total
impunties
+ NB)
% HF
0.35 0.00 0.61
0.12 0.04 0.56
0.23 0.06 0.06
1572
ANALYTICAL CHEMISTRY
% Total impurities 0.34 0.10 0.62
Figure 5. Control panel of comparative condensation pressure analyzer
Analytical Pressure Measuring System. The difference in pressure between the sample condensation pressure P1 and the reference pressure Po is monitored as the analytical signal. The pressure Ptis fed to the high side of the AP signal transmitter, while the reference bulb pressure is fed to the low side. With a 30-mm. Hg range AP transmitter and the reference pressure fixed a t 300 mm. Hg abs. by the temperature control, each division of the differential indicator is equal to about 0.1% impurity. The AP signal is also recorded by one pen of the twcpen pressure recorder. Figure 4 shows the rear view of the instrumental assembly. The chamber on the right is insulated for 60” C. operation. OPElUllNG PROCEDURE
The control panel is shown in Figure 5. (As indicated, it has also been referred to 88 the relative pressure analyzer.) The comparative condensation pressure analyzer is a fully automatic instrument. An operating switch is provided to valve the instrument onand off-stream. When switched to “Analyze” position, the outputs of the control transmitters are directed to the inlet and outlet control valves and the spring closes the bypass valve, PCV 3; solenoid valves S 4 and S 5 also open (see Figure 3). When switched to “Bypass” position, to take the analyzer off-stream, the inlet and outlet control valves. PCV 1
and PCV 2, are closed by the 22 p.s.i.g. air admitted through solenoid valves S 1 and S 2. Simultaneously, 22 p.s.i.g. air is relieved through solenoid valve S 3 to open the bypass, PVC 3; and solenoid vnlves S 4 and S 5 close, locking the control pressures in the ballast tanks. A similar vnlving arrangement is provided for automatic standardization with pure uranium hexafluoride. For intermittent use, the quick startand-stop feature keeps the capillary “primed” and eliminates waiting periods associated with forming a new deposit in the cool capillary upon start-up. PERFORMANCE
The instrument has given troublefree service for several years at the gaseous diffusion plants a t Oak Ridge, Tenn., Paducah, Ky., and Portsmouth, Ohio, where it monitors impurities in flowing uranium hexafluoride systems, Impurities in the systems are nitrogen, oxygen, and hydrogen fluoride. Thermal Compensation. A deliberate shift of 0.2O C. of the detector bath temperature caused an error of less than 0.03% absolute a t 0.4% impurity. Comparison with Other Methods. Using the theoretical calibration curve instrumentally zeroed with a purified sample, the instrument was checked on three occasions by specific analyses for impurities; hydrogen fluo-
ride by infrared, and oxygen and nitrogen by mass spectrometry. Each time, the new analyzer gave total impurity analysis within 0.01% agreement with the other methods. Table I shows the results of these tests. During periods when the hydrogen fluoride concentration is very low, the total impurities measured by the analyzer are in good agreement with mass spectrometer analyses for nitrogen and oxygen. Precision. Using the instrument routinely and standardizing twice a week, a standard deviation of ~ 0 , 0 1 4 ~uranium 0 hexafluoride has been maintained. Results can be improved by refined calculations to
account for the small temperature fluctuations; also, because trace irnpurities could form or be retained in the reference bulb, analytical results might be improved by a technique of direct comparison with the very pure standard sample. LITERATURE CITED
(1) Bogardus, B. J., Ritter, R. C., U . 8. At. Energy Comm. Repi. K-1240 (1959). (2) Gentry, W. O., Zbid,, KD-1560 (TID6529) ..-. (1958). I
\ - - - - ,
(3) Oliver, G. D., Milton, H. T., Grisard, J. W., J. Am. Chem. SOC.75, 2827 (1953). (4)Pappm, W. S., U. S. Patent No. 3,234,780 (Feb. 15, 1966).
(5) Pappas, W. S., Weber, C. W., ANAL. CHEM.37.407 (1965). (6) Rodden; C. J., "Analytical Chemistry of the Manhattan Prolect, p. 44, McGraw-Hill, New York, 1950. (7) Smith, D. F., Spectrochim. Acta 12, 224 (1958).
(8)Sorteberg, Force Bridge, MinneapolisHone well Regulator Co., Philadelphia, Pa., eatalog C 80-1, March 1957. (9) Weber, C. W., Pappas, W. S., ANAL. CHEM.37, 1221 (1965). (10)Werts, R. J., Hedge, W. S., U. S. At. Energy Comm. Rept. K-1418 (1960).
RECEIVEDfor review March 14, 1966. Accepted August 4, 1966. Sixth Conin fererence on Analytical Chemist Nuclear Reactor Technology, Gatlizurg, Tenn., October 1962. Work performed at the Oak Ridge Gaseous Diffusion Plant operated by Union Carbide Corp. for the U. S. Atomic Energy Commission.
Dissociation of 4-Aminopyridinium Ion in 50 Weight Per Cent Methanol-Water and Related Acidity Functions from 10" to 40" C. MAYA PAABO, R. A. ROBINSON, and ROGER G. BATES National Bureau o f Standards, Washington, D . C.
b The dissociation constant of 4aminopyridinium ion in 50 wt. methanol-water has been determined by the electromotive force method from 10' to 40" C. The pK value a t 25" C. is 8.520 compared with 9.1 1 4 in water. The changes in free energy, enthalpy, and entropy in the dissociation process have been calculated. Values of p(aH7cl) and pan* are given for buffer solutions containing equimolal amounts of 4-aminopyridine and its hydrochloride.
%
4-AMINOPYRIDINEIt
is a useful standard for the direct titration of aqueous also provides a acid solutions (9). convenient acid-base system for pH control in the pH range 8.5 to 9.5 where the undesirable side reactions of borate buffers often preclude the use of borax. It could be equally useful in 50 wt. % methanol-water. We have measured the dissociation constant in this solvent medium from 10" to 40" C. and derived pa=* values for solutions containing equimolal amounts of 4aminopyridine and its hydrochloride. I n addition, the measurements enable us t o compare the medium effect for the dissociation of Caminopyridinium ion in 50% methanol and in water (3) with medium effects which have already been observed for ammonium ion (6) and for the protonated form of tris(hydroxymethyl) aminomethane (10) in these two solvents.
EXPERIMENTAL
The cell P t ; HZ (gas, 1 atm.), NHzCsHlN (m), NHzCJ&N.HCl (m) in 50 wt. % methanol, AgCl; Ag was used; m designates molality. The p r e p aration of the electrodes and other experimental details have already been described (1, 7, 10). 4Aminopyridine was available from previous work (3). A stock solution was prepared by dissolving 4aminopyridine in standard aqueous hydrochloric acid. The number of moles of the base added was exactly twice the moles of acid present. Sufficient methanol was then added to adjust the solvent composition to 50 wt. yo. This solution was further diluted to give the solutions which were used in the cell measurements. The e.m.f. values of the cell, corrected to 1 atm. of pressure of hydrogen gas, are given in Table I. The e.m.f. of the cell can be expressed as :
E
=
,Eok log mH+'mcl-'~H+*ycl(1)
or
( E - ,E")/k
+ log mcl- = Pa(aEYCl)
(2)
Here sE" is the standard potential of the silver-silver chloride electrode in 50yo methanol. Its values from 10" to 40" C. have already been determined (8) (see Table 11) ;
k = 2.3026RT/F, and p.(aEycl)
E
- log, (aH+yC1-)
The acidic dissociation constant of Paminopyridinium ion is
where B stands for aminopyridine and BH+ for the aminopyridinium ion. Then, p.K = ( E log
- ,E")/k
mcl-
+
+ log msa+/mB+ log
YCI-YBE+/YE
Here, mcl- = m, mBH+ = m me = m - moa-.
(4)
+ moH-,
The molality of hydroxide ion was calculated with the approximation : log moH+ = 1% KS
+ p.(wcd,
(5)
using a value of the apparent ion product or autoprotolysis constant, Ks, interpolated from the data of Koskikallio (6). The correction amounted to less than 0.001 in.p,K but was made nevertheless. Putting log yci-
*
YBH+/YB =
- 2 L I Z " ~ + 81 (6)
Equation 4 becomes p,K'
G
p,K
- PI
=
+ + + moH-)/(m - moa-) -
(E - B")/k log mcllog (m
2 AZl'Z VOL 38, NO. 1 1 , OCTOBER 1966
(7) 1573
