the development of color. Thus, even impure ferrozine can be used with a reasonable amount of success. The molar ratio of iron(II1) to ferrozine must be maintained close to 1 : 3, at higher concentrations of impure ferrozine blanks become larger and are undesirable. The effect of temperature on the reduction of iron(II1) in the presence of ferrozine by sulfur dioxide is very small in the range from 20 to 45 "C. The absorbance in this range increases by less than lo%, which is quite contrary to the iron(II1)-phenanthroline system. It appears that at room temperature and at 50 "C, sulfur dioxide yields approximately one Fe(I1) per SOz indicating that sulfur dioxide has been oxidized to dithionate, SZO6*- with approximately 95 % efficiency in agreement with the following reaction: ~ C F ~ ( I I I ) A C , I ~ -6~f e r r 2 +2SOZ 2 H z 0+2 F e ( I I ) ( f e r ~ ) ~ - ~Sz062- 2(x)Ac-. 4Hf. Sulfur dioxide in the amounts as low as 3.0 pg and as high as 60 pg have been determined using 1-cm cells as shown in Table I. The relative precision is improved with increasing concentration of SOz; it changes from 4 % at 6.0 pg to 1 % at 15-pg level in the flow system. The negative deviation from Beer's law is observed at higher SO2 concentrations. This deviation probably results because of the incompleteness of SO2 oxidation by iron(III), evaporation, and mechanical losses while passing the air. Evaporation and mechanical spray at room temperature is in the order of 2 %. Amounts of formaldehyde, nitric, nitrous oxide, and mercaptans comparable to sulfur dioxide concentration had no apparent effect on the absorbance. Hydrogen sulfide seriously interferes and must be removed; chloride interferes if present in excess of 100 ppm.
Table I. Absorbance as a Function of Sulfur Dioxide Amount of SO?, pg/25 Absorbance at 562 nm for sample ml of final solution less reagent blank From aqueous bisulfite solution at 22 "C (1-cm cell) Observeda Calculatedb Corn. 3.01 6.02 15.05 21.07 30.10 33.11 36.12
0.050 0.101 0.255 0.345 0.480 0.503 0.535
h 0.004 + 0.003 i 0.002 f 0.002 =k 0.003 =t 0.003 f 0.003
0.052 0.105 0.261 0.366 0.522 0.575 0,627
96.2 96.2 97.6 94.2 92.0 87.5 85.2
+
From permeation tube in gas flow system at 22 "C (1-cm cell) 2.75 5.50 6.60 11.00 16.50 22.00 27.50
0.036 =t 0.006 0.090 0.004 0.110 =t 0.004 0.176 i 0.004 0.259 i. 0.004 0.348 i 0.004 0.425 f 0.005
*
0.039 0.096 0.115 0.193 0.289 0.386 0.481
94.8 93.8 95.6 91.2 89.6 89.5 88.5
Absorbancr: was determined on 25 ml of final solution containing 7.5 pmoles of ferrozine, and 1 ml of one molar acetate buffer of pH 3.8. t Calculated values are based on the molar absorptivity of 2.8 X 104M-1cm-1 for iron(II)(ferz)34-complex and the assumption that SO2 is oxidized to dithionate, S2062-. a
2.5 pmoles of iron(",
the blank and the sample of twice and four times recrystallized reagent correspond to approximately 0.025 and 0.008 absorbance unit/hour, respectively. However, if the blank is run side by side with the sample, the difference between the absorbances of the sample and the blank changes by less than 0.006 absorbance unit per hour. The absorbance change may be minimized by the addition of approximately 10 mg of sodium fluoride to the blank and the unknown after
+
+
+
+
RECEIVEDfor review December 30, 1971. Accepted March 30, 1972. Presented at the 161st National Meeting, ACS, Los Angeles, Calif., April 1971. Taken in part from the M.S. Dissertation submitted by Amir Attari to the Graduate School of Loyola University, Chicago, 111.
Ultrasonic Velocity in Water-Deuterium Oxide Mixtures A Basis for Deuterium Determination in Water Solutions J. G . Mathieson and B. E. Conwayl Department of Chemistry, Unioersity of' Ottawa, Ottawa, Canada INPROCESSES FOR ENRICHMENT of water in the deuterium isotope and in control of D content in heavy-water cooled nuclear reactors, it is desirable to have available a facile and direct method for isotopic analysis of H or D content in the water produced or used. Many of the methods available for analysis of protiumdeuterium oxide mixtures have been reviewed by Kirschenbaum (1). The mass spectrometric method is probably the most accurate available for gas phase or vapor samples; however, it involves a heavy financial outlay and requires, for 1
To whom correspondence should be addressed.
(1) I. Kirschenbaum, "Physical Properties and Analysis of Heavy Water," McGraw-Hill, New York, N.Y., 1951.
injection into the spectrometer, the sometimes technically difficult preparation of samples of elemental Hz, Dz, H D gases by exhaustive decomposition of a liquid aqueous medium. The gases, of course, must be representative of the sample and, hence, must not have suffered any change of isotopic composition during their preparation. Conditioning of the mass spectrometer for H / D analyses is usually required, necessitating virtual restriction of the use of the mass spectrometer to H/D analyses only, if reliable and reproducible results are to be obtained. Infrared spectrometry, on the other hand, is best applicable to solutions of either high deuterium or hydrogen content, while specific gravity methods require lengthy temperature equilibration, very accurate weighing procedures and also great care if they are to offer the required accuracy. Similarly, direct thermal conductivity determinaANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
1517
d - 5
A /
BALL JOINT
REFLECTORS
TRANSMITTINO TRANSDUCER
MAONETIC STIRRER
(b)
Figure 1. Cell and reflection arrangements for ultrasonic velocity measurements on solutions tions ( 2 , 3) depend on preparation of Hz/HD or DZ,or corresponding water vapors, in a high state of purity Le., without contamination with air, Oz, or Nz. Gas chromatographic methods (4) avoid problems of contamination by other gases but require gaseous samples of the input gas mixture in the form of Hz/HD/Dz. Great advantages accrue if direct measurements can be made on liquid mixtures of HzO and DzO, L e . , containing various proportions of HOD. Recently, instruments which can accurately measure the velocity of ultrasound in relatively small samples of liquids have become available ( 5 ) at a reasonable cost. Hitherto, ultrasonic velocity and corresponding adiabatic compressibility measurements (6) have been employed mainly to investigate solvation properties of ions in solution (7) including H z 0 / D 2 0solvent isotope effects in partial molar compressibilities (8). The convenience of the method, however, suggests applications to the problem of in situ direct HzO-DzO analysis, which are now described. The facility and accuracy with which the measurements can be made will lead to various practical applications. EXPERIMENTAL The method employed in the ultrasonic velocity measurements involves the so-called "sing-around'' principle (9). The apparatus used (NUS Corporation, Model 6105) employs two transducers and two reflecting surfaces in the liquid. A pulse of ultrasonic radiation is sent from a transducer, marked A in Figure l a , and reflected twice through the liquid being tested, back to a receiving transducer B. The transducers and reflectors are mounted in a corrosion resistant, stainless steel probe. After transmission of a given pulse, a transmission time t sis involved before the reception of the reflected signal at the receiver. This delay time is deter(2) C. C. Minter and S . Schuldiner, J. Chem. Eng. Data, 4, 223 (1959). (3) A. Farkas and L. Farkas, Proc. Roy. SOC.Loridon, A144, 467 (1934). (4) W. R. Moore and H. R. Ward, J. Amer. Chenz. SOC.,80, 2909 (1958).
(5) Model 6105, NUS Corporation (Velocimeter), Underwater Systems Division, Paramus, N.J. (6) B. E. Conway and R. E. Verrall, J . Phys. Chem., 70, 3952 (1966). (7) L. Laliberte and B. E. Conway, ibid., 74, 4116 (1970). (8) B. E. Conway and J. G. Mathieson, in course of publication. (9) R. Garnsey, R. J. Boe, R. Mahoney, and T. A . Litovitz, J . Chem. Phys., 50, 5222 (1969). 1518
+
u =
RECElVlNO TRANSDUCER
la )
mined by the sound velocity in solution. A further calibrated delay time id is also involved between the reception of the reflected pulse signal and generation of a new pulse from the transmitter transducer. Pulse repetition rate is determined by the total time t a t d and the measurement consists of counting the number of pulses received in a given time. A Philips decade scalar, timer, and printer were used to record the counts. The number of counts per second is the pulse frequency f, typically ra. lo5 sec-'. If I is the length of the sound path at 0 "C and CY is the coefficient of expansion of the stainless steel probe, then the sound velocity u in msec-1 is given by
ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
+ an
If(1 1-
tdf
where T is the temperature in "C. Calibration values of 1 and td were supplied by the manufacturer (IO), but the probe can be calibrated by velocity measurements in two or more pure liquids for which u is independently known. The manufacturer's values were used in this investigation after verification. The probe was fitted with a Plexiglas platform [see Figure lb), so that it could be sealed into a flanged solution container in such a way that no exchange of DzO would be possible with atmospheric water vapor. A further flanged cap could be fixed over the whole apparatus. The assembly was partly immersed in a large water thermostat set at 25 "C and accurately controlled by a 1.5-liter mercury-toluene regulatur, fabricated from a long length of copper tubing wound around the inside of the bath. Temperature control was better than 1 5 X "C. The solutions under investigation were stirred magnetically by a Teflon-coated bar which followed a submerged rotating magnet driven by a motor through a flexible cable. Initial solutions were made up by weight, the minimum sample size being about 85 ml. Much smaller samples could be employed in suitably designed apparatus. Dilution was effected by introduction of pure HzOor D20, using calibrated hypodermic syringes. In this way, the whole concentration range from pure HzO to pure DzO was covered. Initial temperature equilibration took about 15 to 20 minutes but only ca. 5 minutes were required for further composition changes effected by additions of DzO or H20 from the syringe. Forty-second count measurements were taken (4 X lo6 counts) until five successive determinations agreed to 1 4 counts, i.e. i l count in 108. This stability could be maintained only when a voltage stabilizer was connected to the ultrasonic pulsed signal generator and it is better than the i ~ 0 . 0 0 short 1 ~ term (24-hour) stability of the instrument as determined by the manufacturer (IO). The DzO used was Merck, Sharp and Dohme, 99.7 atom % D. The HzO was double-distilled conductance water of normal isotopic composition. RESULTS
A plot of sound velocity US. m o l x (mol fraction X 100) for the HzO-DzO system over the whole range of composiis shown in Figure 2. tions and in the range 0 to 6.2 mol The line, based on a large number of points, is almost straight but shows a small but significant curvature away from the composition axis. However, for practical purposes, the sound velocity may be considered equally sensitive to composition changes throughout the whole concentration range and is, therefore, a most suitable basis for HzO/DZOanalysis. Determination of relatively low concentrations of D in HzO or H in DzO (as HOD, of course) is often a matter of (10) Instruction Manual for Laboratory Velocimeter, Model 6105, NUS Corporation, Underwater Systems Division, Paramus, N.J.
Mol p e r c e n t 100
90
80
70
60
50
I
I
I
I
D20 40
IO
20
30
1,490 1.480
t
1,470
1.460 ‘g 1,45010
E 7.
1,440-
c .-
0
2 1,430‘p
3 1,420 cn
-
1,410
SCOIO
, 0
1,390
1
2
3
5
e
M o l % HpO
0
4
I
I
IO
20
I
I
I
I
I
I
30
40
50
60
70
80
Mol percent
Figure 2. Ultrasonic velocity at 25 “C as a function of mol tween zero and 6 mol H 2 0
z
ACCURACY OF ANALYTICAL MEASUREMENTS The change in sound velocity going from pure D20to pure H 2 0 is 97.7 msec-I or some 7 % of the value for pure DzO. On the basis of the instrumental stability of frequency (*0.001 the change in velocity caused by such a frequency change would be =k0.013 msec-’, which is ca. =t0.013% of the 97.7 msec-1 difference in sound velocity covering the whole composition range DzO to H 2 0 . Thus, the uncertainty in cdncentration associated with such a variation in frequency would be 0.013 % of H or D content. The temperature coefficient of sound velocity in water (13) is 2.4 msec-l “C-I or a negligible 10.001 of H or D content
z
(11) G. Walrafen, “Hydrogen Bonded Solvent Systems,” A. Covington and 1’. Jones, Ed., Taylor and Francis, London, 1968, Chap. 1. (12: B. E. Conway, A / m . Rea. Phys. Chem., 17, 481 (1966). (13) “Handbook of Chemistry and Physics,” 49th ed., Chemical Rubber Co., Cleveland, Ohio, 1969, pp EM.
0
HO ,
H 2 0 in H , 0 / D 2 0 mixtures over the whole composition range and be-
practical importance in following enrichment of HzO with D or adventitious dilution of D 2 0 by H in reactor fluids. Examination of the ultrasonic velocity data obtained in dilute solutions shows that satisfactory measurements can be made down to 0.015 % H in D 2 0 or 0.015 D in H 2 0 , Le., cu. 0.01 mole I.-’. HOD. Enlarged sections of the curve for the high dilution ends (0 to 0.5 mol %) are shown in Figure 3. The points are virtually exactly linear with concentration in this range but lie on different lines for H in DzO and D in H 2 0 . The data also allow the partial molar compressibility of D in H 2 0 or H in D20(as HOD) to be accurately evaluated (8). Such information is of interest in regard to current ideas concerning water structure (11, 12).
z),
I 90
06
Mol percent Component
Figure 3. Ultrasonic velocity at 25 “C as a function of composition (H in D 2 0 and D in H 2 0 ) at low isotope concentrations (0 to 0.5 mol
z)
0
D 2 0 in H 2 0 0 H 2 0 in D 2 0
for a 1 5 X “C temperature change or, for less sophisticated temperature control of 10.01 “C (e.g., by means of a contact thermometer and electronic relay), the above temperature coefficient represents an uncertainty in mol % of ~k0.024 in the composition. Practical applications of this technique may involve analysis of atomic pile heavy water. This may contain dissolved materials but these would not cause serious interference if they remained fairly constant in concentration and such solutions were included in the samples used in the calibration because it may be shown that +O.OlM change in concentration of a typical dissolved electrolyte (e.g., KCl) would be in the H/D content equivalent to an uncertainty of *O.l ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
1519
Table I. Contributions to the Uncertainty in Analysis as Variability of of conditions Pure components Temp. 1 5 X “C Pure components Temp. 1 0 . 0 1 “C Dissolved solute 10.01M in concentration
of H-D Content Dissolved solutes
Instrument stability
Temperature control
10.013
10.001
...
10.014
Xt0.013
10.024
...
10.037
3Z0.013
3Z0.024
10.1
of the solution. One further source of error in the method is possible long-term drift of the instrument, quoted (IO) as *O.Olx (3 months). Errors from this source can be eliminated by periodic redetermination of the calibration curve (Figures 2 and 3), or better by using a v - vo calibration, where vo is the sound velocity in the pure solvent, a figure which can easily be checked each day. It should also be noted that if comparison of results with those from other probes or analyzer units is not to be made, the frequency count itself provides a perfectly good basis for a calibration curve. The rapidity with which measurements can be made by this method depends mainly on the time taken for thermal equilibration. Thus, if two or three probes were in use with one pulse generator instrument, and counter, a rate of one determination in five minutes could be achieved. Table I, summarizes the accuracy which can be achieved in analysis of D20-H20mixtures by this method. The sensitivity of the method also makes it suitable for accurate determination of adsorption of substances from solutions, e.g., on high area electrode materials (14). The method, unlike UV spectrophotometry used previously for this purpose (14), is obviously not limited to aromatic molecules and (14) R. G. Barradas and B. E. Conway, J . Electronnal. Chem., 6 , 314 (1963).
Total
XtO, 15
can be quite generally employed for any substances, provided previous calibrations are made. “CONTINUOUS” SAMPLING In practical applications for analysis of H20-D20 or other liquid mixtures, continuous, on-line operation and analysis is usually advantageous. Strictly continuous measurements are not possible owing to the necessity of taking a sample reading over a 10- to 45-second period after thermal equilibration. However, slug-sampling of a liquid flowing in a line could easily be arranged with the probe held in a locally thermostated by-pass section of pipe or tube with admission of samples from the line controlled by a system of two magnetically controlled valves. A suitable mounting manifold for such purposes is available (5). ACKNOWLEDGMENT The interest of M.C.B. Hotz in this project is acknowledged, as are discussions with W. H. Stevens, Atomic Energy of Canada Ltd. RECEIVED for review December 2, 1971. Accepted January 27, 1972. Support of this and related work on a contract from the Department of Energy, Mines and Resources (Inland Waters Branch), Canada, is gratefully acknowledged.
Apparent Ionization Constants of Water in Aqueous Organic Mixtures and Acid Dissociation Constants of Protonated Co-Solvents in Aqueous Solution Earl M. Woolley Department of Chemistry, Brigham Young Unioersity, Provo, Utah 84601
Loren G. Hepler Department of Chemistry, University of Lethbridge, Lethbridge, Alberta, Canada
WE HAVE RECENTLY described (1) a rapid and convenient method for accurate determination of “apparent” ionization constants of water in aqueous organic mixtures. We have also shown ( 2 ) that these “apparent” ionization constants lead to a convenient method for evaluation of pK, values for ionization of aaueous acids with pKn < 16.
We report here an analogous convenient method for evaluation of pK, values for acid dissociation of protonated aqueous co-solvents with pK, > -2. We have applied this method to the determination of pK, values of protonated aqueous urea, N-methylformamide, and N-methylacetamide.
(1) E. M. Woolley, D. G. Hurkot, and L. G. Hepler, J. Phys. Chem., 74,3908 (1970).
(2) E. M. Woolley, J. Tomkins, and L. G. Hepler, unpublished work, University of Lethbridge. Alberta, Canada, 1970.
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
