with nitroso compound for variable reaction times. Anhydrous acetonitrile, unlike its aqueous solution, was unsatisfactory because of a substantial blank reaction with potassium iodide and hydrogen chloride. All in all, the analyst should adopt a flexible attitude and fit the medium and reaction conditions to the particular N-nitroso compound at hand. With regard to the present determination of N-nitroso compounds, several matters require clarification. At least two standard sources state that such compounds do not oxidize hydriodic acid (16, 17). Clearly, this cannot be generally correct and should be discredited. Another cause for confusion may arise when C-nitroso and N-nitroso compounds are indiscriminately lumped together, as is sometimes done. In a semiquantitative investigation of the effect of hydriodic acid on C-nitroso compounds, Earl and Kenner showed that the production of iodine accompanies hydroxylamine formation (18,19). There are, in fact, at least three reactions which have to be con~
(16) S. Veibel, “The Identification of Organic Compounds,” G. E. C. Gad Publishers, Copenhagen, 1954, p. 286. (17) N. D. Cheronis, J. B. Entrikin, and E. M. Hodnett, “Semimicro Qualitative Organic Analysis,” 3rd ed., Interscience, New York, 1965, p. 406. (18) J. C. Earl and J. Kenner,J. Chem. SOC.,1927,2139. (19) J . C. Earl, F. C. Ellsworth, E. C. S. Jones, and J. Kenner, J . Chem. SOC.,1928,2697.
sidered here, two of which have been used for the analysis of C-nitroso compounds (Ar = aryl) (2, 7-9). R-NO
+ 2HI
+ ArNHOH ArNO + 4HI
ArNO
+ 12 ArN=N(O)Ar + HzO ArNHz + HzO + 21z +
RNHOH
+
+
(5)
Thus, C-nitroso and N-nitroso groups present different analytical problems and should be treated separately. Apart from any problems involved in storing and transferring some of the solutions, or choosing suitable reaction conditions, titrimetric methods involving HI, Cr(II), Ti(II1) appear to be comparable in convenience and accuracy, Certain group interferences, e.g. nitro, azo, double bonds, may preclude analyses with chromium(I1) or titanium(II1) (3). On the other hand, a few compounds which possess functional groups that absorb (or produce) iodine are unsuitable for iodometric analysis. Fortunately, such interferences tend to be specific, so that at least one of the methods can be employed for any given compound. RECEIVED for review August 28, 1967. Accepted October 12, 1967. Work supported by the Petroleum Research Fund of the American Chemical Society. E.R.S. is grateful to the National Science Foundation for an Undergraduate Participation Award. Inquiries should be addressed to S.I.M.
Determination of Oxygen-Zi iconium Ratio in Oxygen-Deficient Zirconia R. E. Heffelfinger, C. T. Litsey, D. L. Chase, and W. M. Henry Battelle Memorial Institute, Columbus Laboratories, 505 King Ace., Columbus, Ohio 43201 Two techniques are described for the determination of the 0 - Z r ratio in oxygen-deficient zirconia. The reaction between oxygen-deficient zirconia and chlorine a t 800° C is discussed as well as the reaction i n vacuum at 650° C between U,Os and oxygen-deficient zirconia. The results from the techniques are checked against thermal-balance data. Precision tests with the UaOsmethod gave oxygen-to-zirconia ratios ranging from 1.9852 to 1.9871 for eight determinations. The average was 1.9861; relative standard deviation was 0.03%.
IN STUDIES directed toward understanding the fundamental properties of compounds, it is necessary to have a precise knowledge of the composition of the materials, including their exact stoichiometry. In such investigations it is usually of interest to study compounds with small stoichiometric deviations. It is unrealistic to base calculations of small deviations on chemical determinations of bulk ratios because the possible deviation would be hidden in the usual precision level of comparing small differences in large numbers. Several chemical methods (1-4) have been described for stoichiometric (oxygen-to-metal ratio) determination of metal
oxides. These chemical methods usually are based on measurement of the oxidation or reduction that occurs during dissolution of the oxygen-deficient metal oxides in an acid. Because of the chemical nature of zirconium, these methods were not applicable to oxygen-deficientzirconia. The thermal-balance method (5) gives good precision when used to measure the oxygen deficiency in zirconia. The assumption used is that zirconia can be oxidized to a final composition of ZrOz.ooo. In actual trials we obtained by thermal balance a value of 2.0023 f 0.0023. As an adjunct to and as a check on the thermal-balance method, we are suggesting two separate methods to measure the oxygen-to-zirconium ratio in oxygen-deficient zirconia. The first of these is based on the reactions at 800” C between chlorine and zirconium and the second is based on the reduction of U30s by oxygen-deficient zirconia at 650” C. The first technique is an adaptation of chlorination techniques used for determining oxygen in refractory materials (6, 7). These methods, like the thermal-balance method, avoid the necessity of making a chemical dissolution and, like every technique we have considered, are based on the assumption that stoichio-
H. B. Sachse and G. L. Nichols, ANAL.CHEM., 33, 1349 (1961). H. B. Sachse, Ibid.,32, 529 (1960). H. J. Allsopp, Analysf, 82, 474 (1957). J. Deren and A. Kowalska, Chem. Anal. Warsaw,7, 563 (1962).
(5) S. Arenson, J . Electrochem. SOC.,108, 312 (1961). (6) J. Corbett, Analyst, 76, 652 (1951). (7) W. C. Lilliendahl, D. M. Wrough, and E. D. Gregary, Trans. Electrochem. SOC.,93, 235 (1948).
(1) (2) (3) (4)
VOL 40, NO. 1, JANUARY 1968
171
Table I. Replicate Analyses of an Oxygen-Deficient Zirconia Using Chlorination Technique Sample Zr evolved, g O/Zr weight, g 0.400 0,0160 1.8933 0.400 0.0166 1.8895 0.400 0,0160 1,8933 0.400 0.0150 1.8999 0.400 0.0146 1,9033 0.400 0.0152 1,8986 Av 1.8963 0.006 Std dev = 0.32 Std dev = ~~
-
Figure 1. Apparatus for determination of 0 - Z r ratio in oxygen-deficient zirconia (1) Tank chlorine (2) Sulfuric acid bubbler
Table 11. Results for 0-Zr Ratio as Measured by Three Techniques Thermal Specimen U308 balance Chlorination S-8696 1,9905 1,998 1,992 S-8697 1,9496 1.963 1.951 ... 1.815 1,771 S-8698 S-8699 ... 1.555 1.553 I . 063 1.041 s-8700 1.0667 ... 1,947, 1.943 s-8110 1.9457, 1.9475 1,9457, 1.9496 ... 1.953, 1.949
metric zirconia will form. The selection of one of these methods would be based on considerations of available equipment although, as will be shown, the technique involving U308may have some advantages. I. TECHNIQUE BASED ON REACTION BETWEEN ZIRCONIA AND CHLORINE The method as adapted consists of volatilizing the excess zirconium (present in oxygen-deficient zirconia) as the chloride at 800" C in a glass system in a stream of chlorine gas dried by use of a HzS04scrubber and a CaCh tower. The reaction is Zr 2C12 ZrC14. The chloride, which has an appreciable vapor pressure at 800" C, is vaporized and condenses in the reaction tube outside the heated zone. A residue of ZrOz remains in a Vycor boat and is taken to be stoichiometric zirconia.
+
-
Experimental. The details of the operation can be conveniently described by reference to the diagram in Figure 1 and the accompanying legend. A 0.2- to 2-gram sample of oxygen-deficient zirconia is weighed into a clean, dry silica boat (6) and put into the Vycor tube (4) which had been previously dried. Tank chlorine (1) is flowed through the sulfuric acid bubbler (2) and the CaC12tower (3), through the Vycor reaction tube (4), and finally into a hydrochloric acid trap (7) which prevents possible back-diffusion of atmospheric air and serves to trap ZrC14 vapor. The chlorine gas-flow rate for the first 20 minutes with the reaction tube cold is 1.5 liters per hour to purge the system. The flow rate is then reduced to 0.3 liter per hour and the furnace temperature brought slowly (about liZ hour) to 800" C and held for 1l/l hours. The furnace is allowed to cool, the chlorine gas flow stopped, the sample boat removed, and the zirconia reweighed. From the weight of the original sample and the weight-loss data, which represents the amount of excess zirconium in the sample, the 0-Zr ratio of the unreacted sample can be calculated. As an alternative, the amount of zirconium evolved from the sample can be determined directly by collecting all the evolved ZrC14 and determining the zirconium present. 172
ANALYTICAL CHEMISTRY
(3) Calcium chloride trap
~
(4) Vycor tube (5) Furnace windings
( 6 ) Silica boat containing sample
(7) Hydrochloric acid trap
Results and Discussion of Chlorination Technique. When oxidation-reduction methods which involve total dissolution of the zirconia failed, it was felt that perhaps the excess zirconium in oxygen-deficient zirconia would be removable by an acid that was known to dissolve zirconium but not atand HzS04 were used tack ZrO?. Subsequently, HCl, "Os, to try to leach out the excess zirconium. That these acids did not react with the excess zirconium but that chlorine gas did react may be attributed to possible differences in diffusion rates at the different temperatures. The acid treatment was done at 100" to 300" C, whereas the chlorine treatment was done at 800" C. The latter temperature should provide a much higher diffusion rate within the zirconia. As was mentioned earlier, the amount of zirconium evolved can be determined two ways. One is to determine the weight loss of a sample as the result of treatment with chlorine gas. Another is to collect all the ZrC14 evolved and determine the zirconium present. The latter is accomplished by bubbling the chlorine gas from the reaction tube through 6N HCI, carefully washing the ZrC14 deposit from the walls of the reaction tube, and finally making a determination of the amount of zirconium collected. For large amounts of zirconium (>lo mg) the determination can be made by precipitation of the zirconium as phosphate, and, for smaller amounts, an emission spectrographic solution spark or atomic-absorption determination is preferable. We have found that both methods of determining the amount of zirconia are useful. However, the weight-loss determination is obviously much faster. The precision of this method was studied by replicate analyses of an oxygen-deficient zirconia powder. The data from these analyses are shown in Table I. The standard deviation was calculated by the equation std. dev.
=
J".,
2
- The agreement between duplicate
runs on other materials is better than the 0.006 standard deviation, which suggests that the sample used in the precision determination may have been somewhat inhomogeneous. To compare the data from the chlorination technique with data obtained by another technique, we determined 0-Zr ratios on several specimens by the thermal-balance method. This involved oxidizing the oxygen-deficient zirconia at 750 O C at 100 torr of tank oxygen in a microbalance system. Cross checks between results from the chlorination method, the U308method and the thermal-balance method are shown in Table 11.
Sample S-8696 S-8697 S-8698 S-8699 S-8700
Table 111. Spectrographic Analysis of Zirconia Materials Impurity, wt A1 Si Fe Mg Cr < O , 003T
