feature of this technique is that it is not necessary to identify the reference peak or any peak other than that of chrysotile. Having selected a reference peak, integrated intensities for the reference peak (ZT)and the 002 reflection of chrysotile (I,) were obtained by the step-scanning procedure described previously. Next, an addition of a known amount of chrysotile (typically 50 pg) was made by filtering a known volume of suspended chrysotile through the sample filter, as already described. The sample was dried and the intensities of the two peaks reevaluated. This process was repeated for 5 additions of chrysotile. Finally, the ratios of the two peak intensities were obtained and plotted (Figure 2) against the amount of chrysotile added to the sample. The graph was extrapolated down to the X axis and the intercept used to estimate the amount of chrysotile in the original sample. RESULTS AND DISCUSSION
The two methods described above can be applied to most situations where an estimate of trace amounts of chrysotile is sought. In the evaluation of environmental airborne chrysotile referred to earlier (2), these particular samples (from 1000 m a of air) had no detectable chrysotile peak and the chrysotile content was therefore below the limits of the X-ray detection methods, Le., less than 0.1 pg/ma. The samples did contain appreciable amounts of quartz and kaolinite but quantitative estimation of these components was not attempted. Electron microscopic methods have now proved sufficiently sensitive to detect amounts of chrysotile of the order of 0.1 to 1.0 ng per cubic meter and will be reported in detail elsewhere. However, the X-ray methods can be used in a number of other cases, such as mixed industrial atmospheric dusts and other instances where the final samples can be obtained on a membrane filter. The ultimate amount of sample on the membrane filter must be small, to ensure that the specimens are thin and thus reduce errors due to specimen thickness. This is of particular importance in the internal standard method, where chrysotile is added to the sample on the membrane. If the sample is too thick, then there are errors due to layering of the chrysotile additions. In practice, additions of 50 pg units of chrysotile to 10 mg of sample hardly affected the intensity of the reference peak, confirming that the errors due to layering are minimal. The detection limits of the two methods were 10 pg for the external standard method and, depending upon the nature of
Mass of chrysotlle pg) added to the s=ample
Mass d ChWSOtlle (!.IO) in the original sample.
Figure 2. Extrapolated graph of additions of chrysotile plotted against diffraction intensity
the sample, between 50 and 100 pg for the internal method. Standard deviations for the external method were 10% for 10 pg and 2 z for 100 pg. The most reliable region is clearly 50-100 pg and in practice this has been found to be the most useful range within which to operate. The methods have been developed especially for evaluation of chrysotile. In principle, however, there is no reason why the methods cannot be applied to any crystalline material, subject to the limitation of producing suitable calibration samples of sufficiently low concentration. Further improvements to the detection limits will require the development of more sensitive electronic equipment and the use of more powerful X-ray tubes. ACKNOWLEDGMENT
The author thanks G . F. Heron and D. V. Badami of the Research and Engineering Division, T.B.A., for their support and encouragement, and the Directors of T.B.A. Co. Ltd. for permission to publish this work. RECEIVED for review February 11, 1972. Accepted April 18, 1972.
Gravimetric Analysis of Uranyl-Orthophosphate Mixtures J. M. Schaekers Atomic Energy Board, Department of Physical Metallurgy, Private Bag X256, Pretoria, South Africa
MIXTURES OF URANIUM AND PHOSPHATE are frequently encountered, especially in connection with the extraction of uranium from low grade ores, and with the study of U(1V) compounds. The analysis of these mixtures, either as a solution or as a solid, is rather tedious, as it normally requires a separation step on a cation exchange resin column in hydrochloric acid medium (1-3). A gravimetric method, in which the UOz2+is ~~
(1) J. A. Goudie and W. Rieman, ANAL.CHEM.,24, 1067 (1952). (2) S. M. Khorkar and K. de Anil, Anal. Chim. Acta, 22, 153
(1960). (3) I. M. Kolthoff and P. J. Elving, “Treatise on Analytical Chemistry,” Part 11, Vol. 9, Interscience Publishers, New York, N.Y.,
1960-63.
-
precipitated as U02NH4P04 XHzO, has been described (4-7) but it has been criticized (8)as not being very accurate. A variation of the same method has also been used for the gravimetric determination (5, 6, 9) of P043- (‘‘P043-”is used (4) A. A. Smales and H. N. Wilson, Rept. BR-150(1943). ( 5 ) L. G. Basset, D. J. Pflaum, et al., Rept. A-2912 (1949). (6) C. J. Rodden. “Analytical Chemistry of the Manhatten Project,” McGraw-Hill Book Co., New York, N.Y., 1950. (7) G. W. C. Milner, D. H. Rowe, and G. Phillips, Rept. AEREA4906 (1965). (8) W. B. Schaaf, L. S. Andrews, and J. W. Gates, Jr., Rept. CD-4002 (1945). (9) E. R. Caley and C. W. Foulk, J. Amer. Chem. Soc., 51, 1664 ( 1929).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
1873
~~
~
~~~
~~~
~~
~~~
Table I. Determination of UOS2+
7
Sample UO*(N03)2solution
Mole useda Mole found Deviation 1.002 X 1.003 X +0.10 1.002 x 10-3 1.001 x 10-3 -0.10 1.002 x 10-3 1.004 x 10-3 1-0.20 1.002 x 10-3 1.002 x 10-3 0.00 U02Cln solutionb 1.001 X 1.014 X +1.30 1.001 x 10-3 1.012 x 10-3 + i . i o 1.001 x 10-3 1.009 x 10-3 1-0.80 1.001 x 10-3 1.018 x 10-3 $1.70 Uo~(N03)2solution 1.019 X 1.035 X $1.57 2 mlHC1 (concd) 1.019 X 1.032 X +1.27 1.019 x 1.035 x +1.57 1.019 X 1.029 X $0.98 UO2(No3)* solution 1.019 X 1.026 X $0.69 2 ml HzSOa(concd) 1.019 X 1.025 X $0.59 0.019 X 1.029 X +0.98 1.019 X 1.024 X $0.49 Uoz(No3)* solution 1.019 X loT3 1.019 X 0.00 2 ml HCI (concd) 1.019 X 1.020 X 10-3 $0. 10 evaporatedtwice 1.019 X 10-3 1.021 X 10-3 +0.20 with HN03 1.019 x 10-3 1.017 x 10-3 -0.20 These are the average values of at least five determinations by the ADU -,U308method. This solution was made by adding an excess of U 0 3 to HCl (concd). The excess solid product was afterwards filtered off.
+
+
+
here instead of "orthophosphate ion", which may in fact have been combined with one or more hydrogen ions, depending on the pH of the solution) but complete precipitation without contamination by UOZ2+ has always been a problem. DETERMINATION OF UOz '+ Criticism of the Existing Method (7, 10). The normal procedure in this method is to neutralize the nitric acid solution containing UOz2+and an excess of Pod3- with an ammonia solution (1 :4). It is evident that this solution must be free from cations which form insoluble phosphates or uranates and also from ions which precipitate under the analytical conditions. The precipitate is filtered after digesting for 30 minutes on a hot water bath, is heated to a temperature between 700 "C (10-13) and 1000 "C (7,14,15) and is weighed off as (U02)2P207. However, by thermogravimetric analysis of carefully prepared samples of U02NH4P04.3H20 and U 0 2 H P 0 44H20, . the following has been established. Pure (UOS)SPZO~ cannot be prepared from U 0 2 N H 4 P 0 43H20 . due to the reducing action of NH3 which is released between 300 and 450 "C. In the presence of burning paper, a reduction takes place and uz03P207, which has a green color, is always formed even below 700 "C. This compound has also been reported by Klygin (12) but he did not discuss its stability. (Uo2)2P207can be prepared from U02HPO4.4HS0 at temperatures between 550 and 650 "C under nonreducing
(10) A. von Weiss, K. Hartl, and U. Hofmann, 2. Naturforsch., 128, 669 (1957). (11) V. I. Karpov and Ts. L. Ambartsumyan, Russ. J . Ztiorg. Chem., 7, 949 (1962). (12) A. E. Klygin, D. hl. Zavrazhnova, and N. A. Nikol'skaya, J . A t i d . Clzem. USSR,16, 311 (1960). (13) V. Pekarek and V. Veseley, J . Inorg. Nucl. Clzem.. 27, 1151 (1965). (14) C. Duval, "Inorganic Thermogravimetric Analysis", sec. ed., Elsevier Publishing Company, Amsterdam, 1963. (15) E. A. Ippolitova, N. I. Pechurova, and E. N. Gribennik, Rept. ANL-Trans-33, p 114-126, (1964). 1874
conditions. Prolonged heating at 750 "C, however, will result in contamination with U20&07,except in a pure oxygen atmosphere. (UOz)zP207is a pale yellow product and very hygroscopic. It does not, however, form a definite hydrate, and it is therefore not suitable as an end product in gravimetric analysis. Uz03P207 cannot be reoxidized, not even in a pure oxygen atmosphere. Uzo3Pzo7 is hardly hygroscopic and a maximum weight increase of only 0.2 %, due to water absorption, could be observed for extremely fine powders, which had been exposed to atmospheric air for more than three weeks. Uz03Pz07is stable in air up to 1200 "C, but in inert atmospheres such as N2 and Ar, further decomposition commences at 950 "C with the formation of (UO)zP20j. A loss of phosphorus may occur in a reducing atmosphere, such as HS, at temperatures >700 "C, with the formation of UOS. Recommended Method. An amount of sample, which will give approximately 0.1500 g US03PzOi,should normally be used. This is dissolved in a suitable medium, preferably nitric acid, and the volume is adjusted between 100 and 200 ml. Phosphoric acid is then added until a small excess of Pod3- is present, nitric acid being added, if necessary, to obtain a clear solution. After addition of a few drops of methyl red indicator, the solution is heated and, while stirring, neutralized by addition of a 1 :4 ammonia solution until the indicator changes to yellow. At this stage all the UOz*+has been precipitated, and the mixture is digested on a hot water bath for 30 minutes. The precipitate is then filtered off on a Whatman No. 42 paper and washed with a 3 to 5 (by weight) NH4N03solution to remove the excess phosphate. After smoking off the paper, the precipitate is heated in a tared porcelain crucible to 850 "C for approximately 30 minutes. After cooling in a desiccator the product is weighed as Uzo3Pzo7. Results and Discussion. The results of a few determinations are given in Table I. It is clear that an accuracy of h0.27, can be obtained if pure nitrate solutions are used. Large quantities of chloride should be avoided because of difficulties in its removal by washing that can lead to high results, as shown in the second and third series of results in Table I. The presence of sulfate ions has a similar effect. DETERMINATION OF ORTHOPHOSPHATE The solubility product of U02NH4P04 (P = [NH4+][UOS2+][P043-]) has been reported (12) to have a value of 2 X At pH 5 it is 1 X indicating that com10-15 at pH 7. plete precipitation of Po43- can still be effected, provided sufficient UOS2+and NH4+are present. However, the excess uranium will be precipitated as ammonium poly uranate, ~ ) 0 ( 4 + 3 ~ )under , these conditions. (This product is (NH4)1U(l+ normally called ammonium diuranate or ADU.) The product starts precipitating between pH 4 and 5 in solutions of approximately 0.1N (15). Due to the fact that the composition of this product depends largely on the pH and ion concentration of the solution, no accurate solubility data are available. It is, however, evident that the product is much more soluble than U 0 2 N H 4 P 0 4and it is therefore possible to prevent ADU coprecipitation by adding a weak complexing agent, without increasing the solubility of UO2NH4PO4too much. Acetate is very convenient for this purpose, In addition to its complexing action it will also buffer the solution, when used together with NH4+. The buffer capacity of such a mixture is suficient to allow a slow adjustment of the pH to 5.5, which is the equivalent point of methyl red. This then also provides a visual method for pH adjustment in the solution. Criticism of the Existing Method. In the method, described in the literature (6), the acid solution is neutralized
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
I 0.5
I 1.0
I 2.0
1,5
x IO-*MOLAR
I
2.5
uo:+
3,O IN
I
3.5
4.0
FINAL SOLUTION
4.5
I 5.0
(200 rnl)
Figure 1. Amounts of NH4Ac required under various conditions
-.-: _.
Absolute lower limit: ADU still formed at pH 7 Recommended lower limit: No ADU formed at any stage during the determination a : 10 ml HNO,(concd) in final solution A : 20 ml HN03(concd) in final solution c : 30 ml HN03(concd)in final solution
with a 1 :4 ammonia solution after addition of sufficient ammonium acetate, The pH of the solution can be determined visually by adding methyl red indicator. However the color tends to fade quickly during neutralization and rather large amounts of indicator are consequently required. This will result in a positive error if the sodium salt of the indicator is used, because the insoluble and thermally stable sodium polyuranate, N a 2 U ~ l + z ~ 0 ~ 4will + 3 z be ) , formed under these conditions. As far as the decomposition of the precipitate is concerned, the remarks made for the U0z2 determination are applicable. Recommended Method. For the determination of phosphate, it is supposed that only orthophosphate is present and that, just as for U022', all interfering ions are absent. The following method can be used when only a small excess of UOZ2+is present and when only the minimum amount of acid is used to obtain a clear solution. A suitable size of sample is dissolved in HNOBand diluted to nearly 200 ml. A small excess of U02*+and 1 g of solid NHdAc are added and the solution is clarified by addition of "03 (concd). The solution is heated and ammonia solution (1 :4) added. As the pH approaches 6, some methyl red indicator is added and more of the ammonia solution until a definite yellow color is obtained. The mixture is then reacidified with acetic acid (1 :10 solution) until the indicator changes to an orange-red. After digesting for 30 minutes, the precipitate is filtered off on a Whatman No. 42 filter paper and washed with 1 % (by weight) NH4Ac solution. The precipitate is further treated as in the U022' determination and also weighed as Uzo3PzOi. +
Results and Discussion. The results of a few deterrninations are given in Table I1 and show that an accuracy of +0.4z can be obtained with pure nitrate solutions. Deviations will be found if chloride or sulfate ions are present, unless in very small quantities. It can be expected that large amounts of U022-+ can not be kept in solution by 1 g solid NH4Ac and it is also evident that the concentration of 4"' will influence the solubility of ADU, and therefore the amount of acetate required to prevent an excess of U022+ to be coprecipitated. The required
Table 11. Determination of Pod3-
?
Sample H3POasolution
Mole useda Mole found Deviation 1.072 X 10-3 1.073 X +0.09 1.072 x 10-3 1.069 X -0.28 1.071 X -0.09 1.072 X 1.072 x 10-3 1.076 X +0.37 H3POasolution + 2 ml 1.040 X 1,088 X +4.23 HC1 (concd) 1.040 X 10-3 1.092 X lo-, + 5 .OO 1.040 x 10-3 1.077 x +3.56 1.040 X 1.083 X t4.13 H3P04solution 2 ml 1.040 X 10-3 1.086 X +4.42 H2S04(concd) 1.040 X 1.079 X +3.75 1.040 X 10-3 1.074 X +3.27 1.040 x 10-3 1.078 x lo-, +3.66 H3P0d solution + 2 ml 1,040 X lo-, 1.044 X + O . 38 HCl (concd) evap1,040 X 10-3 1,042 X loT3 + O . 19 orated twice with 1,040 X 1 0 - 3 1.041 X +O. 10 "03 1.040 X 10-3 1.039 X lo-, -0.10 a These are the average values of at least five determinations by potentiometric titration. Two colorimetric determinations were also done with comparable results.
+
amount of NH4Ac under varying conditions was determined in the following way. Test solutions, containing varying quantities of UOZ2+and H N 0 3 , which is partly responsible for the final NH4+concentration, but no PO4", were prepared. The amount of ",OH which will produce a precipitate at pH 7, after neutralizing with N H 4 0 H ,but still yield a clear solution after reacidifying to pH 5.5 with HAC,was determined. The results, for solutions containing 10 ml HNO, (concd) are given in Figure 1 (dashed line). These values are the minimum values which can be used. However, ADU precipitate may exist as inclusions, which are difficult to dissolve and larger amounts of NH4Ac should be used. The recommended values then, are those which will not produce any precipitate at any stage of the analysis if PO4,- is absent. These values are also given in Figure 1 (solid lines).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
1875
At the end of each experiment a few drops of 0.1M Hap04 were added to the test solutions and each time a precipitate could be observed, indicating that a complete removal of Pod3- could still be effected. The influence of large excesses of NH4Ac was also investigated and up to 12 g extra solid NH4Ac were added (e.g., the following mixtures: 40 ml 0 . 1 M UOzZ+, 10 ml "03 (concd), 3 g NH4Ac 12 g NHdAc, diluted to 200 ml; and also: 10m10.1MUOz2+,10 ml HN03(concd), 1 gNH4Ac 12 g NH4Ac, diluted to 200 ml). A few drops of 0.1M H3P04still produced a visible precipitate in these solutions after they were neutralized to pH 5.5 in the usual way. It can therefore be concluded that, to prevent ADU precipitation, sufficient NH4Ac must be added. The recommended minimum quantities, under different conditions are given by the solid lines in Figure 1. Excessive amounts of NH4Ac can be used as they will not prevent the complete removal of Pod3-.
+
+
PROCEDURES FOR DISSOLVING SOME U-P-0 COMPOUNDS Almost all uranium phosphates can be dissolved in HNOa (concd) and it is recommended that the solutions should be boiled for a short time to ensure complete conversion of U(IV), if present, to UOZ2+. UP207is a very stable compound and cannot be dissolved in H N 0 3 (concd) or in aqua regia, not even by boiling. However it is soluble in an equal mixture of NH40H (concd) and HzOz(30%), especially if the mixture is gently heated. Excess HzOz should be destroyed by repeated evaporation with H N 0 3 until a transparent residue is obtained. ACKNOWLEDGMENT The author thanks Cynthia Bennett for the assistance in the laboratory.
RECEIVED for review, November 19, 1971. Accepted February 22, 1972. The Atomic Energy Board of South Africa gave permission to publish this work.
Acidimetric Titration of Heavy Metal Acetates M a r c G . Mannens Research Laboratories, Ada-Gevaert N . V., Mortsel, Belgium ACIDIMETRIC TITRATION of heavy metal acetates is generally performed in a mixture of glyco1:isopropanol (1 :1) (Le. G : H) or in methanol, with HC1 or HC104 titrant ( I , 2). Hg(OAc)z cannot be titrated in these media with HC104 titrant unless a halide isadded (1,3). Undissociated mercurichalide is formed and the acetate ion can be titrated as a base, according to Equation 1. Hg(0Ac)z
+ 2 KC1
-+
HgClz
+ 2 KOAC
(1)
In acetic acid, very poor end points are obtained for the titration of most heavy metal acetates with HC104, because of the high degree of association of such compounds in this medium (2,4-6). In mixtures of acetic acid and chloroform (4, 7) or of acetic acid and acetonitrile (3, several heavy metal acetates can be titrated very sharply. In the latter case, the metal ion probably forms a complex with CH3CN, thus freeing the acetate ion for reaction with HC101, according to Equation 2. M(0Ac)z
+ x CH3CN * M(CH3CN),Zf + 2 OAC-
Table I.
Analysis of Heavy Metal Acetates HOAc: HOAc: HClOa, HC104, G:H, G:H, Direct Bromide HClO4 HCl
72.8% 73.2% 72.6% Co(OAc)za ... 100.4 100.4 100.4 N ~ ( O A C ) ~ . ~ H Z O. . . 100.8 100.0 99.7 Cu(OAc).HzO ... Not soluble AgOAc 99.7 100.5 Zn(OAc)z,2HzO ... 100.2 100.6 100.8 Cd(0Ac)z. 2H20 ... 100.4 100.5 101.3 Hg(0Ac)z" ... 100.2 ... 100.7 Pb(0Ac)z ... 94.4 94.4 93.5 a CO(OAC)~ and Pb(OAc)z have a water content of 26.7 and 4.5 %, respectively (determined by the Karl Fischer method).
(2)
We now find that most of the heavy metal acetates can be titrated potentiometrically in HOAc, if an excess of cetyltrimethylammonium bromide is added to the solution. This phenomenon can undoubtedly be explained by assuming the formation of sparingly dissociated metal bromide, as shown in Equation 3. (1) M. N. Das, Ziidiarz Chem. Soc., 31,9 (1954). (2) R. B. Rashbrook, Aiialysr (Loiidoii),87, 826 (1962). (3) K. K. Kundu and M. N. Das, ANAL.CHEM., 31,1358 (1959). (4) A. P. Kreshkov and L. B. Kuznetsova, Zh. Anal. Khim., 24, 380 (1969). (5) J. S. Fritz, ANAL.CHEM., 26, 1701 (1954). (6) A. T. Casey and K. Starke, ibid.,31,1060 (1959). (7) C. W. Pifer, E. G. Wollish, and M. Schmall, ibid., 26, 215 (1954). 1876
EXPERIMENTAL
Apparatus. All the determinations with HC104 titrant were performed with an automatic titrator (Titromatic QuCrC) provided with a glass-indicator electrode and a Pt-reference electrode placed in the flowing titrant (8). Titrations with (8) I. Gyenes, "Titration in Non-Aqueous Media," Iliffe Books Ltd., London, 1967, p 303.
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