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 0.1M 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 HC104titrant ( 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 several heavy metal acetates can be acid and acetonitrile (3, 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, HClO4 HCl Direct Bromide
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 (determinedby 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.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
1
I
100 mV
Figure 1. Titration of CU(OAC)~ in HOAc with HC104 0.1N in dioxane A. Direct B. In the presence of bromide ions
Figure 3. Differential titrations in HOAc with HClOd 0.1N in dioxane. Bromide ions are added after the first inflection A. NaOAc and CU(OAC)~ B. Ca(OAc)z and Cu(0Ac)z
Procedure. Samples, 0.20 meq, were dissolved in 60 ml of acetic acid. After addition of 5 ml of cetyltrimethylammonium bromide solution, the titrations were performed automatically, with O.1NHC1o4in dioxane.
RESULTS AND DISCUSSION
Figure 2. Titration of CU(OAC)~in glycol: I . Pr. O H 1.1 A . With HCI 0.1N in G :H; glass-calomel B. With HC104 0.1N in dioxane; glass-Pt
HC1 titrant were performed with a Radiometer pH meter 22 with glass and calomel electrodes. Reagents. All chemicals in our experiments were “pro analysi Merck” and were used without additional purification. The standard solutions of HC104 0.1N in dioxane and HCl 0.1N in G: H were prepared by the usual methods (9) and standardized against diphenylguanidine. Cetyltrimethylammonium bromide (purum Merck) was added as a 4x solution in acetic acid. (9) Ibid.,p 152.
In all cases examined, the potential jump at the equivalence point is greater and sharper than when the titrations were carried out in G:H medium, either with HClOd or with HC1 titrant. This is clearly illustrated in Figures 1 and 2, which show the titration curves for Cu(0Ac)z. The data obtained are reported in Table I and are expressed in percentages of the theoretical value. In some cases a precipitate is formed when adding bromide ions; however, it seems to have no influence on the results. With this method, it is also possible to differentiate mixtures of alkali- or alkaline earth metal acetates from heavy metal acetates in acetic acid medium. The mixture is first directly titrated in acetic acid. A first inflection is obtained when all the alkali- or alkaline earth metal acetate is neutralized. At this stage, cetyltrimethylammonium bromide is added in excess, and heavy metal bromide and acetate ions are formed, which increase the basicity of the solution. Upon continuing the titration, a second inflection is obtained, giving the amount of heavy metal acetate present in the mixture. The differential titration curves for mixtures of NaOAc and CU(OAC)~ and also of Ca(OAc)2are shown in Figure 3.
RECEIVED for review February 1, 1972. Accepted May 12, 1972.
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