(3),which is a time-consuming operation and is not amenable to analytical recovery of' isolated fractions. When the aqueous NaCl developer was reduced t o 2.5%, a complete separation of these two acids was obtained on the Amberlite XAD-2 resin. Figure 3 shows the elution diagram for a mixture containing 25 mg of each acid. The 4-biphenylsulfonic acid is strongly held by the resin and moves only when methanol is added. This behavior is shown by the large
volume of aqueous developer that can be collected free of the monosulfonate species. The resin columns are regenerated by washing with water and have been used for a variety of mixtures for up to 6 months without any indication of loss of separation efficiency.
RECEIVED for review on December 26, 1967. March 22,1968.
Accepted
Separation of Silver and Lead by Precipitation by Cation Release from Homogeneous Solution P. F. S. Cartwright Department of Chemistry, Sir John Cass College, London, E.C.3, England PRECIPITATION FROM HOMOGENEOUS solution by cation release can be effected by complexing the cation with EDTA, adding a solution of precipitating anion and slowly releasing the cation into solution by oxidation of the complex a t constant pH. The method frequently yields improved precipitates, but is not very selective because all the cations present are released into solution ( I ) . When more than one cation is present, selective precipitation can sometimes be achieved by adding an anion which will form a precipitate with only one cation under the conditions of the reaction. Alternatively, precipitation may be brought about in the absence of oxidizing agents by slou!y decreasing the p H of the solution t o reduce the stability of the complex t o a point a t which precipitation will occur. Although this method may appear attractive at first sight, there are few cases in which it has been successfully employed for the separation of cations. Complexes of metals with EDTA d o not break down completely at definite p H values, but over a range of p H depending on the concentration of other ions present and the insolubility of the precipitate that is formed (2,3). Overlapping of ranges of stability frequently occurs resulting in coprecipitation when similar metals are present. This difficulty can be overcome by selecting a mixture of suitable complexing agents. Thus, both silver and lead form complexes with EDTA, but with the former metal the complex is so unstable that precipitation of silver chloride occurs immediately on addition of chloride ion to a solution containing silver/EDTA complex. The method is clearly not suitable for precipitation from homogeneous solution. Attempts were made t o generate chloride ion in homogeneous solution by hydrolysis of ethylene chlorhydrin, but the decrease in p H that accompanies the reaction resulted in some release of lead. The method finally adopted was t o use EDTA t o complex the lead and ammonia t o complex silver. The silver was slowly released by boiling the solution t o volatilize ammonia according t o the method developed by Firsching (4). EXPERIMENTAL
Reagents. All reagents were of analytical grade. Stock solutions were prepared containing about 10 grams/liter of (1) P. F. S. Cartwright, Analyst, 92,319 (1967). (2) A. H. A. Heyn and E. Schupak, ANAL.CHEM.,26,1243 (1954). (3) P. F. S. Cartwright, Analyst, 87, 163 (1962). (4) F. H. Firsching, ANAL.CHEM.,32, 1876 (1960).
Separation of Silver and Lead Ag Rhodizonic taken, mg found, mg acid test 90.7 None detected 500 500 89.9 lo00 89.9 1000 91.1 500 245.6 Trace 500 244,6 None detected loo0 245.4 loo0 247.6 Lead present
Table I. Ag
taken, mg 90.8
245.6
Pb
silver and lead. The actual strengths of the solutions were checked by precipitating silver as chloride and lead as chromate. Method. Aliquot portions of silver and lead solutions were transferred t o beakers and sufficient EDTA solution was added to complex the metals. Ammonia solution was added t o a p H value greater than 10. After the addition of 5 grams of ammonium chloride the solutions were warmed o n a hot plate until the smell of ammonia was only faintly detectable. The solutions were filtered through weighed sintered porcelain crucibles, and the precipitates were washed with hot water. The filtrates were reheated to boiling t o ensure that precipitation was complete. Precipitates were dried t o constant weight at 120 " C and weighed as silver chloride. After weighing, the silver chloride precipitates were examined for the presence of lead by a spot test with rhodizonic acid. The results are shown in Table I . DISCUSSION
The method provides a n example of the use of EDTA in conjunction with a secondary complexing agent. During precipitation the lead/EDTA complex remains intact while the silver-EDTA and silver-ammonia complexes are dissociated because of instability and removal of ammonia. The stability of the lead-EDTA complex permitted a good separation of silver and lead t o be achieved. The amount of lead coprecipitated was reduced t o the point where it could only be detected by a sensitive spot test. The precipitates of silver chloride which were formed from homogeneous solution were dense and readily filtered.
RECEIVED for review November 14, 1967. Accepted February 19, 1968. VOL. 40, NO. 7,JUNE 1968
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