1922
ANALYTICAL CHEMISTRY
titration for a microburet. Suitable amounts of europium are in the range 0.1 to 0.4 meq., which will require 2.5 to 10 ml. of 0.04N potassium dichromate. The rare earth sample must be purified to remove materials reducible by zinc. This is done by oxalate precipitation of the rare earths and ignition of the oxalate to oxide. Dissolve the sample of rare earth oxide in hydrochloric acid, evaporate to a sirup under an infrared lamp to remove excess acid, and dilute to about 20 ml. with distilled water. Flush the Jones reductor column with 150 ml. of 0.05N hydrochloric acid. ;\dd 20 ml. of 0.02N ferric chloride solution to a 500-ml. Erlenmeyer flask. Attach the flask to the Jones reductor with a threeholed rubber stopper fitted with a glass tube for introducing carbon dioxide gas below the liquid level in the flask. Bubble carbon dioxide through the solution in the flask for 7 minutes to remove air. iZdd the 20-ml. sample solution to the top of the reductor, and follow with 150 ml. of 0.05N hydrochloric acid wash solution; 10 minutes are required to pass the sample and wash solution through the reductor. Remove the flask, add 4 ml. of concentrated hydrochloric acid, 5 ml. of 85% phosphoric acid, and 3 drops of 0.3% sodium diphenylamine-psulfonate solution, and titrate with standard 0.04N potassium dirhromate solution. DISCUSSION
Kolthoff (1)states that if ferrous iron is titrated in the presence of hydrochloric acid with diphenylamine compounds as indicators, the end point is poor because the indicator is oxidized before the end point is reached. The addition of phosphoric acid lowers the oxidation potential of the ferric-ferrous iron system by forming a complex ferric phosphate ion, thereby avoiding premature oxidation of the indicator. By trial and error, the quantities of phosphoric acid and hydrochloric acid used were found to give a very sharp end point. The method is designed as a control method for europium purification techniques. The wide range of the method and its reliability are shown by the following typical experiment.
A sample of samarium acetate analyzed by the method was found to contain 2.21% Eu2O3in the rare earth oxide. A solution of 315 grams of the samarium acetate containing 159.7 grams of rare earth oxide was extracted nine times with liquid sodium amalgam to separate europium by the method of Marsh ( 4 ) . The rare earths from the extracts and residue were recovered by precipitation as oxalates, and analyzed for europium.
Table I.
Extract 1 2 3 4 5 6 7 8 9 Residue Total Rwowwd
Separation of Europium from Samarium Rare E a r t h Oxide, Grams In Sample for extract analysis 2.40 0.1419 1.90 0.1814 1.19 0.1778 5.27 0.4621 1.80 0.1872 3.56 0.2551 0.4609 30.65 0.1716 2.03 3.40 0.2719
0.0124.V KdhOT, All. 13.80 13.77 4.83 0.61 0.12 0.12 0.06 0.07 Si1
EuzOi in Rare Earth Oxide, 5% 72.55 56.62 20.30 0.98 0.48 0.35
Eui01 In
Extract, Grams 1.71 1.10
0.08
0.30 Nil
98.00
0.24 0.0.5 0,008 0.012 0.024
0.006 Si1
-~
150.26
3 18
X o t all of the rare earths taken n e i c recovered, because of
unavoidable incomplete precipitation of rare earth oxalate and mechanical losses in the eutractions. The data are given in Table I. The precision of the method is adequate. With rare earth ouide samples containing 2 to 3% EulOa, analyses can be duplicated to within 1% precision, and with essentially pure europium oxide the difference between duplicate samples is less than 0.5%. The method was checked against pure europium oxide prepared by McCoy (5) and results were consistently 3.3 to 3.5% low. Europium materials prepared in this laboratory known to contain a maximum of 0.2% impurities by spectroscopic tests also analyzed about 3% low. The cause of these low results could not be found, but this does not prevent the procedure from being a very useful tool for following a europium separation. It is possible to standardize the potassium dichromate solution empirically against pure europium compounds. LITERATURE CITED
(1) Kolthoff, J. A I , and Sandell, E. O., "Textbook of Quantitative Inoreanic Analvsis." u. G08. Sew York. Nacmillan Co.. 1935. (2) McCoy, H. N., A h . C h e m ' S o c . , 58, 1577-8 (1936). ' 13) I b i d , 59, 1131-2 (1937). (4) Marsh, J. K., J . Chem. Soc., 1943, 531-5.
RECEIVED for review March 9, 19%. Accepted August 13, 1953
Polarographic Analysis of lead Driers D. A. SKOOG AND ROBERT L. FOCH'I" Stanford University, Stanford, Calif. soaps are widely used in the paint industry as driers, and L numerous methods of determining the lead content of such EAD
materials have been reported. I n general, these methods require a preliminary separation of the metallic ion from the organic acid, followed by the determination of the lead by one of the standard procedures. I n the American Society for Testing Materials method ( I ) , the sample is wet oxidized with nitric and sulfuric acids, followed by determination of the lead as sulfate. Gracis ( 4 ) recommends separation of the lead by hydrolysis of the soaps with nitric acid. The lead is then determined in the aqueous solution by titration with molybdate. Castiglioni ( 2 ) hydrolyzed lead naphthenates in acetic acid and then removed the naphthenic acid by extraction with ether. The lead was then determined as the chromate. Gavarret (5) analyzed lead driers by dissolving in benzene or trichloroethylene and extracting the lead with nitric acid. The lead was determined in the extract by the ferrocyanide method. 1 Present address, International Minerals a n d ChemioaL Corp., San Jose, Calif.
411 of these procedures are time-consuming and tedious, and in the search for a simpler method the polarographic behavior of lead soaps dispersed in aqueous medium by means of a detergent was investigated, Preliminary experiments showed that most of the common lead driers could be readily suspended in aqueous dodecylamine acetate to give clear solutions. Such dispersions gave typical polarographic waves for lead, and it was found that the wave heights were directly related to the concentration of the lead in the driers. These experiments led to the development of a very simple and rapid procedure for the analysis of lead driers which does not require the preliminary isolation of the metallic ion from the organic substrate. This paper gives a detailed description of the method and the important variables affecting the results obtained by the procedure. MATERIALS Ah-D APPARATUS
Lead Soaps. Several lead driers were obtained from commercial sources. These included lead naphthenate, octoate, linoleate, and resinate from Frederic A. Stresen-Reuter, Inc., Chicago, I11 .; and lead naphthenate, octoate, and linoresinate from Advance
V O L U M E 2 5 , NO. 1 2 , D E C E M B E R 1 9 5 3 Solvents and Chemical Corp., New York, X. Y. These were analyzed for lead content by the ASTM method ( 1 ) for liquid driers. Dodecylamine Acetate, 0.1M. Solutions were prepared from dodecylamine or dodecylamine acetate obtained from Armour and Co., Chicago, 111.. under the trade names of .4rmeen 12D and Arniac 12D, respect,ively. To prepare a 0.1M solution from the dodecylamine acetate, 25 grams of the solid were dissolved in hot water and made up to 1 liter. Solutions were also prepared from the dodecylamine hy adding 19 grams of the reagent to 5.8 ml. of glacial acetic acid and diluting to 1 liter. The solutions were clear when first prepared; however, with standing a haze slowly developed. This did not appear to affect the results in any way. Lead Acetate Solutions. A standard 0.02M lead acetate solution was prepared for calibration purposes by dissolving 6.5 grams of the reagent in water and diluting to 1 liter. This solution was analyzed for lead gravimetrically as lead sulfate and then used for preparation of more dilute standards by accurate dilutions with the dodecylamine acetate solution. Apparatus. Most of the current-voltage measurements were made Lvith a Fisher Elecdropode. The galvanometer scale was calibrated at the various sensitivities, and where half-wave potentials were to be determined, voltages were measured with an auxiliary potentiometer and corrected for the ZR drop of the cell. A few curves i\-ere obtained with a Sargent Model XXI polarograph. The polarographic cell was an H-type with a sintered-glass d a t e in the cross arm. One side of the cell was a saturated calohe1 electrode which was used as the reference electrode. The cell temperature : ~ q inaiiitained at 25' & 0.3' C .
Table I.
Half-Wave Potentials for Reduction of Lead Soaps (Solution- 0.1.V in dodecylamine acetate) E l / ? DS. Saturated Lead Compound Calomel Electrode Lead acetate -0.484 Lead naphthenate -0.494 Lead octoate -0.50.; Lead linoleate -0.497 Lead linoresinate -0,484 Lead rwinate -0.484
EXPERISIEZITA L
All of the driers investigated, with the exception of leiid resinate, wei'e rapidly dispersed in 0.1X solutions of dodecylamine acetate to give clear stable solutions with no undispersed residues. I n the case of the lend resinate, dispersion was incomplete and small particles could be detected even after long agitation and heat.ing. These particles, \~-l-lien removed by filtration, were found to contain lead, and it. IRIS concluded that polarographic anal?-sis of this particular soap from detergent solut'ions would le:td to low results. This W:LP mbsequently confirmed. The solutions containing the dispersed driers were found to yield well-defined polarograms for lead. Current masima were never encountered, and no suppressors were required in any of this Tvork. The half-n-sve potentials for the lead wave varied slightl). for the different soaps, as is illustrated in Table I. However, the potentials are the same as. or only slightly more negative than, the half-warp potential for lead acetat,e in 0.1X dodecylamine acetate solution. Furthermore, the slopes of the vaves for lead soaps were found to be the same as for the lead acetat.e solutions. These data and observations indicate that the lead soaps are appreciably dissociated in the detergent media and that the dissociation equilihriiim is rapidly attained. An investigation of the effect of detergent concentrations on the lend polarograms n-:ts undertaken. I t was found that sufficient mnounts of the driers for analytical purposcs could be dispersed even in 0.005X solutions of the dodecylamine acetate. However, the more dilute detergent solutions gave high values for limiting current's because insufficient electrolyte was present t,o eliminate the migration current. This is shown in Table 11. Constant current's are obtained for detergent concentrations above 0.05.V. Attempts were made to reduce the migration cur-
1923 rent in the more dilute detergent solutions by addition of such eleot'rolytes as sodium acetate and potassium nitrate. However, coagulation of the dispersed soaps occurred and these esperiments were abandoned. The diffusion currents for lead from 0.131detergent solutions were found to be directly proportional to the lead concentration; the proportionality constant was the same for lead acetate and the various lead soaps investigated with the exception of the lead resinate. This is illustrated in Table 111, where the values for the diffusion current over concentrat'ion for lead acetate and lead naphthenate are compared a t different lead concentrations. Similar values w r e found with the other soaps studied. Thus, standard solutions of lead acetate can be used for calibration purposes for t.he malysis of any of the common lead driers. RECORISIENDED PROCEDURE
Kcigh out samples of 0.05 to 0.1 gram into 50-ml. glassstoppered Erlenmeyer flasks. Measure exactly 25 nil. of the 0.1JI dodecylamine acetate into the fla i \%-itha pipet, and shake the mixture vigorously until disper on is complete and a clear solution is obtained. Rinse the polarogralihic cell with this solution and finally fill the cell. Remove the oxygen from the solution by bubbling with nitrogen. During this process, foaming will occur and some of the solution may be lost. However, this does not alter the results in anv Tray. After the on-gen has been removed and the solution brought to ronstnnt temperature, a polarogram is obtained between -0.2 and -1.0 volt versus the sat,urated calomel electrode. The height of the lead wave a t -0.5 volt is detwniiiied and blank correction is made by running a solution of 0.1.11 dodec>.lamine acetate under the same conditions. The 1e:rd concentration in the solution is calculated from t,he diffusion current hy reference to calibration data obtained with a ntand:irtl solution of lead acetate in 0.1:11 dodecylamine ac'ctate or from samples of driers of known lead concentration which have h e n treated by the above procedure. The diffusion current is dirrctlj, proportional to t h p c3oncentr:ition of lead. RESULTS AND DISCUSSIOU
h number of commercial driers were analyzed by the above procedure and the results are given in Table IV. The data are compared with results by the American Society for Testing hIaterials procedure ( 1 )involving wet ashing and gravimetric estiniation of the lead as lead sulfate. The results compare favorably in every case but the lead resinate. This material could not be completely dispersed in the medium and the method cannot be applied satisfactorily to this type of sample. The main advantage of the procedure lies in its simplicity and speed. A single analysis can be completed in 20 minutes and when
Table 11. Effect of Dodecylamine Acetate Concentration on Limiting Currents for Lead (2 X 10-3.V solutions of Pb-') Dodec ylamine Limiting Current, pa. Acetate Concn., .?I 0.005 20.5
n nin
18 7
0 050 0.080 0.090 0 100
16.9 16.7 16.6 16.7
Table 111. Relationship between Diffusion Current and Concentration for Lead Acetate and Lead Naphthenate in Dodecylamine Acetate Solutions Lead -4cetate Lead concn., C , millimoles/liter
Av.
Id/C
8.35
Lead Naphthenate Lead concn., C , milli moles/liter id/C
8.38
ANALYTICAL CHEMISTRY
1924
TabIe IV. Analysis of Lead Driers Type of Drier
h‘aphthenate Ootoate Linoresinate Linoleate Resinate
Lead Analysis, % Polarographic Gravimetric 24.6 24.8 23.5 23.8 23.8 23.7 17.1 16.9 23.4 23.4 23.5 23.3 24.6 24.6 27.9 27.7 19 6 21.4
Deviation from Gravimetric Method, &I. % -0.8 -1.3 +0.4 +1.2 0.0 $0.9
+o.o
10.7 -8.4
polarographically to determine whether this technique might not have wider application. These included iron, cobalt, manganeae, copper, and zinc naphthenates. Although a wide variety of conditions was tried, only copper gave a satisfactory wave. Apparently the remainder are so slightly dissociated in aqueoua suspensions that no reduction occurs below the decomposition potential for the supporting electrolyte. Further work on the polarographic analysis of copper soaps is now in progress and the results of this work will be published subsequently. LITERATURE CITED
several analyses are to be done, an even smaller time per sample is needed. The procedure should be particularly useful in the control of dilute driers (4 to 6% lead) where the errors of the method wouId not be serious. The naphthenates of several other metals were investigated
( 1 ) American Society for Testing Materials, Philadelphia, Pa., “Book of A.S.T.M. Standards,” Part IV. p. 251, 1952. (2) Castiglioni, A., Ann. chim. applkata, 39, 197 (1949). (3) Gavarret, J., Congr. tech. intern. ids. peintures id. msoc., 1 , 203
(1947). (4) Gracis, E., Ind. vernice 5, 58 (1951).
RECEIVED for review May
12, 1953.
Accepted September 8, 1953.
Calculation of Amount of Tracer Carried with Precipitates of Its Radioactive Parent FAUSTO W. LIMA Escola Politecnica, Universidade d e Szo Paulo, s“ao Paulo, Brasil
receiit note Kirby showed how to calculate the proporIgenetically tion of measured radioactivity due to each nuclide, when related radioisotopes are employed to follow a chemi(1)
N A
cal reaction. A similar type of reasoning can be used to study carrying of genetically related radioisotopes when they are in equilibrium before fractionation by precipitation. Consider the case of the following radioactive disintegration: E1
+
E2
E, +
Defining A&’AY = a as “entrainment,” we can write
AI = A: [,(,-’It
- e - X z f ) + a e-xzt 1
Since, from now on, only the activity of element E2 will be considered, we shall use A as meaning the activity, &, measured a t time t after separation of precipitate and filtrate. Measurement of A a t two different times t’ and t”-i.e., A ‘ and A”-and substitution in Equation 3 will give two equations and two unknowns, A: and a. Solving for the entrainment a, one gets:
Suppose that a suitable absorber is used to screen out all radiations except that from the fiist decay product, Ez. The activity of nuclide E2 a t time tis ( 3 ) :
(4)
I n a special case where X2 > > XI, m 1 ; and if t is sufficiently small so that --e-ht is very nearly equal to unity, for the times ts of measurement, Equation 4 will simplify to
where
A:
(3)
activity of the parent a t zero time A i = activity of the daughter a t zero time A, and X p = decay constants of parent and daughter, respectively m = A,/(& - A:) =
“Zero time” is taken as the time when separation is completei.e , the precipitate is no longer in contact with the solution Suppose that A, is much smaller than A2 and that El and E2 are in secular equilibrium and consequently A? = A : , where AI is the activity of the parent at time t. Suppose that element E1 is precipitated and that some E2 is entrained with the precipitate. We wish to calculate the amount of E , cariied with El. Rather than measure the activity of E2 before and after precipitation, we shall use Equation 1 and measure the activity of the precipitate only. Since Az/AI = 1 (or is equal to m if equilibrium is not secular) before precipitation, the deviation fiom unity (or from m ) , of A42/d1,measured in the precipitate, will indicate the percentage of E2 carried with El. We shall see that it is not necessary to measure AI; it is sufficient to measuie As a t two different times. Equation 1 can be written alternatively as
A z --. : 4 [m(e-xlt
- e-hZt)
+ ,-ht
A;IA?I
(2)
A’ a = l -
A‘e-Azt”
- A” - A”e-X2t’
In fact, absolute activities are not measured, and all values for A should be multiplied by a constant of proportionality; however, since counting of a sample, a t two different times, is done under the same conditions the constant of proportionality will cancel out. Kirby ( 2 ) called attention to the fact that the condition of equilibrium before fractionation or precipitation is not stringent and that the ratio of parent to daughter in a nonequilibrium mixture could be calculated with the same Formula 4 by counting an unfractionated aliquot of the mixture. Also, he showed (2) that Equation 4 could be applied when the parent had not been quantitatively precipitated or carried. I n this last case the required algebraic transformations are:
where AI is the total activity of parent at the time of fractionation. Al/A: is calculitted from Equation 8 of Kirby’s work ( 1 ) :
(7)
