The radiometric titration of trace amounts of zinc

International Journal of Applied Radiation and Isotopes, 1968, Vol. 19, pp. 23-26. Pergamon Press. Printed in Northern Ireland

The Radiometric Titration of Trace Amounts of Zinc A. R. LANDGREBE* N a t i o n a l B u r e a u of Standards, W a s h i n g t o n , D.C. 20234 and W. O. McSHARRYt and M. CEFOLA'~ F o r d h a m University, Bronx, N e w York 10458

(Acceptedfor publication 1 August 1967) A m e t h o d o f r a d i o m e t r i c titration for the d e t e r m i n a t i o n of trace a m o u n t s of metals has b e e n developed. T h e E D T A complex of the m e t a l to be d e t e r m i n e d is formed, a n d the excess, u n c o m p l e x e d m e t a l ions are r e m o v e d b y m e a n s of a cationic exchanger. A titration curve is t h e n constructed b y plotting the activity of the liquid phase, vs. the a m o u n t of E D T A added. Both the b a t c h a n d c o l u m n m e t h o d were tried in this laboratory, a n d it is our opinion t h a t the b a t c h m e t h o d is easier from a m a n i p u l a t i v e p o i n t of view. A new m e t h o d of d e t e r m i n i n g the e n d p o i n t was developed. I f b o t h the resin a n d solution are counted, this is a simple way of d e t e r m i n i n g the h a l f equivalent point. M e t h o d s are u n d e r w a y to m a x i m i z e the sensitivity for zinc b y chelating radiometric titration. LA TITRATION

RADIOMI~TRIQUE DE QUANTITES DE ZINC

DE TRACE

O n a mis 5. p o i n t u n e m 6 t h o d e de t i t r a t i o n r a d i o m f t r i q u e p o u r la d 6 t e r m i n a t i o n de quantit6s de trace d e m6taux. O n forme le compl6xe E D T A d u mdtale ~ mesurer et o n 6te le surplus &ions non-compldxds a u m o y e n d ' u n 6changeur cationique. Ensuite on construit u n e c o u r b e de titration, soit de l'activit6 de la p h a s e liquide contre la q u a n t i t 6 d ' E D T A ajoutde. O n a essay6 dans ce l a b o r a t o i r e les mdthodes et de groupe et de colonne, et il est n o t r e avis que la m d t h o d e de g r o u p e est plus facile d u p o i n t de r u e manipulatif. O n a mis ~ p o i n t u n e nouvelle m 6 t h o d e de r e c o n n a l t r e le p o i n t de terminaison. Si on c o m p t e et la r6sine et la solution, cela r e n d u n e fa~on simple de trouver le p o i n t de mi-dquivalence. Des mdthodes sont sur c h e m i n de maxlmiser la sensibilit6 p o u r le zinc p a r la titration de ch6lation radiom6trlque. PA,~HOMETPHHECKOE THTPHPOBAHHE MHHPOt~OJIHLIECTB I~HHHA t3LIJI paapa6oTaH MSTO~ pa/II~OMeTpn~ecHoro TnTpllponaHna ~Iff onpe~eaeHna MnHpoHO~IatleCTB MewaJIJIOB. O6pa3yewc~ O ~ T A HOMIIZleHC nO~JlemalIlero onpe~e~eHnio MevaJiJia, n n3JInliiHlle nOHI,I HeKOMnJIeKcHoro MewaJiaa ycwpaH~IOWCa n p n imMOmn I~aW~10HO06MeHn~tKa. 3aWeM C0CTaBJmeTClt Kpnnaa TI4wpHpoBannff nocTp0eHeM aHTnBHOCTI~ mn/lHOl~ ~aBb! IIpOWnB npH6aBJIenHoro HOoInqeCTBa O,~TA. HaK nopunonnLifi, TaK rt HOJIOnnt~I~ cnoc06 ncnhIWl~Inaancl~ n OToft Jia6opaTopm~, n no H3ILIeMy MHenlltO IIOpIInOHHhli~ CLI0CO6 nponle c TouHn * W o r k p e r f o r m e d in R a d i o c h e m i c a l Analysis Section of the I n s t i t u t e of M a t e r i a l s Research. Present address : R a d i o a c t i v i t y Section, I n s t i t u t e of Basic S t a n d a r d s , N a t i o n a l B u r e a u of Standards, W a s h i n g t o n , D.C. J" F o r d h a m University, Bronx, New York. 23

24

A. R. Landgrebe, [IV. O. ~IcSharry and M. Cefola 8peHgg MaHHny~mp0BaH~g. ]3H~ pa3paSoTaH ~OBH~ cnoco5 onpe~eaein~g KoHeqHofi TOqHII. E C ~ I CqlITaTB CMO~y ~ pacTBop, DTOT CHOCO~ ftB2IHeTeg IIpOeTbIM MeTOJIOM onpe;leaetmn 3HBIfBaJIeIITHOi~ TOtIHII, PaapafavHBa~OTCg MeTO;~bI ;~oBeReHHg ~0 MaHCgMyMa qyBeTBI~Te2IBHOCTII II;[HRa 1]pI4 IIOMOHIH BHyTpHHOMH~eKcHpyIoII~ero pa~r~OMeTp~qeet¢oro THTpOBaHHH.

DIE R A D I O M E T R I S C H E T I T R I E R U N G VON Z I N K IN SPUREN Ein Verfahren zur radiometrischen Titrierung ftir die Bestimmung yon Metallen in Spuren ist entwickelt worden. Es wird der EDTA-Komplex des zu bestimmenden Metalls gebildet und die tiberschtissigen nicht-komplexen Metallion en werden durch einen Kationenaustauscher entfernt. Man konstruiert dann eine Titrierkurve durch Auftragung der Aktiviffit der fltissigen Phase als Funktion des zugeffigten Betrages von EDTA. Sowohl das Posten- als auch das Kolonnen verfahren wurde in unserem Laboratorium versucht und u.E. ist das Postenverfahren einfacher zu hantieren. Eine neue Bestimmungsweise ffir den Endpunkt wurde entwickelt. Wenn man sowohl das Harz, als auch die L6sung z~ihlt, so ist das ein einfacher Weg zur Bestimmung des halben Aquivalenzpunktes. Es sind Verfahren im Gange, um die Empfindliehkeit ftir Zink dureh radiometrische Chelattitrierung auf ihren H6chstwert zu steigern. INTRODUCTION IN GENERAL, r a d i o m e t r i c titrations are a p p l i c a b l e to a n y r e a c t i o n w h e r e the p r o d u c t s m a y be s e p a r a t e d fi'om the u n r e a c t e d m a t e r i a l . T h e processes most c o m m o n l y used to effect this s e p a r a t i o n are p r e c i p i t a t i o n a n d solvent extraction. I n the case o f p r e c i p i t a t i o n reactions the sensitivity is l i m i t e d b y the solubility of the r e a c t i o n p r o d u c t . I n the case of solvent extraction, however, because of the high stability of the complexes, the limit d e p e n d s o n l y on the o r d e r of m a g n i t u d e o f the i m p u r i t i e s present a n d on the specific a c t i v i t y of the r a d i o a c t i v e isotope used for labelling, m A n a d v a n t a g e of r a d i o m e t r i c titrations is t h a t t h e y lend themselves well to a u t o m a t i o n . I n most o f the p r e c i p i t a t i o n methods, specially designed e q u i p m e n t is used in w h i c h a p o r t i o n o f the r e a c t i o n m i x t u r e is d r a w n u p t h r o u g h a filter into the c o u n t i n g c h a m b e r after each a d d i t i o n of titrant. I n the n o n a u t o m a t e d p r o c e d u r e s the p r e c i p i t a t e m a y be r e m o v e d b y o r d i n a r y filtration methods. I n tile case o f solvent extraction, it has not been possible as y e t to evolve a continuous process, a n d as a result a u t o m a t i o n has been d e l a y e d (1). A m e t h o d of r a d i o m e t r i c titration for the d e t e r m i n a t i o n of trace metals has been d e v e l o p e d b y STARY, RUZlCKA a n d ZEMAN(2). T h e E D T A c o m p l e x of the m e t a l to be d e t e r m i n e d is formed, a n d the excess, u n c o m p l e x e d m e t a l ions a r e r e m o v e d b y m e a n s of a c a t i o n exchanger. A t i t r a t i o n curve is then constructed b y p l o t t i n g

the r a d i o a c t i v i t y of the liquid p h a s e versus the a m o u n t of E D T A a d d e d , a n d the end p o i n t is i n d i c a t e d b y the p o i n t of intersection of the two straight lines thus o b t a i n e d .

EXPERIMENTAL A. Reagents and purification 1. I o n - e x c h a n g e resin: Dowen 50W-X8 20-50 mesh, s o d i u m form was used. T h i s was w a s h e d with 10 b e d volumes o f 6 M HC1 followed b y a distilled-water wash until the effluent was only w e a k l y acidic to i n d i c a t o r p a p e r . T h e resin was then c o n v e r t e d to the a m m o n i u m form w i t h 10 p e r cent N H 4 O H , a n d a g a i n w a s h e d with water, followed b y acetone. A f t e r this the resin was allowed to soak o v e r n i g h t in 1 M EDTA m a d e strongly alkaline w i t h N H 4 O H . I t was finally washed w i t h 20 to 30 b e d volumes of distilled w a t e r a n d d r a i n e d o n l y until excess surface w a t e r was r e m o v e d . 2. S t a n d a r d zinc solution: T h i s was m a d e b y dissolving 0-6538 g o f high p u r i t y (99"98 p e r cent) zinc in 3 M H N O a a n d d i l u t i n g to 100 ml with distilled water. 3. S t a n d a r d E D T A solution: This was p r e p a r e d b y dissolving 3"7228 g o f Na2C10HI4N2Os'2H20 in 100 ml of distilled water. This was then s t a n d a r d i z e d using the zinc solution as a p r i m a r y s t a n d a r d (3). T h e m o l a r i t y was f o u n d to be 0.1000 ± 0"3 p e r cent.* O t h e r E D T A * All expressions of this type used herein represent the relative standard error of the mean.

25

The radiometric titration of trace amounts of zinc

solutions were made by diluting this with distilled water. 4. Buffer solution, p H 10 : Concentrated buffer was made using 3 5 m l of N H 4 O H (28-30 per cent NHa) and 8.0 g of NH4C1 in enough water to make 100 ml. This was diluted to make the other concentrations. 5. 0-1 M N a O H : Four grams of N a O H were dissolved in 1000 ml of water. 6. Radioisotope: Zn 65 in HC1 containing 105 to 106 counts, m -1. m1-1 was used. The amount of zinc present was determined by the dithizone method (a) after masking the copper.

i

!,o

__/__

A series of test tubes, usually ten, was prepared containing varying amount of E D T A and the desired amount of buffer in 1 ml of solution. One milliliter of zinc solution, containing the tracer, and 1 ml of 1 M H N O a per 100ml were added, followed by enough (previously determined) 0"1 N a O H to maintain the p H between 9 and 10. Then 0"5 ml of moist resin was added, and the tubes were agitated for 4 min, after which an aliquot of the liquid was drawn off for counting. Two points in this procedure are worthy of emphasis. One is that the zinc solution must be kept below p H 6 until it has been added to the reaction vessel. Experiments have shown that at low concentrations significant quantities are absorbed on the glass at higher pH. Moist resin from which only excess surface water has been removed must be used. Otherwise, concentration of the solution occurs due to absorption of water by the resin. Equal portions of resin were measured as follows. A piece of polyethylene tubing was fitted over a glass rod to give a sliding fit. The resin was measured out by pressing it into the open end of the tubing to the desired length, and then extruding it by pushing on the glass rod. C. Calculations

After subtracting the background, the count rate for each aliquot was plotted against the amount of E D T A added (see Fig. 1). The end point was first approximated from the graph. Any points lying on the nonlinear portion of the graph (the portion near the end point) were ignored in further calculations. A least squares

0

0

2o

i 2

X

I5 0-

B. Procedure

0

MILLIMOLES

OF

EDTA

ADDED

FIG. I. Titration curve of 0"706/~g/ml zinc with EDTA. calculation was used to obtain the intercept and slope of the best straight line passing through the first set of points. The slope of the horizontal portion of the graph was assumed to be zero, and the intercept was calculated by taking the average of the points. The two linear simultaneous equations thus obtained were solved to find the point of intersection. R E S U L T S AND D I S C U S S I O N Results of analyses are shown in Table I. The " A m o u n t Present" includes the zinc carrier of the tracer. At first a problem was encountered at concentrations below 67/~g/ml At these concentrations the nonlinear portion of the curve began somewhat ahead of, and extended far beyond the end point, i.e. the maximum activity in the solution was not reached until a large excess of E D T A had been added. This introduced TABLE 1. Summary of results Amount present (/*g of Zn/ml)

Number of determinations

654 65.45 6-59 1.013. 0.706

4 4 2 2 2

Amount found (~g/ml)

Relative error

650_K1.0%T -0.6% 67.0 :t:0.3% 2.4% 6.974-0.1% 5.8% 1.03 4-1.0% 1.7% 0.760 4-0.9% 7.6%

* Determined by dithizone method TM Standard error

26

A. R. Landgrebe, W. O. 3IcSharty and M. Cefola

a high degree of uncertainty in the end point. I t was found this could be overcome by reducing the buffer concentration to about one order of magnitude above the zinc concentration, down to a m i n i m u m of 10-3 M. However, another problem then arose. Due to the necessity for keeping the zinc solution acid, i.e. there was too much acid for the buffer to neutralize and the p H dropped too low. U n d e r these conditions, results were obtained which gave excellent graphs and had good precision, but which had very large positive errors, e.g. 30 per cent too high at the 0-7 #g/ml level. This was overcome by adding enough 0"1 M N a O H to maintain the p H at 9 to 10 after shaking. T h e p H was checked by adding a drop of 0.125 per cent 5-(p-nitrophenylazo) salicyclic acid sodium salt (Alizarin Yellow R) to the aliquot after it had been removed, and comparing it to a set of color standards. Several runs were tried in which the end point was calculated from the activity of the resin, but this did not give as good a precision as the method which measured the solution.

CONCLUSION The batch method has been found to be applicable to chelating radiometric titration of trace amounts of zinc by ion exchange. T o the best of our knowledge, this is the first time the batch method has been applied to this technique. Both the batch and column method were tried in this laboratory, and it is our opinion that the batch method is easier from a manipulative point of view. Although errors increased to considerable values at the one-to ten-microgram level, there is no definite evidence that this is the limit of sensitivity, and it is hoped that further modification, perhaps including an end-point correction, will bring improvement. REFERENCES 1. BRAUN T. and TOLGYESSYJ. Talanta 11, 1277 (1964). 2. STARYJ., RUZlCKAJ. and ZEMANA. Talanta 11, 481 (1964). 3. VOGEL A. A Textbook of Quantitative Inorganic Analysis, pp. 433-434, Wiley New York, 3rd edition (1963). 4. DEVoE J. R. NBS Technical Note 401 (1966).