Bd. 187 (1962)

241

Central Research Institute of A.K.U. and Affiliated Companies, Arnhem, Netherlands

Determination of Trace Amounts of Calcium in Caustic Soda Complexometrie Titration after Separation on Dowex A-1 Chelating Resin By A. J, VAN DER REYDEN a n d R. L. M. VAN LINGEN W i t h 2 Figures in t h e T e x t

(Received September 22, 1961) I n a study of the effects of certain metals in technical processes a simple method for the determination of microgram amounts of calcium in commercial 500/0 caustic soda solutions was required. Usually, the amounts encountered v a r y between 2 and 10 ppm. Other metals, such as iron, copper, barium, manganese, aluminium, nickel, mercury, and magnesium m a y be present in metal/calcium ratios varying from 1:2 to 5:1. Though m a n y methods have been published on the determination of calcium, none, as far as we know, deals with microgram quantities in technical chemicals. As it m a y be assumed t h a t for any method to be applicable the sample must first be neutralized, our problem was to determine calcium in the presence of large amounts of, say, sodium chloride. I n view of the minute amounts to be determined, speetrophotometrie methods were a first consideration. Those best known are based on the use of such reagents as: murexidelS, 2~ ehlorardlic and bromanilie acids s,21,24, alizarine 15, lithium loretinate 10 picronolie acid is, 8-hydroxyquinoline in the presence of n-butylamine 2.5, sodium naphtha]hydroxamate2, la, and molybdate, involving measurement as molybdenum thio-

oyana,te11,19. Though any of these methods might be applicable to caustic soda, they all involve tedious and time-consuming precipitative or extractive isolation of calcium. The same applies to methods using an E D T A titration with spectrophotometrie endpoint detection1,5,14. The only method found in the literature for a direct determination of calcium in caustic soda 4 is based on turbidimetrie determination of calcium sulforieinoleate. According to our previous experience, this fails for the small amounts to be considered. An indication of an elegant solution of the problem was found in a brochure on a new chelating resin, Dowex A-1 7. According to the Z. analyt. Chem., Bd. 187

16

242

A. J, VAN DER ~:~EYDEIg and R. L. M. van LINGEN

Bd. 187

manufacturers this is a styrene-divinylbenzene copolymer matrix with imino-diacetate active groups, exhibiting preference for polyvMent ions over univalent ones. With this resin calcium and magnesium are alleged to be separated from even concentrated sodium chloride solution by passage through an ion-exchange column. I n principle this provides a very welcome means of isolating and concentrating calcium from large volumes of solution. After elution with hydrochloric acid the metal could then be determined by any convenient method. I t was therefore decided to assume work on this resin. For determining calcium in the eluate titration with EDTA was considered most attractive. Such titration procedures have been published for the use of various indicators, e.g. Erioehrome Black T 9, but this involves a subsequent separation, as magnesium is co-titrated. Of the calcium specific indicators murexide, ealeein, ealcon, methylthymol blue, and Alizarin Black SN were compared by BELCHERet al. a. They decided upon ealcon as giving the best results at magnesium/calcium ratios of 1 :l and higher. So did T6T~ERMANsy, who compared cMcein, eal-red and ealcon in the determination of calcium in pulp. In view of earlier experiences and in accordance with DIEHL and ELnlNGBOE6 who advocate ealcein (Flnorescein eomplexone, Fluorexone) for titrations in the presence of sodium salts, we chose this indicator, in admixture with thymolphthalein, as recommended by TucKE~23. I t seems expedient to mention in conclusion the very simple and rapid method of HoL and LEFE~IN~: 12: based on a direct titration of calcium in a large volume of magnesium-free sodium chloride solution with EDTA, using transmitted light for observation of the Eriochrome Black T endpoin~.

Experimental Principle As the method to be developed should be easily applicable in routine control work, it was decided to titrate the calcium with 0.02 N EDTA solution from an ordinary 5-ml microburette. For the determination of as little as 2 ppm, a sample size of 50 g was considered convenient. This would then result in 0.1 mg of calcium to be titrated. This sample contains 25 g of sodium hydroxide, which on neutralization produces about 35 g of sodium chloride. To keep the salt coneentratio~l before ionexchange at a safe level, dilution to a volume of 250 ml was chosen. The principle to be tested in the following would then be as follows: A 50 g sample is neutralized with concentrated hydrochloric acid, diluted to 250 ml, and the solution passed down a Dowex A-1 resin column. The effluent is rejected, the catcium eluted with 2 N hydrochloric acid, and titrated at p~ 12.2 with 0.02 3/EDTA solution to cMeein/thymolphthaleh~ indicator.

1962

Determination of calcium in caustic soda

243

E//ect o/Salt on Titration To d e t e r m i n e t h e a m o u n t of s o d i u m chloride t h a t m a y be p r e s e n t in t h e solution to be t i t r a t e d w i t h o u t causing interference, solutions o f 0.48 m g of calcium a n d increasing a m o u n t s of s a l t were d i l u t e d to 200 ml, a d j u s t e d to p ~ 12.2 b y a d d i t i o n of 2 N s o d i u m h y d r o x i d e , a n d t i t r a t e d w i t h 0.02 57 E D T A solution to t h e m i x e d i n d i c a t o r . The Table 1. E//eet o/NaCl on Titration results are l i s t e d in Table 1. Present Found Present Found g SaC1 m g Ca g SaC1 m g Ca The e n d p o i n t in t h e presence of 6 a n d 9 g of sodium 0 0.48 6 0.51 chloride was h a r d to d e t e r m i n e . 0.48 0.51 I t a p p e a r s f r o m the d a t a given 3 0.48 9 0.53 0.48 0.61 t h a t 3 g of salt, i.e. a b o u t 50 m i l l i e q u i v a l e n t s , does n o t interfere w i t h t h e calcium t i t r a t i o n . Consequently, n o t m o r e t h a n 50 meq. of s o d i u m chloride m u s t be f o r m e d b y n e u t r a l i z a t i o n of t h e a c i d c o l u m n eluate, which implies t h a t t h e a m o u n t of h y d r o c h l o r i c a c i d t o be used for elution m u s t n o t be higher t h a n 25 m l 2 h r. I t will be shown in T a b l e 5 t h a t t h e a m o u n t o f calcium t h a t is i n t r o d u c e d as an i m p u r i t y of t h e s o d i u m chloride used, is so s m a l l t h a t it c a n n o t be responsible for t h e high results o b t a i n e d in t h e presence o f 6 a n d 9 g of salt. These m u s t therefore be a t t r i b u t e d to t h e well-known salt effect.

E//ect o/Foreign Ions As a p a r t from calcium o t h e r p o t y v M e n t m e t a l s w o u l d be r e t a i n e d b y t h e resin, a n d s u b s e q u e n t l y eluted, t h e i r possible interference was checked as follows: To aliquots of a standard calcium solution, each containing 0.5 mg of calcium, each of the metals listed in Table 2, likely to be encountered in technical caustic soda, were added in ratios increasing from 1 : 1 to 5: t. Calcium was then titrated at pu 12.2 with 0.02 N EDTA solution. The results are given in Table 2. Table 2. E//ect o] Separate Metals on Calcium Recovery (0.50 mg Ca present) m g Ca recovered Metal

rag Ca recovered

A m o u n t of m e t a l added 0,5 m g

1.0 m g

Metal

2.5 m g

Mg 0.51 0.50 -Cu 0.52 0.50 0.49 Fe 0.46 0.48 * Ni 0.50 0.49 0.47 A1 0.50 0.50 0.50 9 Endpoint blurred; discoloration. 9* Endpoint not detectable; turbidity.

Zn Hg

A m o u n t of m e t a l a d d e d 0.5 rag

1.0 m g

2.5 m g

0.50 0.50 0.56

0.50

0.50

0.49

16"

244

A. J. VAN DER REYDEI~ and R. L. M. vA~r LI~cOE~r

Bd. 187

I t is seen t h a t w i t h the exception of iron a n d m a n g a n e s e none of the m e t a l s a t t h e 0.5-rag level interferes, t h a t m o s t m a y be p r e s e n t in a 2 : 1 ratio, a n d some in a 5 : 1 ratio. The t e s t s w i t h 1 m g a n d 0.5 m g of i r o n a n d m a n g a n e s e were r e p e a t e d b u t n o w 10 ml t0~ t r i e t h a n o l a m i n e was a d d e d as a m a s k i n g a g e n t 26. R e s u l t s were as in Table 3. t:inally, 0.5 m g o f each o f t h e m e t a l s l i s t e d in T a b l e 2, b u t 1.25 m g o f iron a n d 12.5 m g of a l u m i n i u m , were a d d e d to a solution o f 0.5 m g o f calcium. The solution was t i t r a t e d in Table 3 the presence o f t r i e t h a n o l a m i n e . TripliE]]ect o/Fe and Mn on Calcium cate results are given T a b l e 4. Recovery in Presence These d a t a show t h a t t h e c o m b i n e d o/ Triethanolamine m e t a l s do n o t affect t h e results for cal(0.50 nag Ca present) cium, when t r i e t h a n o l a m i n e is u s e d an mg Ca recovered a complexing agent. I t should be n o t e d ~Ietal Amountof metal added t h a t t h e a m o u n t s o f m e t a l s a d d e d in 0.5 mg 1.0 mg T a b l e 4 are in excess of those u s u a l l y Fe 0.49 0.50 p r e s e n t in a 50-g s a m p l e o f caustic soda. Mn 0.50 0.49 Resin Capacity Table 4 T h o u g h t h e resin c a p a c i t y could be Effect o/Combined Metals on Calcium Recovery in Presence e x p e c t e d t o be a m p l y sufficient for t h e of Triethanolamine a m o u n t s of calcium concerned, it was (0.50 nag Ca present) d e t e r m i n e d for g r e a t e r c e r t a i n t y as ~1etals added mg mg Ca found follows : A solution of 400 nag of calcium and 400 g ~g 0.5 0.49 of sodium chloride in 2 1 of water at PH 4 was 0.5 0.49 Cu passed at a flow rate of 2 nal/nain through a 1.25 0.48 ~'e 1-ena inside dianaeter column containing 10 g 0.5 Ni of resin in the Na-form. The effluent was colM i2.5 lected in 50-nal fractions, which were titrated Zn 0.5 at PH 12.2 with 0.02 N EDTA. 0.5 Hg Fig. 1 shows t h e results when p l o t t e d Mn 0.5 Ba 0.5 a g a i n s t t h e a m o u n t of effluent. The t e s t was r e p e a t e d a t b o t h p~ 6 a n d pI~ 8 to y i e l d a curve p r a c t i c a l l y coinciding w i t h t h a t o f Fig. 1. A p p a r e n t l y b r e a k - t h r o u g h occurs a f t e r 750 m l of feed has passed, from which a resin c a p a c i t y o f 15 m g p e r g r a m is calculated. E v e n i f t h e resin is n o t to be loaded to more t h a n 10~ o f its c a p a c i t y , a few g r a m s w o u l d suffice for the a m o u n t s of calcium to be considered. As pE v a r i a t i o n b e t w e e n 4 a n d 8 has no effect on t h e calcium retention, i t follows t h a t t h e pH before passage t h r o u g h t h e column need n o t be e x a c t l y a d j u s t e d b u t m a y be set a t a n y v a l u e b e t w e e n 4 a n d 8, e.g. b y n e u t r a l i z a t i o n of t h e s a m p l e solution to l i t m u s p a p e r .

i962

D e t e r m i n a t i o n of c a l c i u m i n c a u s t i c s o d a

245

A m o u n t of Resin to be Used

I t follows from the m a x i m u m amount of salt t h a t m a y be present during the titration t h a t the amount of hydrochloric acid to be used for elution must not exceed 25 ml of 2 N. To find the amount of resin t h a t can be stripped of retained calcium with this amount of acid, a solution

,1-

,

i

d8

,

'/

),0

/

--

~ 60 5O

---

geI 30

I

~ 800

/aoa

/200 /~o ~]. EPflurnf

o

I

[o

20

30 40 mL Elua/e

:Fig. 1

Fig. "2 Fig. 1. :Resin break-through curve F~g.~. Amount of acid required for elution of 3.9~ mg of Ca from i0 g of resin

of 3.94 mg of calcium was placed in a 1-era diameter column filled with 10 g of resin in the Na-form. The column was washed with 100 ml of deionized water, and the retained calcium eluted with 2 N acid. The eluate was collected in 5-ml fractions, and tested for calcium b y titration at p~ 12.2. The results were plotted against per cent. calcium recovery, as illustrated in Fig. 2. I t is shown t h a t at least 35 mt of 2 N acid is required for recovering the calcium from 10 g of resin. Since for a given column the amount of acid required is mainly determined by the resin bed height, it m a y be eonchded t h a t half the amount of resin, i.e. 5 grams, will require half the amount of acid, i.e. 17.5 ml. Using 25 ml of acid for elution f r o m 5 g of resin will thus provide a sufficient safety margin to ensure complete recovery of calcium. Blan]~ Values Two possible sources of calcium must be considered; (a) the hydrochloric acid used for neutralization of the sample and for elution, and (b) the sodium hydroxide solution used for pH adjustment. I f 25 ml of 2 N hydrochloric acid is used for elution, the amount of alkali required for raising the pH to 12.2 in a final volume of 200 ml is about 30 ml 2 N. This amount of alkali was prepared from an analy~icM

246

A.J. VANDER REYDE:~ and R. L. M. VANLING~

Bd. 187

reagent grade caustic soda, a n d stored i n a p o l y t h e n e bottle. The E D T A c o n s u m p t i o n of 30 ml of this solution was zero, which means t h a t the calcium c o n t e n t is lower t h a n detectable b y m e a n s of the ~itration procedure. The hydrochloric acid was tested for calcium b y e v a p o r a t i o n of 100 ml of c o n c e n t r a t e d solution to near dryness, d i l u t i o n of the residue w i t h water, a n d t i t r a t i o n a t p~ 12.2 with 0.02 57 E D T A . Duplicate results were 0.01 a n d 0.00 mg of calcium. These findings show t h a t i n tests on technical samples no corrections need be made for alkali a n d acid blanks. U n k n o w n chemicals, however, should be tested previously, a n d results corrected accordingly.

Method On the basis of the preceding sections the following m e t h o d could be d r a w n up. Apparatus. a) Ion-exchange column o] borosilicate glass, 1 cm inside diameter, 15 cm long, with 100-ml tap funnel at the top, and small-bore stopcock at the bottom. b) 5 ml Microburette, graduated to 0.01 ml. c) pn-Meter with glass and saturated calomel electrodes, e.g. Beckman Model H2.

Reagents. a) Water for all purposes: completely demineralized. b) EDTA Solution, 0.02 N. Dissolve 3.7200 g of disodium dihydrogen ethylenediaminetetraacetate dihydrate (Siegfried, Zofingen, or equivalent) in water and dilute to a volume of 1000 ml in a volumetric flask. If carefully prepared the solution is exactly 0.02 N; in case of doubt standardize against standard calcium solution at Pu 12.2 to ealcein/thymolphthalein indicator. c) Indicator. Thoroughly mix in a mortar 0.5 g of calcein (British Drug Houses Ltd), 0.3 g of thymolphthalein, and 50 g of sodium chloride (Merck). d) Hydrochloric acid, 12 N (sp. gr. 1.19), Merck or equivalent, free of calcium. e) Hydrochloric acid, 2 N, prepared from d. ]) Sodium hydroxide solution, 2 N, prepared from Merck or equivalent caustic soda, stored in a polythene bottle. g) Triethanolamine, 10~ analytical reagent grade. h) Dowex A-1 chelating resin, 50 mesh. Resin Preparation. Stir about 50 g of resin as received with 50 ml of 2 N hydrochlorie acid, allow to settle, decant the acid, and wash with water. Stir again with 50 ml of 2 N sodium hydroxide solution, and allow to stand for about 15 min. It will be noted that the resin volume has nearly doubled. Place a plug of borosilieate wool in the column, and transfer so much of the resin that the column is filled to a height of about 10 era, which corresponds to about 5 g of resin in the as-received state. Wash with water until the effluent is free of alkali. Keep the resin covered with liquid at all times, to prevent channelling. Procedure. Dilute a 50-g sample of caustic soda with water to a volume of about 100 ml, and carefully neutralize with 12 N hydrochloric acid to litmus paper. Add water to a volume of about 250 ml, mix, and pass the solution down the resin column at a rate of 2 ml/min. Follow with 100 ml of water, and discard the effluents. Elute the calcium with 25 ml of 2 N hydrochloric acid, wash with 50 ml of water, collecting the eluate and w~shings in a beaker, Add 10 m! of tr~ethano!amine and

t962

Determination of calcium in caustic soda

247

30 ml of 2 N sodium hydroxide solution, and dilute with water to a volume of about 200 ml. Add some 50 mg of indicator, and titrate with 0.02 N EDTA solution until the green fluorescence has just disappeared and a light purple remains. In the same manner titrate 30 ml of 2 N sodium hydroxide solution as a blank. Tests on Simulated Sample. The m e t h o d as described was applied to 50-g a m o u n t s of sodium chloride before a n d after a d d i t i o n of k n o w n a m o u n t s of calcium. The results are listed i n Table 5. Table 5. Recovery o] Calcium added to 50 g _NaCl rag Calcimn Found

Added

Expected

Found

Difference

0.11 0.10 0A1 0.11

0.25

0.36

0.36 0.34 0.36 0.36

0 --0.02 0 0

0.5

0.61

0.61 0.63 0.60 0.59

0 +0.02 --0.01 --0.02

1.0

1.11

1.12 1.08 1.11 1.12

+0.01 --0.03 0 +0.01

1.5

1.61

1.63 1.58 1.60

+0.02 --0.03 --0.01 +0.01

1.62

mean stand, dev.

--0.002 0.015

Application of S t u d e n t ' s t-test to the m e a n difference between expected a n d f o u n d values does n o t i n d i c a t e a n y significant difference from zero, which m e a n s t h a t the a d d e d a m o u n t s of calcium are recovered q u a n t i t a t i v e l y . The s t a n d a r d d e v i a t i o n of a single d e t e r m i n a t i o n , calculated from the results o b t a i n e d before a n d after a d d i t i o n of calcium, is 0.015 m g of calcium. Tests on Technical Samples. The procedure was applied to three 50-g samples of technical 500/0 caustic soda solutions. K n o w n a m o u n t s of calcium were t h e n added, a n d the tests repeated. I n T a b l e 6 the results are g i v e n ; values expected were calculated from the m e a n values found. The two s t a n d a r d deviations calculated from the results before a n d after a d d i t i o n of calcium do n o t differ significantly. The t - t e s t applied to the m e a n difference b e t w e e n expected a n d f o u n d values does n o t indicate a significant difference from zero.

248

A. J. VAND~.RRErI)EN and R. L. M. VANLING~r

Bd. 187

Table 6. Ca in Technical Samples be/ore and a/ter Addition o] Calcium ppm Ca

Sample

mean st. dev.

Found

Added

Expected

Found

Difference

5.7 4.6 4.9 5.1

3.8

8.9

10.4 9.4 8.8 8.7

+1.5 +0.5 --0.1 --0.2

5.0 5.1 6.1 5.4

3,8

9.2

9.4 8.9 9.0 8.8

--0.2 --0.3 --0.2 --0.4

3.6 3.5 2.8 2.3

3.8

6.9

6.4 6.4 7.2 6.8

--0.5 --0.5 +0.3 --O.i

0.52

+0.02 0.56

0.53

From the standard deviation it follows that the lower limit of deterruination of the method is 1.6 ppm, so that the method m a y be applied to the determination of down to about 2 ppm of calcium.

Discussion The method as developed allows the determination of as little as 2 ppm of calcium in a 50-g sample of technical 50~ caustic soda solution. The accuracy of the method is strongly evidenced by the recovery values shown in Tables 5 and 6. The precision of the results, as indicated by t h e standard deviation, is quite satisfactory for our purpose; the value calculated from Table 6, 0.52 ppm, i.e. 0.026 mg of calcium, compares well with that obtained from Table 5, 0.015 rag. The precision of the method may obviously be further improved, if required, e.g. by using a larger sample, a more dilute EDTA solution, a mierQmeter type burette, and/or instrumental techniques of endpoint detection. For work on a routine basis these refinements were not considered desirable. I n Table 1 on the effect of sodium chloride on the calcium titration 3, 6 and 9 g of salt were added. I n Table 5 this salt was shown to contain 0.11 mg of calcium per 50 g. The highest amount of calcium introduced in the tests of Table 1 therefore is 0.02 mg, which is below the limit of titratability with the present method. The effect shown in Table 1 is consequently entirely due to the presence of the salt. A point frequently missed in discussions of methods for trace amounts 0f elements is the fact that the reagents to be used often contain the elements in amounts as high as, or higher than, the sample. Fortunately,

1962

Determination of calcium in caustic soda

249

we have not met with this problem in the present work, as both the alkali for p~ adjustment, and the hydrochloric acid for neutralization and elution were found to be free of titratable calcium. Yet, in every series of tests the alkali and acid used should be checked. According to our reported findings Dowex A-1 chelating resin presents an excellent means of separating and concentrating small amounts of calcium from relatively large sample volumes. I f treated as described after every use, one column filling may be used for months without any appreciable loss of capacity. As it obviates lengthy and cumbersome separating methods, otherwise required, its application in the determination of other metals from salt solutions deserves consideration.

Addendum Since completion of the work described in this paper, the authors took cognizance of a publication by OLsv~, DIe.L, CoLLI~S, and ELLESTAI)17 on the determination of microgram quantities of calcium in lithium salts by titration with very dilute EDTA solution to a fluorimetrically determined calcein endpoint, using preliminary separation on Dowex A-1 chelating resin. This, as far as we know, is the fn'st communicated application of the resin to an analytical problem.

Summary A procedure is described for the determination of as little as 2 ppm of calcium in 500/0 commercial solutions of caustic soda. I t is based on a preliminary separation and concentration of calcium from a large volume of neutralized sample on Dowex A-1 chelating resin, followed by elution with dilute hydrochloric acid, and titration at p~ 12.2 with standard EDTA solution to the ealeein/thymolphthalein mixed indicator visual endpoint. Magnesium, iron, copper, manganese, barium, nickel, mercury, and aluminium do not interfere when triethanolamine is used as a masking agent.

Zusammenfassung Es wird eine Methode f'dr die Bestimmung yon mindestens 2 ppm Calcium in 500/oigen, kguflichen Atznatronl6sungen beschrieben. Die Methode beruht auf einer Vortrennung und Anreicherung yon Calcium aus einem grogen Volumen der neutralisierten Probe mit Dowex A-1 Chelating Resin, anschliel3ender Elution mit verdtinnter Salzs/~ure und Titration mit JkDTA-L6sung bei p• 12.2 zum visueUen Endpunkt des Mischindieators von Calcein und Thymolphthalein. Magnesium, Eisen, Kupfer, Mangan, Barium, Nickel, Quecksflber and Aluminium stOren die Bestimmung nicht, wenn Trigthanolamin als Maskierungsmittel verwendet wird.

"250

vA~rDER ~EYDE~ e~ al. : Determination of calclum in caustic soda

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