Potentiometric Titrations Involving Chelating Agents, Metal Ions, and

Chelometric indicator titrations with the solid-state cupric ion-selective electrode. James William. Ross and Martin S. Frant. Analytical Chemistry 19...
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Potentiometric Titrations Involving Chelating Agents, Metal Ions, and Metal Chelates SIDNEY SIGGIA, DALE W. EICHLIN, and RICHARD C. RHEINHART Central Research Laboratory, General Aniline

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Film Corp., Easton, Pa.

This study was initiated to find a suitable niethod for analjAing chelating agents in general and also nietal chelates. The method can be used for determining mixtures of chelating agents and, in reverse, for determining metals and mixtures of certain metals. The general approach is the potentiometric titration of the chelating agent with a metal ion (or vice \ersa) using an electrode system that indicates an) excess of metal ion in the system. Conditions, metal ion, and electrode ‘)stems can be varied so that mixtures of chelating agents can be determined. The chelating agent, coiiditions, and electrode s>stern can be varied so that niixtures of metal ions can be determined. Metal chelates ran also be analyzed for unchelated metal ion present, excess chelating agent, and the amount of metal chelate present. The method is simple to apply, and the precision and accuracy are generally better than =kly‘for the S J stems where good titration cur\es are obtained.

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H1:IATIIVC; ageiitt have iouud their way into iiicin~diverse applications. The nuniber of chelating agents in nse in industry has increased beyond the ell-known ethylenedianiinetetia,wetic acid [ E D T h ; also known as (ethylenedinitri1o)teti aacetic arid]. Induqti y no\\- employs mixtures of chelating agents to achieve certain desirable characteristics. Metal chelates have become industrially important in recent years, esprrially in the agricultural field. This diversification of applicatioiis and the increase in the scope of chelation chemistry have necessitated the development of analytical methods to folloii the proresses and investigations being carried ollt. Chelating agents have, i n recent years, become of great importance from the standpoint of analysis. Biedermann and dchw arzenbech ( I ) devised a method for determining various nictallic ions with EDTA. involving the use of a special indicator. The procedure can be reversed to determine EDTA by titrating n i t h magnesium ion. The piocedure cannot be applied to porn(’ of the other chelating agents n hich are produced today. Furness, Cramhaw, and Davies (6) determined E D T A polarogi ‘iphically. Blaedel, Knight, and Malmstadt (2-4) used high frequency titration of metal ions I\ ith E D T I . Laitinen and Sympson ( 9 ) employed anipeiomctric titrations, and Hall and others ( 7 ) rmd conductonietric titiations. Pfihil, Iioudela, and 11atyska (IO) used potentiometrir titrationq for determining 11011 by direct titration n ith CDT-3. solution. Platinum was used as the indicating elertrode in an aqnrous medium. Iron was the only metallic ion that could be cietei mined directly. Aluminum, copper, cadmium, zinc, nickel. lead. and bismuth were a190 determined, but indirectly, b i addition of cxcesb E D T A and back-titration of the excess n i t h a standard solution of ferric chloride. I t was found that by varying the solvents and especiall? the electrodes employed, the potentiometric approach described by Pfibil could be evtended not only to the direct titration of a nun1 ber of metals other than iron but also to the titration of chelating agents other than EDTB. The electrode systems given in the tables were the best of the several tried for detecting the metal in question. The platinum electrode worked in only a few cases. Pyridine intensified the magnitude of the titration breaks and also made possible the solution of the acid forms of the chelating

:ipent!: anti of inany tlif’crcirt types of sample. 11ixtnrc.s of rhelatiug agents also could be determined if the proper electrodes solvent systems, and pH mere used. This approach was found to be suitable for the analysis of metal chelates, making possible the determination of any excess chelating agent or metal ion that may be present and also the amount’of metal chelate present. Potentiometric titration, in addition to being a versatile approach in chelating agent-metal analysis, is a simple tool t o use. .\ pH meter may be used for the titrations, and the electrodes used are easily made. This puts the approach a t the disposal of laborat~oriesand plants with limited equipment. The accuracy aiid precision of the method vary with the chelating agent, metal ion, conditions, and elect,rodes used; however, arcurac?. and l)rwision within f1yoare often obtainable. UE‘I’EKI.IINATION OF CHEL.4TING AGENTS AND METAL IONS

Thr’ same procediirw ;ire involved i n the determinat,ion of dirlating agents and metal ions, except that for determining c.helating agents a standard eolntioii of the metallic ion is used, a n d for determining the niet.allic ion a standard solution of rhelating agent is used. The electrode and solvent syst,enis arc the same in 11-hicheverdirection t.he titration is performed. Typicaal titration curves are shown in Figure 1, and results are shown in Table I. Table I s h o w that EDTA chelates well with all the met& ti,ied. 1 $harp titration break appears to indicate a small dissociiitioii ronstant for the metal chelates formed on t,itration. hmmoriia-triacetic acid gives sharp titration breaks with copper, zinr, and lead. Iron gives sharp breaks only under certain conditions. Mercury chelates only weakly, and under some c*onditionsno break at all is visible. Calcium and magnesium do not noticeably chelate with this agent under the conditions employed. .~,r.S-di(p-hydroxyethyl)glycine exhibits relatively poor chelating properties under t,he conditions tried. Only copper and ~ lead caould be used to tit,rate this agent, and the precision w a not too good, as only fair titration rnrves were obtained. Table I shows that chelation varies depending on three fac-

‘\\“e

\

I

~

1

I

I

ml. Figure 1. Curves of ethylenediaminetetraacetic acid titrated with copper, manganese, and magnesium Mercur) on platinum US. calomel electrode system and 1 to 1 pyridine-water solvent system

1745

ANALYTICAL CHEMISTRY

1746 Table I. Metal Used

Titrant, 0.l‘V

Titration Results with Various Rletal Ionsa

Solvent System

p H Range During Titration

Found, %’ob

Electrodes

Remarks

HOOCCH?

>xc H?-c

Titration of Ethylenediarninetetraacetic Acid

Hzx

HOOCCH?/ FS

cu

Hg

FeCh

Hz0 Hz0 Hz0 Hz0 HzO Hz0 Hz0 Hz0

Cu(N0a)n

Hg(’4c)z

++ 51 g.nil.SazCOJ 0.5‘\- NaOH

++ 41 ml. of 1 N NaOH ml. 1 N NaOH + 1 g. NaAc HzO + pyridine (50-50) Hz0 + 1 g. SazCOs HzO

-

HzO

-+ pyridine (50-50)

pyridine (50-50)

++ pyridine + 0.1 ml. 0 . 5 N NaOH pyridine + 1.0 ml. 0.5N NaOH + pyridine + 4.0 ml. 1.O.V NaOH + pyridine f 5.0 ml. 1 . O S NaOH pyridine + 3.0 ml. 1.ON NaOH

Hz0 Hz0 Hz0 Hz0 Hz0 f Zn

Zn(N0dz

HzO

Pb

Pb(NOs)z

HlO

\In

31ni‘ln

HzO

l\ln(hc)z

ca

CaCOy

+ IIC‘1

+ pyridine

(50-50)

- pyridine (50-50)

4.70- 1 . 7 8 10.17-10.00 12.03- 7 . 0 0 4.7 - 1 . 8 4.7 - 1 . 8 4.*5- 1 . 7 8.2 - 2.1 5.9 - 4.2

99.95 100.08 100.0 100.0 99.4

7.0110.2 7.1 7.1 7.1 -

- pjridine (50-30)

Hz0

+ pyridine + 2 rnl. of

Si

H20

+ pyridine (50-50)

C O

HzO

+ pyridine (50-50)

cu Hg

HzO H20 Ha0

++ NazCOa NaAc Hz0 + pyridine + 5 ml. of l.O.V NaOH H20 + pyridine + 4 rnl. 1.ON NaOH Hz0 + pyridine (50-50) HsO + pyridine + 2 ml. 1.O.V KaOH

tors-the agent used, the metallic ion used, and the conditions, usually pH of the titration. [Sullapons (hntara Chemicals, 435 Hudson St., New York 14, X.Y.) indicates the relative chelating power of various metals with EDTA a t various pH’s.] Using a certain metallic ion and certain titration conditions, one chelating agent can be determined in the presence of another. An example is shown in Table 11. I n this case there is a mixture of EDTA and ammonia-triacetic acid which is a mixture sometimes obtained in the production of EDTA. The EDTA in the mixture can be titrated with mercuric ion under conditions which exclude the titration of ammonia-triacetic acid. Then the total of EDTA and ammonia-triacetic acid can be determined by

Very good break Small but sharp break Fair breakc Fair breakc Good breakc Good break Good break Good break Good break Good break Good break Good breakc Good breakc Good breakc

99.0 100.3 100.0 99.03 99.01 99.05

7.657.658.016.686.7 6.7 -

6.75 6.75 7.72 6.63 6.6 6.6

Hg Hg Hg Hg Hg Hg

on on on on on on

Pt Pt Pt Pt Pt

7.7 - 6.8 7.7 - 6.8 7.7 - 6.8 7.03- 6.50 7 . 0 3 -6.50 7.03- 6.50

Hg Hg Hg Hg Hg Hg

on on on on on on

P t us. cal P t us. cal P t UB. cal Pt us. cal P t u s . cal P t u s . cal

Very good break Very good break Very good break Good breake Good breake Good breakc

-

6.6 6.6 6.6 7.4 7.5

Hg Hg Hg Hg Hg

on on on on on

Pt Pt Pt Pt Pt

Good break Good break Good break Good breakc Good breakc

7.31 7.31 7.31 6.83 6.83 6.83

H g on Hg on H g on Hg on Hg on Hg on

-

100.0 100.0 98.9 99.3 99.3

7.7 2.7 t .7 9.0 9.0

100.23

2.42t .427.427.157.157.15-

100.00 100.0 99.716 99.54d 99.726 99.16 100.21 100.04

- 7.4 - 7.4

9.2 9.2

..... ,.... .

.

.

.

I

..... ..... .....

HOCHzCH: (“ocHzcH; ,..d ..... ..... . . .d ...d ..... 98.6d 96.2d , . . d

...e ...6

11.9 12.1

- 8.1 - 7.7

.. .. .. .. .. .....

Pt us. oal us. us. us. us. us.

us. us. us. us. us.

cal cal cal cal cal

cal ea1 cal cal cal

Ag us. cal A g u s . cal Ag us. ea1 P t u s . cal P t u s . cal P t u s . cal

Very Very Very Very Very Very

Good Good Good Good Good Good

good good good good good good

break break break breakc breake breake

break break break break break break

H g o n P t u s . cal Hg on P t u s . cal

Good break Good break

Hg on P t us. cal H g on P t u s . cal Hg on Pt us. cal

Very good break Very good break Very good break

H g on P t us. cal Ag u s . cal P t u s . cal

Small break Small break Small break

‘x--CH2COOH

Tit rat ion of N , N - Di (P-hydroxyethyl) glycine Fe

P t u s . cal P t u s . cal P t us. cal P t u s . cal Hg on P t us. ca Agus. cale Ag us. cale d e u s . cale & u s . cal’ Agus. c a l e .4gus. cala Agus. cal Agus. cal Agus. cal

100.45

1 N KaOH

Fair break c Good breakc Good breakc

7.2 - 6 . 5 7.2 6.5 8.9 - 7.2 8.9 - 7.2 9 . 2 - 7.3 9.6 - 7.4 8.3 - 7.1 9.3 - 7.4 7.8 - 7.1

99.81 99.55 99.96 98 95

hI g

7.20 9.9 6.7 6.7 6.7

‘CH2COOH P t u s . cal P t u s . cal P t u s . cal P t us. ea1 P t us. ea1 P t u s . ea1 P t u s . cal P t us. cal

100.9d 100.1 100.2f 99.li 100.2f 99.51 99.2 99.8 100.4

99.5 99.95 99.83 99.51 99.66 99.60

+ pyridine (50-50) pyridine + 2 rnl. 1 S KaOH H20

Hz0

101.0 KObreak N o break 99.8 98.5 99.7 99 7 99.9

/CHzCooH

)

P t u s . cal P t us. cal P t us. cal

No break0 No breakc h-o breakc

Hg on P t u s . cal H g on Pt us. cal

Good break C Fair breakc

P t u s . cal Ag u s . cal Ag us. cal

No break0

No break” breakc

using an ion, such as copper, which titrates both chelating agents The content of ammonia-triacetic acid is then obtained by difference. It was ascertained later in this work that a zinc titration on such a mixture yields two breaks in the curve (see Figure 2); the first break represents the titration of the EDTA, and the second break represents the titration of the ammonia-triacetic acid. Values shown for the analysis of mixtures using zinc are given in Table 11. Conversely, chelating agent and conditions can be varied so that mixtures of metals can be determined. For instance, mixtures involving iron can be titrated a t alkaline pH’s where iron

V O L U M E 2 1 , NO. 11, N O V E M B E R 1 9 5 5

1747

_Table I. Metal Used

Titration Results with Various -Metal Ions" (continued) p H Range During Titration

Found,

Titrant, 0.1N

Solvent System

%b

Remarks

Electrodes

Titration of 1~,.~--Di(B-hydroxyethyl)glycine Pb

Pb(N0s)z

Ca

CaCOs

+ HC1

HOC H E H2 7 . 7 - 0.8 7.7 - 6.8

+

25% H20 25% pwidine 50% acetone 50% pyridine t 50% acetone

H?0

+ pyridine

99.8Rd

100.7W

...

(50-50)

.....

H g on P t u.3. cal Hg on P t u s . ea1

Fair break Fair break

Hg on P t u s . cal

S o breakc

P t 7:s. P t us. P t us. P t us.

Good break Good hreakc Poor breakc Good breakc

HOOCCH?, Titration of Ammonia Triacetic Acid Fe

H20 Hz0 HzO

+ 2 ml. 1 . O S S a O H + 2 ml. 1 . 0 s S a O H

cu

Hz0 Hz0

+ pyridine (50-50) - pyridine + 1 ml. 1.O.V

Hs

H20

+ pyridine

Zn

H20

+ pyridine (50-50)

+ pyridine (50-50)

Ph

Pb(N0:)s

H20

RIn

hl n ( Ac) 2

H 2 0 + pyridine

Ca

CaCOa

11%

MgC12

Xi

Nl(NO2)z

a

a

d a

f

+ HC1

H20

+ 4 ml.

4

SaOH

1 . 0 s KaOH

7.639.1 3.0 9.1 -

99.7ld 99.0 99.0

7.00- 6.88 8.8 - 6.8 9.1 - 7.0

99.8 99.8 99.8

2 1111. 1 . 0 s S a O H

1.73 2.4 2.1 2.4

11.6 - 7 . 8 11.6 7 . 9 11.8 - 8.1

-

cal cal cal cal

P t us. ea1 P t us. cal Hg on P t u s . ca

Very sharp break Fair breakc Good breakc

Ag u s . cal Ag u 8 . cal Ag u s . cal

Poor break" Poor breakc Poor breake

us. ea1 us. cal us. cal u s . cal us. cal u s . cal

99.0 98.6 99.0 99.44 99.40 99.44

7.637.637.607.907.907.90-

7.35 7.35 7.30 7.30 7.30 7.30

Hg Hg Hg Hg Hg Hg

on on on on on on

Pt Pt Pt Pt Pt Pt

98.82 99.01 98.82 97.5

7.427.427.4210.5 -

6.88 6.88 6.88 7.9

Hg Hg Hg Hg

on on on on

Pt us. cal P t us. cal P t u s . cal P t u s . cal

Good Good Good Good Good Good

break break break breakc breakc breakc

Good breakc Good breakc Good breakc Fair hreakc

+ pyridine (50-50)

...

.....

Hg on P t u s . cal

KO breakc

+

...

.....

99:41 99.13 99.25

.....

Hg Hg Hg Hg Hg

S o breakc S o breake Good break Good break Good break

H20 pyridine t 2 nil. 1 . O S S a O H Hz0 H 2 0 4 pyridine (50-50)

..... ..... .....

on on on on on

P t us. cal Pt us. cal Pt u s . cal P t u s . ea1 P t u s . cal

+ pyridine (50-50)

99.72 ..... H g on P t u s . cal Good break 99.72 Hg on P t u s . cal Good break 99.66 He on Pt us. cal Good break All chelating agents were prepared and recrystallized in the free acid form. Their purity was checked b y C. H , and N analysis and by titration with standalkali. All chelating agents used were in form of disodium salt. except where designated otherwise. All salt prepared from pure free acids. R u n on macro scale. All other determinations run on semimicro scale. Free acid. Calomel electrode with KKOi bridge ( 5 ) . Tetrasodium salt.

CO

ard

H?0

FeClr

S-CHzCOOH (HOOCCH; 99.91 99.4 99.8 99.4

Co(N0a)z

Table 11.

HzO

Titration of Ethylenediaminetetraacetic Acid-Ammonia Triacetic Acid Mixture with Various Metal Ions*

Metal Used Hg

Titrant, 0.1N Hg(Ac)r

cu

Cu(Ac)r

Compn. of Mix, W t . % EDTA ATA 66.9 33.1 83.49 16.51 95.29 4.71

Mole 0,0003m3 0.0001714 0.0001028

0.0005000 0 0003000 0.0003000

79.64 ?& 20.46 84.74 15.26 91.81 8.19 a Solvent system used in this series, 1 t o 1 pyridine-H20.

Zn

Zn(NOdz

Found, % EDTA ATA 66.3 ... 84.10 ... 95.99 ...

p H Range During Titration 7.3-6.4 7.2-6.5 7.2-6.5

Alole 0.0008419c 0.0004705 C 0.0004005C

oi /o

79.86 20.46 7.2-6.: 85.25 15,21 7.2-6 a 92.18 8.24 7.2-6.5 b Calomel electrode using KNOa bridge ( 5 ) .

does not chelate. If the mixtures involve mercury, calcium, or magnesium, the titrations can be run using either ammonia-triacetic acid or N,N-di(P-hydroxyethyl)glycine, which show no chelation for these ions under certain conditions. Table I11 shows results of analysis of mixtures of copper-calcium or copperiron using these described approaches. It is possible to choose electrode systems which are specific for certain metals (8) in a mixture, although this was not tried in this series of experiments. A mixture of calcium, lead, and zinc ions was titrated with EDTA to see if a differential titration could be obtained. I n pyridinewater all three were titrated with only one break in the curve. I t may Fell be possible, honever, to obtain a differential titration

C

Electrodes Ag u s . cal b A g u s . calb A g us. cal b

Remarks 1 good break 1 good break 1 good break

P t u s . cal P t us. cal P t us. cal

Very good break Very good break Very good break

H g on P t u s . cal H g on P t us. cal H g on P t u s . cal Total moles EDTA

2 very good breaks 2 very good breaks 2 very good breaks and .ITA.

of mixtures of ions by selecting the proper titrant, solvent, electrodes, and conditions. METAL CHELATES

The approach described makes it possible to analyze a metal chelate sample to determine the amount of any free chelating agent or any free metal ion that may be in the sample. I n titrating any free chelating agent in a metal chelate, the metal ion used as titrant should be the same metal uaed in the chelate. This is necessary because if a metal ion used as titrant forms a more stable chelate with the agent used in the metal chelate, the metal used as titrant will replace the metal in the chelate, and the final result of the titration will not represent the free chelating agent

1748

ANALYTICAL CHEMISTRY

hut the total chelating agent (free pluq conihined) in the sample. The same is true in titrating free metal ion in a metal chelate; the chelating agent used as titrant should be the same as that uqed in the metal chelate. EDTA-metal complexes may also contain the ammonia-triacetic acid-metal complex, for most commercially available CDT.1 contains some ammonia-triacetic acid. If an EDTA qolution is used to titrate the excess metal ion in an EDTA-metal complex containing ammonia-triacetic acid-metal complex, then the EDTA added will not only complex the free metal ion but also remove the metal from the ammonia-triacetic acid causing the value for free metal t o be high. This is due to the fact that EDTA is a much stronger complexing agent than ammoniatriacetic acid. I n the case, then, of EDTB-metal complexes it would be advisable to titrate the excess metal with a standard solution of ammonia-triacetic acid, as this chelates only the free metal and cannot break up any of the complexes in the svstem. However, titrating a metal with ammonia-triacetic acid results in poorer titration breaks than titrating this chelating agent with a metal For best results, a known amount of ammonia-triacetic acid iq added to the sample, and the excess is titrated with the same metal used in the complex. The amount of ammonia-triacetic acid consumed is a measure of the free metal in the sample. Metals such a s calcium, magnesium, or mercury, which show little or no chelation 15 ith ammonia-triacetic acid, cannot be handled in thiS manner

2

0

4

6

8

mi. Figure 2. Titration of ethylenediaminetetraacetic acid-ammonia-triacetic acid mixtures w-ith zinc Mercur) on platinum DS. calomel electrode system and 1 to 1 pyridine-water solvent system

The metal chelate content of a sample can be determined in one of several m-ays. The free metal ion in a sample of metal chelate is first determined as just outlined. Then a separate Metal qample is digested by an acid (sulfuric acid or a mixture of sul- Metal Used, hlillimole Found, Titrant, 0.1.v cu Ca Fe llillimole Remarli furic and nitric acids) digestion to destroy the organic portion of Copper-Calcium Mixturesa the sample, and the total metal is determined by standard inorEDTA 0,3018 0.3096 . 0 . 6 0 8 8 total Good break ganic methods or by titration xyith EDTA. The difference be, . 0.5081 total Good break 0.3018 0.2064 tween free metal ion and total metal present is taken as the metal 0.3018 0.1032 0.4059 total Good break .4TA 0,3018 0.3096 0.2966 Cu Poor break chelate. Another approach is the determination of free chelat0.3018 0.2064 .... 0.3001 Cu Poor break , , . 0.2984 Cu Poor break 0.3018 0.1032 ing agent as outlined, then titration of a separate sample with a metal that forms a more stable complex with the chelating agent Copper-Iron hlixtures than the metal already on the chelate. For example, the calcium0 3162 0 3018 Cu E D T A b 0 3018 Good break 0 6324 0 3056 Cu 0 3018 Good break EDTA complex can be titrated n i t h iron a t somewhat acid pH’s, 0 1054 0 3013 Cu 0 3018 Good break 0 1956 0 4930 total Good break AT.4 C 0.3027 and the iron replaces the calcium on the chelate (see Figure 3 1. 0 3162 0 6211 total 0 3066 Good break Some rewlts are shown in Table IV. 0 0978 0 3988 total Good break 0.3027 A modification of this approach is to vary the titration condia Ca content can be obtained b y difference. Pyridine-vater, 1 t o 1, was used as solvent, and H g on P t us. calomel electrodes were used. tions to destroy the metal chelate and then titrate the freed b Pyridine, 1 t o 1, was used a s solvent during tiration with E D T A . This permits titration of copper alone. Water alone was used as solvent during chelating agent with a metal ion that will complex under these $TA titration. Ag us. calomel electrodes were used in both titrations. Iron conditions. For euamplc, iron-EDTA under certain alkaline content can be obtained from difference between ATA and E D T A titrations. c Excess ATA added and excess back-titrated n i t h 0.1.V cupric acetate. conditions is not stable, and the iron will form the hvdroxide. The freed EDT.1 Table IV. Titration of Ethylenediaminetetraacetic .4cid Metal Chelates can be titrated under alkaline Solvent Svstem conditions using copper (see Metal Titrant, HD. Pyr, TSPa, Used, Found, ElecUsed 0.1‘2’ id. ml. 0. Mole 7% trodes Remai hs Table IT7). Mercuric ion can also be used instead of copper. Ethylenediaminetetraacetic Acid-Iron Chelate (XaFe-EDTA) 98.56 .Ig us. calb Verv good break When the metal chelate conHp Hg(Ac)z 35 1.5 1.0 .. 85 0 2.0 100.30 -4g us. cal b Pogr break tains a mixture of chelating 35 15 1.5 100.25 Ag us. calb Very good break 35 15 2.0 99,25 Ag US. calb Very good break a g e n t s s u c h a s EDT.1100.02 Cu Cu(Ac)z 35 13 1.0 Pt us. cal Very sharp break 9 9 . 8 3 Pt us. cal Very sharp break ammonia triacetic acid, care 35 13 1.5 .. 35 15 2.0 99.54 P t us. cal Very sharp break should be exercised in choosing Ethylenediaminetetraacetic .Icid-Calciriin Chelate (EDTA-CaNa2) conditions and titrants. In Fe FeCla Hz0 9 8 70 Pt us. cal Very good break the case of an iron chelate .. 98 81 Pt u s . ea1 Very good break made from EDTA containhlixture of Ethylenediaminetetraacetic Acid-Iron Chelate and Ammonia-Triacetic Acid-Iron Chelate ing ammonia-triacetic acid, E DT.4ATANaFe Fe Mole the amount of each chelate Hg Hg(Ac)z 35 15 2.0 0,000247 0.000128 0.000244C Ag u s . cal Very good break can be determined as follows. Cu Cu(Ac)l 35 15 2.0 0.000247 0.000128 0.000378d Agus. cal Poor break Any free iron or excess chea Trisodum phosphate. E D T A complex alone. lating agent is determined as b Calomel electrode, using KNOa bridge (6). d Total of E D T A a n d ATA complexes. _______ __ . ___ described above. A separate Table 111.

Analysis of Copper-Calcium and Copper-Iron Mixtures

~

C

~

1749

V O L U M E 27, NO. 11, N O V E M B E R 1 9 5 5 sample is treated with trisodium phosphate in a pyridine-water system. The alkalinity of this system breaks the iron chelate. The liberated chelating agents can be titrated with mercury which yields only the EDTA content. h separate, similarly treated sample can be titrated with copper to yield the total of EDTA and ammonia triacetic acid. The various components of the sample can then be obtained from these data. Table 11indicates some results obtained on synthetic mixtures of an iron chelate composed of a mixture of the EDTA and ammoniatriacetic acid complexes. Preparations of Titrants

27 grams ferric chloride hexahydrate dissolved in m t e r . 6 grams of copper diwolved in con1. 0 . 1 s CU(N0B)Z centrated nitric acid. If pure copper is used for this solution, the strength of the solution ran be determined from the weight of copper used. 3. 0.1-V Hg(CH,C00)2 31 grams mercuric acetate dissolved in m t e r and a few milliliterP of acetic acid to clear solution. 4. Zn( " J J ) ~ 30 grams zinc nitrate hexahydrate dissolved in i n t e r . 33 grams lead nitrate dissolved in 5. 0 . l S Pb(S03)Z water. 20 grams manganese dichloride tetra6. 0.1.)- MnCl? hydrate dissolved in water. 0.1S Mn(CH3C00)2 24.5 grams manganous acetate tetrai. hydrate dissolved in water. 10 grams calcium carbonate dib8. 0. LV CaCO3 solved in concentrated hvdrochloric acid. 25 grams magnesium sulfate hepta9. 0.1N MgSOa hydrate dissolved in water. 29.1" grams cobaltous nitrate hexa10. 0.1*\- Co(X03)2 hydrate dissolved in m t e r . 29.1 grams nickel nitrate hexa11. 0.l.V Ni(SOd)2 hydrate dissolved in water. In each case the titrant was diluted to 1 liter x i t h water. 1. 0.1S FeCI3

acid form, it can be converted to the disodium salt by adding 2 moles of sodium hydroxide per mole of agent. The sample is dissolved in the sodium hydroxide solution (heating will help in most cases). Converting to the disodium salt is done for solubility purposes. When pyridine plus water IS used as the solvent for the titration, converting t o the disodium salt is not necessary. although in most cases it improves the breaks obtained. For determining the amount of chelating agent in EDTA-iron chelate the procedure is varied somewhat: One-tenth gram of iron chelate is dissolved in 35 ml. of watei . From 1 to 2 grams of trisodium phosphate is added. After the chelate and the trisodium phosphate are dissolved, 15 ml. of pyridine is added and the solution heated a t 60" to 70" C. for 5 minutes. After cooling to room temperature, the solution iq titrated m-ith mercury or copper ion using the electrodes obtained from Table I. Further details and results can be seen in Table IV.

-

2

m I.

3

4

Figure 3. Ethylenediaminetetraacetic acid-calcium chelate titrated with iron(II1) chloride

STANDARDIZATION OF TITRANTS

All the titrants used can be standardized by inorganic means. A more convenient and much more rapid method of standardization is to titrate a known amount of disodium EDTA dihydrate potentiometrically, using the electrodes and solvent system for each metal from Table I. This EDTA salt can be used as a primary standard ( 2 , 3 ) . PREPARATION OF ELECTRODES

Mercury on Platinum Electrode. A 1.5 X 1.5 cm. piece of platinum foil is welded to a piece of platinum wire about 10 to 12 em. long. Two of these electrodes are placed in a 1 to 3% mercuric acetate solution containing 3 to 5 ml. of concentrated nitric acid per 100 ml. The electrodes are then connected to a 4- to 6-volt direct current supply and allowed to plate from 15 to 20 minutes. The electrode connected to the negative terminal will then be covered n-ith a smooth coating of mercury. The platinum should be cleaned thoroughly before attempting to plate n ith mercury. This can be done by dipping the electrode into concentrated nitric acid and burning in a Bunsen burner flame. Frequent replating is advisable to keep the breaks as large and sharp as possible. Platinum Electrode. The platinum electrode connected to the positive terminal when preparing the mercury-platinum electrode is cleaned by dipping in concentrated nitric acid and burning in a Bunsen burner flame. This is then used for all the titrations invoIving n platinum electrode. Silver Electrode. A piece of silver wire 10 to 12 cm. long is used for all titrations involving the silver elertrode. Calomel Electrode. For all titrations, except the semimicro mercury titrations, a Beckman S o . 1170 saturated calomel electrode is used. Calomel Electrode with Potassium Nitrate Bridge. This e l e r trode ( 6 )is used to keep the solution free of chloride ion contamination, which could give erroneous results in the micro scale method when mercury(I1) ion is used as titrant. On the macro scale, the calomel electrode described above was used for mercury titrations and operated well. PROCEDURE

For semimicroanalyses, a 0.1-gram sample is used; for macroanalyses, a 1.0-gram sample is taken. If the agent is in the free

For semimicroanalyses the sample solution should consist of 30 to 40 ml. in a 150-ml. beaker. The electrode systems used can be obtained from Table I. The potentiometric titration is carried out using a semimicroburet. For macrodeterminations, 60 to 80 ml. of solution is used. A ;\lode1 H2 Beckman pH meter is used for the titrations. ACKhOWLEDGRlERT

The authors are indebted to Max E. Chiddix and C. R. Enyeart, Central Research Laboratory, General hniline 8: Film Corp., foi the preparation of the pure forms of the chelating agents used in this work. The contribution of Fred G. RIaiscli in the early stages of this work is also acknowledged. LITERATURE CITED

Biedermann, IT., and Schwarsenbach, G., Chimiu (Switz.), 2, 56 (1948). Blaedel, W.J., and Knight, H. T., ARAL.CHEM.,26,741 (1954) Ibid., p. 743. Blaedel, W.J., and Malmstadt, H. V., I b i d . , 2 4 , 455 (1952). Edsberg, R. L., Eichlin, D., and Garis, J. J., Ibid., 25, 798 (1953).

Furness, TV., Crawshaw, P., and Davies, W. C., Analyst, 7 4 , 629 (1940). , \ - - -

Hall, L. H., Gibson, J. A . , Jr., Wilkinson, R. P., and Phillips H. O., ASAL. CHEM.,26, 1484 (19.54). (8) Kolthoff, I. bf., and Furman, N. H., "Potentiometric Titrations," 2nd ed., Wiley, New York, 1931. (9) Laitinen, H. A., and Sympson, R. F., A x . 4 ~ .CHEM.,26, 55fi (1954). (10) Piibil, R., Koudela, Z . , and Blatyska, B., Collection Czechoslol,. Chem. Communs., 16, 80 (1951). RECEIVED for review September 3, 1954. Accepted June 29, 1955.