Trien and tetren as titrants in potentiometry with a silver indicator electrode

Tcrlonfcr, Vol.20,pp. 1117-l 125.Pergamon Press.1973.Printedin GreatBritain

TRIEN AND TETREN AS TITRANTS IN POTENTIOMETRY WITH A SILVER INDICATOR ELECTRODE ADAM HULANICKI, MAREK TROJANOWICZand JOANNA DOMANSKA Institute of Fundamental Problems in Chemistry, University of Warsaw, Warsaw, Poland. (Received 19 February 1973. Accepted 29 April 1973)

Sununary-The possibility of application of triethylenetetramine (TFUEN) and tetraethylenepentamine (TETREN) in metal titrations with the silver electrode as indicator was investigated. Copper, cadmium and zinc were determined in the presence of calcium, magnesium, ahuninium and iron(II1) in the concentration range from 0.02 to 2mM. The errors did not exceed 1%. On a similar basis copper and iron may be successfully determined in their mixtures under carefully controlled conditions. Copper is titrated with TRIEN, and both metals with EDTA at pH 7.58.0 in sulphosalicylate medium. The results obtained were in good agreement with those theoretically predicted.

In parallel to the development of new potentiometric sensors, interesting and analytically promising results in titrimetry may be obtained by the use of various complexing titrants. In this respect aliphatic polyamines show advantageous properties and may be used for determination of transition metals in the presence of elements of the main groups, which have lower affinity for nitrogen donors. Reilley and Sheldon’ have used triethylenetetramine (TRIEN), and Reilley and Vavoulis’ have used tetraethylenepentamine (TETREN), as metal titrants, with the mercury indicator electrode. They have shown that calcium, magnesium, aluminium and lanthanum do not interfere in the determination of transition metals. The selectivity of amines as titrants is connected with the stability constants of the metal complexes, which were investigated by several authors ‘-’ (Table 1). The higher stability of the silver complex should favour the use of the silver electrode for monitoring the course of titration, improving the end-point potential jump. 6 The silver indicator electrode has Table 1. Stability constants of polyamine complexes (Ringbom)16 TRIEN

TETREN

Cation log B, Ag+ Cd2+ cu*+ Hg2+ Zn*+

7.7 10.75 20.4 25.26 12.1

log 192 log K:HL log 81 log K&IL 13.9

8.1 6.4 3.6 5.6 5.2

7.4 14.0 24.3 27.7 15.4

8*3+ 5.6

* Additional constants for protonated complexcs16

K-&“2L = 10’54, K’H AI”BL= lo21.3.

1117

been used several times in EDTA titrations, even for concentration as low as 10-5M.7-9 Among other titrants EC3TA was claimed as advantageous for calcium titration in the presence of magnesium. “*tl DTPA was found useful in automatic determination of alkaline earths,12 and for lead in the presence of aluminium.‘J In t&s perpcr the titration with TRIEN and TETREN of invade metal ions and their mixtures, using the silver indicator elect&c, is discussed. The titration curves are considered in terms of the equation previously derived’ and its validity for these systems is checked. EXPERIMENTAL TRIEN solutions wcrc prcparcd by dissoking TRIEN sxdphatc in W?4M sodium hydroxide. The reagertt was purifki according to R&Icy and Sh&don’ and the sohuions were standardized potentiomctrioahy, using the mcroury indicator electrode and standard ooppcr nitrate solution. TETRRN solutions were prepared similarly by dissolving TETREN sulphate in water, and adding sodium hydroxide tili the pH was 74. TETREN (Koch Light) was puritlcdaccording to Rcilley and Vavoulis,z and the sohttions were standardized as for TRIEN. EDTA and D’I’PA so&ions wcrc prepared as usuai and standardized with zinc nitrate soIution. Met& ion sohztks were prepared as nitrates and standard&cd ~p~orne~~y with EDTA, using the mercury indkator &cot&c. Ah sohrtions were prepared Arom ana&tkally pure rcagccts and doubly distilkd water. The saturated borax solution wuaprepared from the twice-crystalked rcagcnt. The siivcr ckctrode was prcparcd as previously dcscribcd.9 The reference electrode was a Radiometer saturated calomel electrode K 401. Tie samph:should contain Wtl2-0~2 mmole of the metal to be dctcrmkcd, in the form of a salt ofwhich the anion dots not react signitifflntly with siivcr. To the sample in a 233-ml beaker I mi of 10-%f silver nitrate is ad&d. This makes it ncccmary to take into account a cornaction equal to 0.1 pmole of the Want. For more dilute solutions it is recommended to add the indicator ion in the form of its mIIN or TETREN complex. ‘To the sample SOml of saturated borax solution is added and after dilution to the total vohune of about 100 ml the sample is titrated with a 040144WW Want depending on the concentration of the sample. The end-point is detcrmincd graphically from the intemcetion of the two bran&cc of the asymmetrical titration curve, In the case of the symmctricai titration eurvc the end-point shouId be obtained by using the first derivative method or the Gran proeedurc. DISCUSSION

Choice

AND

RESULTS

oftitration canditions of individual ims

ions, Cu2 ‘, Cd” + and Zn’ + were titrated with TRIEN and TETREN. In our previous paper’ it was found that a borax buffer of pH 940 is the most convenient medium for titrations in which the silver indicator eiectrode is used, because silver forms rather weak complexes with the ligands used. I4 A decrease in the borax concentration improves the potential jump and a half-saturated solution was found the best. Further decrease of the borax concentration significantly influences the rate of potential establishment, and a threefold dilution ,makes the titration impossible. Nevertheless titration is also possibie in unbufFered sotution, but seems rather impractical because of instability of the conditions (see Fig. 7). In all the titrations mentioned the titration curves were asymmetrical. Results correct within 4 1% may be obtained for amounts from 04025 to 0*2 mmole in 100 ml (Table 2). The asymmetry of the titration curve is most pronounced in the case of copper, but the end-point can easily be found by extrapolation of the two nearly linear parts of the curve, even for 2 x IO-sM solution. Some titration curves of Zn2+ with TETREN are shown in Fig. 1. As rqxesentative

TRIEN Table 2. Polyamine

and TETREN

titrations

as Wants

1119

in the absence of interfering ions Titrant

TRIEN Ion determined

Taken, mmoIe

Cd2+

0.1977 0.0988 0*04940

Found, mmole 0.1981 0.2011 0.0986 0.0987 0.04973 0.04928

TETREN Error,

%

Found, mmole

+0*2 +1.2 -0.2 -0.1 +0.8 -0.2

0.00988 oGO494 cuz +

0.2066 0.1033 0.05165 0.01033 OGO250

Znz+

0.2002 O*lOOl 0*05005 0~01001 0*00500 OGO250

Theoretical

treatment

0.2063 0.2059 01036 0.1038 o-05154 0.05163 0*01031 0*01024 0.002570 0.002530

-0.15 +@3 -0.3 +0*8 -0.2 0.0 -0.2 -0.8 +2-Q $-I*1

0.1991 0.2001 0*1004 0.05009

-0.5 -0.1 +0*3 +0.1

oGM.83 o+lO489 0.00248

-3.4 -0.7 -0.8

of experimental

Error,

0.1976

-0

0.0985

-0.3

0*04920

-0.4

0.00986 0*00500

-0.2 $1.2

0.2065

-0

0.1026 0.1034 0.05165

-0.6 t-o.2 0

0.01045

+1*1

0.2000

-0.1

09999

-0.2 0.0 -0.7 +0*6

0~05003 o*OOQQ4 0~00503

%

data

In determination of a given ion the symmetrical or asymmetrical shape of the titration curve depends on whether the conditional stability constant of the indicator ion complex is greater or smaller than that of the ion determined. For the silver electrode used in our experiments the values of the conditional constants of the silver complex (QrY) are rather small (Fig. 2) and do not exceed the values for the Ct.?+, Cd’+ and Zn2+ complexes. Therefore6 the potential change AE, ,2 ,z obtained in titrations of metals of total concentration CM is given by the equation: A& 12,~

=

$

ln( CM - f&.)

indicating that it is best to perform titrations with TETREN and TRIEN at pH 9-10. The effect of the conditional constant for the silver complex is clearly seen when titration curves of the same ion but for different titrants (forming silver complexes of different stability) are compared (Fig. 3). The good agreement of experimental and calculated potential changes (A&2.2) is shown by a linear regression line, which may not be limited to polyamine titrants but also extend to some polyaminopolycarboxylic titrants.

1120

ADAM

HULANICKI. MARERTROJANOWICZ and JOANNAD~MANSKA

\b

-+0,005

0

I

I

0.5

1.0

Fraction

I

rnM

0,Ol mM

,05mM 0,20mM

I

1.s

2.0

titrated

Fig. 1. Titration of zinc in various concentrations with TETREN at pH 94 in half-saturated borax medium, C, - 1O-6Ikf, Y- 100 ml, silver indicator electrode.

__ .___._., --2

.-_

3

-4

6

I

I

7

6

I 9

I IO

I II

PH

Fig. 2. Change of conditional stability constants of silver complexes with pH. I-TETREN, 2-DTPA, 3-EDTA, AmEN.

1121

TRIEN and TETREN as titrants

Fraction

titrated

Fig. 3. Titration of @lM copper with various Wants: I-EDTA, 2--TRIEN, ADPTA. Other conditions as for Fig. 1.

I0

I

0.5

I

I

I.0 Fraction

3-m,

1.5

titrated

Fig. 4. Titration of O*lmM cadmium with TFSEN in the presence of interfering ions: I1GmM Caz+ + 1GnM Mg*+, pH 8-O; 2--O*lmM Al”+, pH 9.2; 3-l@mM Al’+, 0.OSM acetylacctonc, pH 8.7; 4-O.1mM Fe 3+, O-01M sulphosalicylate, pH 90. In all titrations: C, = 10e6M, V= 100 ml, half-saturated borax, silver indicator electrode.

ADS H~LANI~KI,MAFSICTAWANOWICZ and JOANNA DOMANSKA

1122

Titration of Cu’ +, Cd’+ and Zn2 ’ in presence of other ions From theoretical consideration it follows that Ca2+, Mg2+, Al’+ and Fe’+ should not interfere in titration of ions which complex preferentially through nitrogen donors. TNEN. Tenfold excess of calcium and magnesium does not interfere in titration of Cu2 +, Cd2 + or Zn2 + at pH 8. The use of higher pH values is rather undesirable because the reaction becomes sluggish, even at pH 8.5. The presence of aluminium in equimolar amounts interferes, except for titration of Cu2 +, because the reaction becomes slow. In the case of Cd2+ determination, this can be prevented by the use of acetylacetone as a masking agent, but not for Zn2+, which is too strongly complexed by this masking ligand. The presence of iron(lII) needs addition of sulphosalicylic acid for masking, which also complexes copper significantly, making the titration curve symmetrical. Some examples of titration curves are given in Fig. 4, and the results in Table 3. Table 3.

TIUENtitrations in the presenceof interferingions Amount of ion

Ion determiaed

Foreign ions, mtwle

Masking agent, M

determined mmole

pH

Found

Error %

04968 04966 04965 0.0966 049f53 04971

-0.2 -0.3 -0.2 -0.5 +0*3

Taken

Cd’+

1.0 W+ + 1.0 MB’+ 0.1 Al3+ 0.1 AP+ 1.0 A13+ 0.1 Fe3+

8.0 9.2 9.2 0.05 acac* a-7 0.01 sulphosalt 9.0

Cuz+

1.0 Caa+ + 1.0 Mg’+ 0.1 Al3+ 0.1 Fe”+ @l Fe3+

0.01 sulphosal 0.01 sulphosal

8.0 9.1 8.8 8.8

0.1033 0.1031 0.1027 0.1030 0.1031

-0.2 -0.5 -0.3 -0.2

Zn2+

1.0 Ca’+ + 1.0 Mgz+ 0.1 A13+ @l Fe”+

0.01 sulphosal

8.0 9.1 8.9

04994

+0*3 -0.9 -0.2

04997 04985 0.0992

acac= acctylacctone. t sulphosal = sulphosalicylic acid.

l

TETREN. In general the conclusions are similar to those for TRIEN. Some improvement in the rate of Cd’+ titration may be observed when succinate is added first, which complexes the interfering elements in a small but sufficient degree. When iron(II1) is present sulphosalicylic acid should be used. In Table 4 corresponding results are given. For both titrants the error does not exceed 1% and is < 0.5 % for 70 % of measurements. Potentiometric titration of Cu2 + and Fe3 + in mixtures At pH 8.8 copper may be titrated successfully with TRIEN when comparable amounts of iron(III) are masked with sulphosalicylic acid (Fig. 5). With EDTA as titrant both metals may be titrated when conditions are carefully controlled. At pH 7.5-8-O and sulphosalicylic acid concentrations from @Ol to O*OSM the yellow iron(II1) complex is

TRIEN and TETREN as titrants

1123

Table 4. TETILEN titrations in the presence of interfering ions

Ion determined

Amount of ion determined, mmole

Mashing agent, M

Foreign ions, mmole

pH

Taken Found

Error, %

10.8 0.0969 0.0965 -0.4 0.0977 +0.8 0.05 succinate 8.6 10.0 oGJ66 -0.3 9.0 0~0971 +0*2 8.6 O-05acac 0.0972 +0*3 9.0 0.0975 +0.6 0.0967 -0.2 0.05 sulphosal 8.0 0.004 TEA 9.1 0.0978 +o*v

Cd2+

1.0 Ca*+ 1.0 Ca*+ 1.0 Ca*+ + 1.0 Mg*+ 0.1 A13+ 1.0 AP+ O-1Fe3+ 0.1 Fe3+ 0.1 Fe3+

cu* +

1.0 Ca*+ + I.0 Mg*+ 8.0 0.1033 0*10&l +0.7 1.0 Ca*+ + 1.0 Mg*+ 8.0 0.1042 +o*v 0.1 Al’+ 9.1 0.1031 -0.2 0.1 Fe3+ 0.01 sulphosal 8.8 0.1026 -0.7

Zn*+

1.0 Ca*+ +1-O Mg*+ 1.0 Ca*+ + 1.0 Mg’+ 0.1 AIS+ O-05 acac 0.1 Fe3+ 0.01 sulphosal

9.0 o+lQ95 oGQQ5 0 8.0 0.1007 +1*2 9.0 0.1002 +0*7 9.0 0+992 -0.3

200 r t v)

I50

z

s I

E 100

3

‘0 a. 50 0

3 Volume

I5

IO of

0,Ol

wont

rolution,

20

ml

Fig. 5. Titrations of copper and iron(m) using silver indicator electrode. L-O~lmM Fe3+, O*OlMsulphosalicylate, pH 8.0, titrant EDTA; 2-0lmM Cu*+ +O.lmM Fe3+, O$lOlM sulphosalicylate,pH 8.8, Want TRIEN; 3-4l*lmM Cu*+ + O-l& Fe’+, O~OSMsnlphosalicylate, pH 8.0,titrant EDTA. In all titrations Cti = 10m6M,half-saturatedborax, V- 100ml. sufficiently stable to prevent formation of the hydroxide but it still reacts quantitatively with the titrant. At pH < 7.5 the potential change at the equivalence point is too small, whereas at pH > 8.0 the formation of hydroxo-complexes significantly inhibits the rate of titration, even at 50-60”. On this basis both ions in mixtures can be titrated, the sum with EDTA as titrant, and TRIEN for copper.

1124

ADAM HULANICKI,MAREKT~o~~~owrcz and JOANNADOMANSKA

Table 5.

Determination of copper and iron in mixtures

Taken, mm&

[sulphosalicylate]. T&rant

Error, %

Fe

pH

M

8.2 7.0 7,s 8.0 8.0 8.0

0.01 0.05 0.05 0.05 0.05 0.05

O-1006 Fe

0.1034 0.1034

0.1016 0.0998 0.0998 0.0998 0.1016 0.1016

0.0999 Fe 0.0996 Fe O-0987 Fe O-2066 Cu + Fe 0.2060 Cu + Fe

-1.0 +0*1 -0.2 -I*0 +0*8 +0*5

0.1033 0.1033

0.0998 0.0998

8.8 8.8

0.01 0.01

0~1030 cu 0.1031 cu

-0.3 -0.2

EDTA

TRIEN

Found, mmole

Cu

CONCLUSIONS The application of the silver indicator electrode in titrations with polyamines offers several advantages compared to methods previously used. TRIEN and especially TETREN give a greater potential change than EDTA, approaching the magnitude of the end-point jump found with DTPA. Compared with the polyaminopolycarboxylic acids as titrants, polyamines permit determination of ions in several mixtures which are difficult to analyse with EDTA as titrant.

Fraction titrated

Fig. 6. Titrations with TETREN as titrant, using various indicator electrodes. l-mercury electrode, pH 4.6, acetate but&r; 2-silver electrode, pH 9.0. half-saturated borax; 3-silver electrode, pH at the end-point 9.1, unbuffered solution. In ail titrations V= 100 ml.

TRIEN and TETREN as Wants

1125

The replacement of the mercury electrode by the silver electrode bears some advantages. The potential change is in general greater (Fig. 6), improving the precision. The potential readings with the silver electrode are usually more stable,“, improving the process of titration. The negative potential range is more easily accessible even when the solution is not carefully deaerated. Finally, no undesirable precipitates are formed on the electrode surface, such as those observed on the mercury electrode surface and causing positive err0rs.l’ The one drawback of the silver indicator electrode is the difficulty of masking, and application of the procedure to mixtures of ions. In this respect the mercury electrode seems to be superior, but the use of polyamines described in this paper still offers some advantage. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

C. N. Reilley and M. V. Sheldon, Tulnntu, 1958, 1, 127. C. N. Reilley and A. Vavoulis, Anal. Chem., 1959, 31, 243. H. B. Jonassen, J. A. Bertrand, F. R. Groves and J. R. Stearns, J. Am. Chem. Sot., 1957,79,4279. C. N. Reilley and J. H. Holloway, J. Am. Cbem. Sot., 1958,80,2917. G. Schwarzenbach, Helv. Chim. Acta, 1950,33,974. A. Hulanicki and M. Trojanowicz, Talunta, 1969, 16,225. F. Strafelda, Collection. Czech. Chem. Commun., 1962, 27, 343; 1963, 28, 3345; 1965, 30,232O. J. S. Fritz and B. B. Garralda, Anal. Chem., 1964, 36, 737. W. Kemula, A. Hulanicki and M. Trojanowicz, Chem. An&it. (Warsaw), 1969, 14,481. T. Nomura and G. Nakagawa, Bunseki Kagaku, 1976,16,1309. I. E. Lichtenstien, E. Capbola and D. A. Aikens, Anal. Chem., 1972,44, 1681. E. D. Olsen and F. S. Adamo. ibid.. 1967. 39. 81. J. P. Cummings, Tulantu, 1976, 17, iO13. ’ ’ S. Hietanen and L. G. Sill&, Arkiv Kemi, 1970,32, 111. H. Wikberg and A. Ringbom, &omen Kern., 1968,418, 177. A. Hulanicki and M. Trojanowicz, Roczniki Chem., 1973,41,279. U. Hannema, G. J. van Rossum and G. den Boef, Z, Anal. Chem., 1970,250,302. Zusammenfassung-Die Verwendbarkeit von Triiithylentetramin (TRIEN) und Tetrtithylenpentamin (TETREN) bei Metalltitrationen mit der Silberelektrode als Indikator wurde untersucht. Kupfer, Cadmium und Zink wurden in Gegenwart von Calcium, Magnesium, Aluminium und Eisen(II1) im Konzentrationsbereich 0,02 bis 2 mM bestimmt. Die Fehler iiberstiegen 1% nicht. Auf tinlicher Grundlage kijnnen Kupfer und Eisen in ihren Mischungen unter sorgfiiltig kontrollierten Bedingungen mit Erfolg bestimmt werden. Kupfer wird mit TRIEN, beide Metalle werden mit EDTA bei pH 7,5-8,0 in Sulfosalicylat-Medium titriert. Die erhaltenen Ergebnisse stimmten mit den theoretisch vorhergesagten gut i&rein. R&sum&-On a 6tudiC la possibilitk d’application de la tri&hylbnet&ramine (TRIEN) et de la t&aQthylbnepentamine (TETREN) dans le titrage de m&aux avec 1’6lectrode d’argent comme indicateur. On a dad les cuivre, cadmium et zinc en la presence de calcium, magn&sium, aluminium et fer (III) dans le domaine de concentration de 0,02 g 2 mM. Les erreurs n’exddent pas 1 %. Sur une base semblable, on peut dCterminer avec suc& le cuivre et le fer dans leurs melanges dans des conditions soigneusement contr&es. Le cuivre est dod par le TRIEN, et les deux mttaux par I’EDTA & pH 7,5-8,0 en milieu sulfosalicylate. I_es r6sultats obtenus sont en bon accord avec ceux p&us thCoriquement.