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Acid-base equilibria in ethylene glycol—III selection of titration conditions in ethylene glycol medium, protolysis constants of alkaloids in ethylene glycol and its mixtures
SHORT Talanra. Vol
23, pp 587-590
Pergamon
COMMUNlCATlONS
587
Press, 1976. Prmted m Great Bntam
ACID-BASE
EQUILIBRIA IN ETHYLENE GLYCOL-III SELECTION OF TITRATION CONDITIONS IN ETHYLENE GLYCOL MEDIUM, PROTOLYSIS CONSTANTS OF ALKALOIDS IN ETHYLENE GLYCOL ITS MIXTURES
AND
P. ZIKOLOV, T. ZIKOLOVA and 0. BUDEVSKY Faculty of Pharmacy, Academy of Medicine, Ekz. Josif No 15, Sofia, Bulgaria
(Received 27 November 1975. Accepted 16 January 1976)
The main purpose of this series of papers’,’ is the development of a general approach for the selection of the most appropriate conditions for volumetric determination of acids and bases in ethylene glycol (EG). For many nonaqueous solvents this selection is usually made empirically, but in the present treatment a somewhat different approach is proposed. EG has properties very similar to water: amphiprotic behaviour, relatively high dielectric permeability, tendency to form hydrogen bonds etc., which is why some methods used for the selection of suitable titration conditions in water could, in principle, be applied to EG. A number of methods based on examination of the titration curve are used for this purpose,3-6 but the graphical method5s6 IS probably the easiest. For its application, the acid-base constants of all protolytes taking part in the titration equilibrium have to be known. That is why m the previous papers the autoprotolysis constants of EG and its mixtures’ and the constants of some acid-base indicators’ were determined. The present paper describes the method for selection of the titration conditions. Since EG is most frequently used as a solvent medium for the titration of water-insoluble weak bases,’ a number of alkaloids were studied in this investigation, and this necessitated determination of their protolysts constants in EG medium. EXPERIMENTAL
Reagents
Ethylene glycol, its mixtures and the titrants were purified, prepared and standardized as described earlier.i*2 Reagent-grade codeine and ephedrine were used after drying in a vacuum desiccator, without purification. Morphine, papaverine and quinine were prepared by making alkaline aqueous solutions of their hydrochloride and recrystallization from ethanol-ether mixture.
The experimental arrangement and the procedure was the same as before.’ The temperature was kept at 25” + 0.2”. Standardized O.lM hydrochloric acid was used to titrate ca. O.OlM alkaloid solution in O.lM potassium chloride. The titration was continued after the equivalence point so as to yield data which were used to calibrate the electrochemical cell for the particular experiment. The e.m.f. of the cell (E) at 25” is given by the equation E = E”’ - 0.05916 log[SH;]
E - E”
PCH= -1wCSH:l = o.05916
The protolysis constants were determined by potentiometric titration using a cell without liquid junction, equipped with a glass and a silver-silver chloride electrode:
e,;;g;de
O.lM(B + KC1 + HCI), SH AgCl-Ag /1
* In this work as in the previous one,’ the EG mixtures are treated as separate solvents with then own acid-base properties and dielectric constant.
(2)
The protolysis of the conjugate acid (BH+) of the alkaloid (B) in EG or EG mixture (SH)* can be presented by the equation BH+ +SH=B+SH:
(3)
The concentration (KcaH+)and the thermodynamic protolysis constants are given by the equations:
(&a+)
K’BH+ = CSH:l (CBI/CBH+I) Ken+ = {SH:}({B;/{BH+}) = K’a,, +fSH;
(4) (f&H
*)
(5)
Since [BH+] = [Ha] + [S] and [B] = [B],,, - [HCl] - [S-l, from (2) and (4) a relationship can be written, which is used in practice for calculation of the pK’,,+values: E - E”
pK’,n+ = o.05916 + log Apparatus and procedure
(1)
The E”‘-value of the cell was calculated from the titration data after the equivalence point by means of equation (1). From this equation it follows that
[HCl] + [S-l
CBlmt - WC11- K-1
where [HCl] is the molar concentration of the added acid, is- the total concentration of the alkaloid base deterCBlmt mined by a Gran plot,* and TS-1 is the molar concentration of the lyate ibn of the solvent. The latter was calculated only for pen > 10, from the values of the concentration autoprotolysis constants.’ As an illustration of the method used for calculation of the protolysis constant, the experimental data from a potentiometric titration and their treatment for the determination of the pK ‘a”+-value of codeine in EG are shown in Table 1.
588
SHORT
Table
1. Experimental
COMMUNICATIONS
data and their treatment for the calculation in ethylene glycol
of pK&,+
of codeine
v“Cl> ml
-losCBH+l/CBI
PCH
0.600 0.800 1.000 1.200 1.400 1.600 1.800 2.300 2.400 2.500 2.600 2.800 3.000 3.200 Mean
f95.4 + 85.5 + 16.2 + 66.9 +569 f45.4 + 30.4 -293.5 -307.2 -315.8 - 322.0 -331.6 -338.5 - 343.9 E” = -482.7
-482.9 -482.9 -482.7 -482.4 -482.6 -482.7 -482.8 If- 0.2*
9.773 -0.415 9.597 - 0.230 9.448 - 0.064 9.291 + 0.097 9.112 + 0.265 8 92-l +0.456 8.669 + 0.699 Mean pKE,u + = 9.376 kO.01 l*
PK&+ 9.358 9.367 9.384 9.388 9.387 9.383 9.368
Co~idrttorzs: 20ml of 1.08 x 10V3M codeine tttrated with 0.1001 M hydrochloric acid m ethylene glycol O.lM m potassmm chloride. The base was neutralized with 2.160 ml of hydrochlortc acid (this volume was determined by Gran plot). * Confidence interval (p = 0.95)
If it is assumed that the activity coefficients m equatton con(5) arc&n; = Jau + andf,, = 1, then the concentration stants determined in the described manner are equal to the thermodynamic ones. The identical values obtained for the constants by measurements at two different ionic strengths (0.1 and 0.05) confirm the validity of this assumption. The protolysis constants of the conjugate acids of some alkaloids (pK,,+) determined in the present work in both EG and its mixtures are given m Table 2. In the same table the protolysis constants for the corresponding bases relation the well-known computed by (PK,) pK, = PK, - PKHH+ are also given. The KS values are the autoprotolysis constants of the solvent, determined m the previous work.’
OUTLINE
(pK,) of the solvent. In Fig. 1 the construction of a titration curve for O.OlM codeine is shown. The left hand part represents an lc-diagram. Note that the length of the pH-scale, m accordance with the autoprotolysis constant of EG, is 15.7 units. The basic principles and construction details of the Ic-diagram are discussed in full by Sillen’ and other authors,“~” and therefore no details will be given here. It will only be mentioned briefly how titration curves are constructed by using an Ic-diagram. Point 1 in the lc-diagram (see Fig. 1) represents the initial pH-value of the EG solution 0.01M m alkaloid (B). This point is transferred to the right-hand part of the figure at position 1’ at Y = 0% added titrant. Point 2 represents the pH-value of a O.OOlM solution of BH+, hence 10% of the alkaloid B present has been converted into its conjugate acid BH+. Therefore point 2 1s transferred to 2’ at Y = 10%. The crossing point 3 corresponds to the equality [B] = [BH+], hence this point is transferred to Y = 50% (3’). Point 4 corresponds to a pH-value of a solution which contains 10% of the initial concentration of the free base and point 4 is transferred to 4’ where Y = 90%. Similar considerations transfer point 5 to 5’ at Y = 99%, 6 to 6’ at Y = 99.9%. Pomt 7 is the pH-value of a O.OlM solution of BH+, hence 7 is transferred to 7’ at Y= 100%. After this point the lyonium ions are in excess, consequently their concentration can be read from the SH:-line. It is obvious then, that point 8 corresponds to a surplus of the acid, or Y = 100.1%. Similarly, points 9, 10 and 11
OF THE METHOD
The theoretical titration curves of all the alkaloids were constructed. The curves were drawn by a graphical method,6 based on the logarithmic concentratton diagram, (lc-diagram). Thts type of diagram is widely used for the presentation of acid-base equilibria in aqueous media, but so far has not been applied to a non-aqueous medium. An essential difference when the lc-diagram is applied to a non-aqueous solvent is the length of the pH-scale, which, as well known, is defined by the autoprotolysis constant Table 2. Protolysis
constants*
of alkaloids
Quinine I Papaverine Codeine Morphine Quinine II Ephedrine
PK,,
+
5.23 7.25 9.38 9 51 9.90 11.29
glycol
EG-E
EG Alkaloid
in ethylene
and its mixturest EG-EMK-Ch
EG-W
PK~
PK,,, +
PK,
10.49 8.47 6.34 621 5.82 4.43
5.25 7.30 9.42 9.47 9.94 1121
10.66 861 649 6.44 5.91 4.70
* Confidence mterval pK,,,, . & 0.01 (p = 0.95) t Symbols: EG--ethylene glycol, E-ethanol (IO%), W-water (5 + 5%).
PK,, 4.49 6.52 8.71 8.81 9.18 10.46
(5%)
+
PK,
PKBH+
PKB
7.14
8.67
9.31
6.50
10.36 8.33 6.14 6.04 5.67 4 39
EMK-Ch+thy1
methyl
ketone-chloroform
SHORT
COMMUNICATIONS
are transferred to Y = 101, 110 and 200%. After the main points of the titration curve are known, it is an easy matter to draw the full line as shown in Fig. 1. The part of the titration curve which covers the pH-values corresponding to Y = 99.991OO.l’A may be called the equrvajence region of the titration curve. For end-points within this range of pH-values the error of the titration is less than or equal to +O.l%. It must be added that the equivalence region is easily found from the k-diagram as shown by an arrow in Fig. 1. After the theoretical curve is known, an appropriate acid-base indicator can be chosen for the detection of the end-point. Obviously the transition interval of the mdicator must coincide with the equivalence region of the titration curve. As can be seen from Fig. 1, Bromophenol Blue (BPB) is a very suitable indicator for the accurate ( f 0 1%) determmation of codeine in EG. There is quite good agreement between the theoretical titration curve and the experimental data as shown in Fig. 1, where the open circles denote the experimentally measured pH-values. Also in accordance with theory, a very sharp colour change was observed with Bromophenol Blue as indicator. Figure 2 shows the theoretical curves for O.OlM solutions of the alkaloids investigated, constructed similarly.
08
589
50
loo
150
200
250
3 0
Y. % Fig. 2. Theoretical titration curves of O.OlM solutions ephedrine (I), quinine (2), morphine (3) and papaverme in ethylene glycol.
of (4)
DISCUSSION
The results from this series of investigations show that acid-base equilibria in EG are simple and similar to those in water. This allows the application of methods commonly used in investigations of acid-base equilibria in water solutions. The utility of EG and its mixtures as a medium for the determination of water-msoluble weak bases has been proved by means of a detailed study. From the theoretical curves (Fig. 2) tt is shown that the visual titration error is kO.l% for 0.01 or O.lM solution of these alkaloids in EG and its mixtures, except for quinine and papaverine where the error may be up to +0.5x. The water content (5%) of EG, though it makes the titration conditions worse by shortening the equivalence region of the curve, does not affect the accuracy of the determination. It is found both theoretically and experimentally that the most suitable indicator for titration of Bromophenol Blue the alkaloids investigated was (pKh, = 6.49). except for quinine, for which Bromocresol Green (pK’,, = 7.38) was best.
,_ 14 -
u Fig. 1. Titration acid in ethylene
According to theoretical considerationsi the titration of bases IS improved in solvents that decrease the Ks/Ka ratio, i.e., increase the pKsn+ -value. The pKan+-values of the alkaloids in EG and EG mixtures, compared with the correspondmg values in water, 10% acetone-water, methanol and formic acid” show that EG as medium provides better titration conditions. The pK,n+-values in EG (Table 2) are on average 1.5 units higher than those in water. Since the protolysis of charged acids of the type BH+ does not involve any charge separation [equation (3)], the decrease of the acid strength can be attributed to the lower basicity of EG.13 Because EG is more acidic than water,‘.” a greater basicity of the alkaloids should be expected. Nevertheless, Table 2 shows that the pK,-values are on average 0.3 units higher in EG than in water. Since the protolysis of an uncharged base (B) is accompanied by charge separation, this difference can be explained by the influence of the lower dielectric permeability of EG. The addition of ethanol (10%) and ethyl methyl ketone+hloroform (5 + 5%) does not affect significantly the strengths of the bases and their conjugate acids. The addition of water, as expected, leads to an increase in the acid strength-the pKau+ -values are on an average 0.7 units lower in EG-water (5%) mixture than in pure EG. The results obtained in this series of papers provide a means of avoiding the empirical approach for the selection of titration conditions for this non-aqueous solvent. The convenient potentiometric method permits an accurate determination of the constants needed for the construction of the theoretical titration curves. These curves constructed by the lc-diagram method allow a rapid and accurate enough prediction of the titration conditions, ciz. evaluation of the titration error, choice of a proper indicator, appropriate concentration of the base to be titrated, etc. There is reason enough to expect that the same approch could be extended to other amphiprotic solvents possessing properties similar to those of EG.
Y. %
curve of O.OlM codeine with hydrochloric glycol. -_t theoretical titration curve o experimental data
Acknowledgemenrs-The authors are indebted to the High Medical Scientific Council, Ministry of Health of P. R. Bulgaria for financial support for this work.
SHORT COMMUNICATlONS
590
I.
REFERENCES
1. P. Zikolov, 22, 511. 2. P. Zikolov
A. Astrug
and 0. Budevsky,
and 0. Budevskv.
Talanta,
1975, 8.
ibid.. 1973. 20. 487.
9,
T. A. Khudyakova and A. P: Kreshkov, j. Eiectroanal Chem., 1971. 29, 181. J. -P Escarfad. Untrrsuchungen an Stiure-Basen-Gleichgewrchtrn in organischen Lkungsmitteln, Promotionsarbeit, ETH, Ziirich, 1963. F. Seel, Grundlagen der analytischen Chemie, IV Aufl., Cherme, Wemheim, 1968. 0. Budevsky, Fundamentals of Analyt~al Chemistry, (in Bulgarian), Medicina i Fizkultura, Sofia, 1974.
10. ll. 12. 13.
S. R. Palit, M. N. Das and G. R. Somayajulu, Nonaqueous Titration, (m Russian), Goskhirmzdat, Moscow, 1958. F. J. Rossottl and H. Rossottl, J. Chem. Educ., 1965, 42, 315. L. G. Sill&n, Graphic Presentation of Equilibrium Data, in Treatise on Analytical Chemistry, eds. I. M. Kolthoff and P. Elving, Part I, Vol. I. Interscience, New York, 1959. G. H&g, Die theoretischen Grundlagen der analytwhen Chemie, BirkhPuser, Basel, 1950. J. N. Butler, Ionic Equilibrmm; (in Russian), Khlmla, Leningrad, 1973. N. A. Izmailov, Selected Works, (in Russian), ed. Naukova Dumka, Kiev, 1967. K. K. Kundu and M. N. Das, J. Chem. Eng. Data, 1964, 9, 82.
Summary-Theoretical titration curves are used for the selection of appropriate conditions for the acid-base volumetric determination of weak bases in ethylene glycol medium. The theoretical curves for titration of some alkaloids are deduced graphically on the basis of the logarithmic concentration diagram. The acid-base constants used for the construction of the theoretical titration curves were determined by potentiometric titration in a cell without liquid junction, equipped with a glass and a silver-silver chloride electrode. It is shown that the alkaloids investigated can be determined accurately by visual or potentiometric titration. The same approach for the selection of titration conditions seems to be applicable to other non-aqueous amphiprotic solvents.
Tulamu. ‘401 23. pp, 590-593. Pergamon Press. 1976Prmted m
CONCENTRATION
GreatBntam
AND SEPARATION OF TRACE METALS WITH AN ARSONIC ACID RESIN* JAMES S. FRITZ and
Ames Laboratory-ERDA
ELIZABETH M. MOVERS
and Department of Chemistry, Ames, Iowa 50010, U.S.A.
Iowa State University,
(Received 10 November 1975. Accepted 19 January 1975)
Because of their selectivity for heavy metals, chelating resins have been used for the concentration and separation of trace metals in food extracts,‘.’ oil-field brine,3 industrial waste waters,4 geological samples,’ and sea-water.6-8 Most of this work was performed on Dowex A-l (Chelex IOO), which has an iminodiacetic acid functional group. Dowex A-l is a gel-type resin which undergoes pronounced swelling and shrinking. This resin has rather slow kinetics and a comparatively high affinity for calcium and magnesium.6z9 To meet the growing needs of ecological research in pollution control” and elsewhere, new chelating resins are contmually being developed. Among those showing prormse for trace metals are dithiocarbamate,“,” natural polymer, I3 8-aminoquinoline,‘4 polyamine-polyurea” and lsothiouronium’ exchangers. Hirsch et a1.16 started with a macroporous polystyrene-DVB resin and prepared a resm containing the iminodiacetic acid functional group and another contaming an arsenic acid group. Macroporous resins are much less susceptible to swelling and shrinking and appear to have faster reaction kinetics than gel-type chelating resins. A series of macroporous arsomc acid resins has now been prepared by a synthetic method similar to that used *A non-exclusive royalty-free license in and to copyright is retained by the U.S. Government.
by Hirsch et al. I6 The effect of varying pore diameter and surface area on the properties of the final resins has been studied. These arsomc acid resins are now proposed for the concentration of trace metal ions from hard water and sea-water.
EXPERIMENTAL
Apparatus A Milton Roy Pump No. 19-60029-003 or hehum pressure was used to maintam a constant flow-rate through the resin column. An RIDL AEC 320-3 single-channel analyser was used for counting the activity of the radiotracers, at a wmdow setting of 1.3 MeV. Plasma emission analyses were performed on the ICP-OES system built at Ames LaboratorIes. Reagents XAD-1, -2, and -4 macroporous resins were obtained from Rohm and Haas. The 15&200 mesh resm was prewashed with acetone and concentrated hydrochloric acid For all trace analyses 0.5 g of resin IV was packed m a column measuring 2.8 x 0.6 cm. with 997” radio54Mn, 65Zn, and 59Fe gamma-emitters metric purity were obtained from New England Nuclear
23, pp 587-590
Pergamon
COMMUNlCATlONS
587
Press, 1976. Prmted m Great Bntam
ACID-BASE
EQUILIBRIA IN ETHYLENE GLYCOL-III SELECTION OF TITRATION CONDITIONS IN ETHYLENE GLYCOL MEDIUM, PROTOLYSIS CONSTANTS OF ALKALOIDS IN ETHYLENE GLYCOL ITS MIXTURES
AND
P. ZIKOLOV, T. ZIKOLOVA and 0. BUDEVSKY Faculty of Pharmacy, Academy of Medicine, Ekz. Josif No 15, Sofia, Bulgaria
(Received 27 November 1975. Accepted 16 January 1976)
The main purpose of this series of papers’,’ is the development of a general approach for the selection of the most appropriate conditions for volumetric determination of acids and bases in ethylene glycol (EG). For many nonaqueous solvents this selection is usually made empirically, but in the present treatment a somewhat different approach is proposed. EG has properties very similar to water: amphiprotic behaviour, relatively high dielectric permeability, tendency to form hydrogen bonds etc., which is why some methods used for the selection of suitable titration conditions in water could, in principle, be applied to EG. A number of methods based on examination of the titration curve are used for this purpose,3-6 but the graphical method5s6 IS probably the easiest. For its application, the acid-base constants of all protolytes taking part in the titration equilibrium have to be known. That is why m the previous papers the autoprotolysis constants of EG and its mixtures’ and the constants of some acid-base indicators’ were determined. The present paper describes the method for selection of the titration conditions. Since EG is most frequently used as a solvent medium for the titration of water-insoluble weak bases,’ a number of alkaloids were studied in this investigation, and this necessitated determination of their protolysts constants in EG medium. EXPERIMENTAL
Reagents
Ethylene glycol, its mixtures and the titrants were purified, prepared and standardized as described earlier.i*2 Reagent-grade codeine and ephedrine were used after drying in a vacuum desiccator, without purification. Morphine, papaverine and quinine were prepared by making alkaline aqueous solutions of their hydrochloride and recrystallization from ethanol-ether mixture.
The experimental arrangement and the procedure was the same as before.’ The temperature was kept at 25” + 0.2”. Standardized O.lM hydrochloric acid was used to titrate ca. O.OlM alkaloid solution in O.lM potassium chloride. The titration was continued after the equivalence point so as to yield data which were used to calibrate the electrochemical cell for the particular experiment. The e.m.f. of the cell (E) at 25” is given by the equation E = E”’ - 0.05916 log[SH;]
E - E”
PCH= -1wCSH:l = o.05916
The protolysis constants were determined by potentiometric titration using a cell without liquid junction, equipped with a glass and a silver-silver chloride electrode:
e,;;g;de
O.lM(B + KC1 + HCI), SH AgCl-Ag /1
* In this work as in the previous one,’ the EG mixtures are treated as separate solvents with then own acid-base properties and dielectric constant.
(2)
The protolysis of the conjugate acid (BH+) of the alkaloid (B) in EG or EG mixture (SH)* can be presented by the equation BH+ +SH=B+SH:
(3)
The concentration (KcaH+)and the thermodynamic protolysis constants are given by the equations:
(&a+)
K’BH+ = CSH:l (CBI/CBH+I) Ken+ = {SH:}({B;/{BH+}) = K’a,, +fSH;
(4) (f&H
*)
(5)
Since [BH+] = [Ha] + [S] and [B] = [B],,, - [HCl] - [S-l, from (2) and (4) a relationship can be written, which is used in practice for calculation of the pK’,,+values: E - E”
pK’,n+ = o.05916 + log Apparatus and procedure
(1)
The E”‘-value of the cell was calculated from the titration data after the equivalence point by means of equation (1). From this equation it follows that
[HCl] + [S-l
CBlmt - WC11- K-1
where [HCl] is the molar concentration of the added acid, is- the total concentration of the alkaloid base deterCBlmt mined by a Gran plot,* and TS-1 is the molar concentration of the lyate ibn of the solvent. The latter was calculated only for pen > 10, from the values of the concentration autoprotolysis constants.’ As an illustration of the method used for calculation of the protolysis constant, the experimental data from a potentiometric titration and their treatment for the determination of the pK ‘a”+-value of codeine in EG are shown in Table 1.
588
SHORT
Table
1. Experimental
COMMUNICATIONS
data and their treatment for the calculation in ethylene glycol
of pK&,+
of codeine
v“Cl> ml
-losCBH+l/CBI
PCH
0.600 0.800 1.000 1.200 1.400 1.600 1.800 2.300 2.400 2.500 2.600 2.800 3.000 3.200 Mean
f95.4 + 85.5 + 16.2 + 66.9 +569 f45.4 + 30.4 -293.5 -307.2 -315.8 - 322.0 -331.6 -338.5 - 343.9 E” = -482.7
-482.9 -482.9 -482.7 -482.4 -482.6 -482.7 -482.8 If- 0.2*
9.773 -0.415 9.597 - 0.230 9.448 - 0.064 9.291 + 0.097 9.112 + 0.265 8 92-l +0.456 8.669 + 0.699 Mean pKE,u + = 9.376 kO.01 l*
PK&+ 9.358 9.367 9.384 9.388 9.387 9.383 9.368
Co~idrttorzs: 20ml of 1.08 x 10V3M codeine tttrated with 0.1001 M hydrochloric acid m ethylene glycol O.lM m potassmm chloride. The base was neutralized with 2.160 ml of hydrochlortc acid (this volume was determined by Gran plot). * Confidence interval (p = 0.95)
If it is assumed that the activity coefficients m equatton con(5) arc&n; = Jau + andf,, = 1, then the concentration stants determined in the described manner are equal to the thermodynamic ones. The identical values obtained for the constants by measurements at two different ionic strengths (0.1 and 0.05) confirm the validity of this assumption. The protolysis constants of the conjugate acids of some alkaloids (pK,,+) determined in the present work in both EG and its mixtures are given m Table 2. In the same table the protolysis constants for the corresponding bases relation the well-known computed by (PK,) pK, = PK, - PKHH+ are also given. The KS values are the autoprotolysis constants of the solvent, determined m the previous work.’
OUTLINE
(pK,) of the solvent. In Fig. 1 the construction of a titration curve for O.OlM codeine is shown. The left hand part represents an lc-diagram. Note that the length of the pH-scale, m accordance with the autoprotolysis constant of EG, is 15.7 units. The basic principles and construction details of the Ic-diagram are discussed in full by Sillen’ and other authors,“~” and therefore no details will be given here. It will only be mentioned briefly how titration curves are constructed by using an Ic-diagram. Point 1 in the lc-diagram (see Fig. 1) represents the initial pH-value of the EG solution 0.01M m alkaloid (B). This point is transferred to the right-hand part of the figure at position 1’ at Y = 0% added titrant. Point 2 represents the pH-value of a O.OOlM solution of BH+, hence 10% of the alkaloid B present has been converted into its conjugate acid BH+. Therefore point 2 1s transferred to 2’ at Y = 10%. The crossing point 3 corresponds to the equality [B] = [BH+], hence this point is transferred to Y = 50% (3’). Point 4 corresponds to a pH-value of a solution which contains 10% of the initial concentration of the free base and point 4 is transferred to 4’ where Y = 90%. Similar considerations transfer point 5 to 5’ at Y = 99%, 6 to 6’ at Y = 99.9%. Pomt 7 is the pH-value of a O.OlM solution of BH+, hence 7 is transferred to 7’ at Y= 100%. After this point the lyonium ions are in excess, consequently their concentration can be read from the SH:-line. It is obvious then, that point 8 corresponds to a surplus of the acid, or Y = 100.1%. Similarly, points 9, 10 and 11
OF THE METHOD
The theoretical titration curves of all the alkaloids were constructed. The curves were drawn by a graphical method,6 based on the logarithmic concentratton diagram, (lc-diagram). Thts type of diagram is widely used for the presentation of acid-base equilibria in aqueous media, but so far has not been applied to a non-aqueous medium. An essential difference when the lc-diagram is applied to a non-aqueous solvent is the length of the pH-scale, which, as well known, is defined by the autoprotolysis constant Table 2. Protolysis
constants*
of alkaloids
Quinine I Papaverine Codeine Morphine Quinine II Ephedrine
PK,,
+
5.23 7.25 9.38 9 51 9.90 11.29
glycol
EG-E
EG Alkaloid
in ethylene
and its mixturest EG-EMK-Ch
EG-W
PK~
PK,,, +
PK,
10.49 8.47 6.34 621 5.82 4.43
5.25 7.30 9.42 9.47 9.94 1121
10.66 861 649 6.44 5.91 4.70
* Confidence mterval pK,,,, . & 0.01 (p = 0.95) t Symbols: EG--ethylene glycol, E-ethanol (IO%), W-water (5 + 5%).
PK,, 4.49 6.52 8.71 8.81 9.18 10.46
(5%)
+
PK,
PKBH+
PKB
7.14
8.67
9.31
6.50
10.36 8.33 6.14 6.04 5.67 4 39
EMK-Ch+thy1
methyl
ketone-chloroform
SHORT
COMMUNICATIONS
are transferred to Y = 101, 110 and 200%. After the main points of the titration curve are known, it is an easy matter to draw the full line as shown in Fig. 1. The part of the titration curve which covers the pH-values corresponding to Y = 99.991OO.l’A may be called the equrvajence region of the titration curve. For end-points within this range of pH-values the error of the titration is less than or equal to +O.l%. It must be added that the equivalence region is easily found from the k-diagram as shown by an arrow in Fig. 1. After the theoretical curve is known, an appropriate acid-base indicator can be chosen for the detection of the end-point. Obviously the transition interval of the mdicator must coincide with the equivalence region of the titration curve. As can be seen from Fig. 1, Bromophenol Blue (BPB) is a very suitable indicator for the accurate ( f 0 1%) determmation of codeine in EG. There is quite good agreement between the theoretical titration curve and the experimental data as shown in Fig. 1, where the open circles denote the experimentally measured pH-values. Also in accordance with theory, a very sharp colour change was observed with Bromophenol Blue as indicator. Figure 2 shows the theoretical curves for O.OlM solutions of the alkaloids investigated, constructed similarly.
08
589
50
loo
150
200
250
3 0
Y. % Fig. 2. Theoretical titration curves of O.OlM solutions ephedrine (I), quinine (2), morphine (3) and papaverme in ethylene glycol.
of (4)
DISCUSSION
The results from this series of investigations show that acid-base equilibria in EG are simple and similar to those in water. This allows the application of methods commonly used in investigations of acid-base equilibria in water solutions. The utility of EG and its mixtures as a medium for the determination of water-msoluble weak bases has been proved by means of a detailed study. From the theoretical curves (Fig. 2) tt is shown that the visual titration error is kO.l% for 0.01 or O.lM solution of these alkaloids in EG and its mixtures, except for quinine and papaverine where the error may be up to +0.5x. The water content (5%) of EG, though it makes the titration conditions worse by shortening the equivalence region of the curve, does not affect the accuracy of the determination. It is found both theoretically and experimentally that the most suitable indicator for titration of Bromophenol Blue the alkaloids investigated was (pKh, = 6.49). except for quinine, for which Bromocresol Green (pK’,, = 7.38) was best.
,_ 14 -
u Fig. 1. Titration acid in ethylene
According to theoretical considerationsi the titration of bases IS improved in solvents that decrease the Ks/Ka ratio, i.e., increase the pKsn+ -value. The pKan+-values of the alkaloids in EG and EG mixtures, compared with the correspondmg values in water, 10% acetone-water, methanol and formic acid” show that EG as medium provides better titration conditions. The pK,n+-values in EG (Table 2) are on average 1.5 units higher than those in water. Since the protolysis of charged acids of the type BH+ does not involve any charge separation [equation (3)], the decrease of the acid strength can be attributed to the lower basicity of EG.13 Because EG is more acidic than water,‘.” a greater basicity of the alkaloids should be expected. Nevertheless, Table 2 shows that the pK,-values are on average 0.3 units higher in EG than in water. Since the protolysis of an uncharged base (B) is accompanied by charge separation, this difference can be explained by the influence of the lower dielectric permeability of EG. The addition of ethanol (10%) and ethyl methyl ketone+hloroform (5 + 5%) does not affect significantly the strengths of the bases and their conjugate acids. The addition of water, as expected, leads to an increase in the acid strength-the pKau+ -values are on an average 0.7 units lower in EG-water (5%) mixture than in pure EG. The results obtained in this series of papers provide a means of avoiding the empirical approach for the selection of titration conditions for this non-aqueous solvent. The convenient potentiometric method permits an accurate determination of the constants needed for the construction of the theoretical titration curves. These curves constructed by the lc-diagram method allow a rapid and accurate enough prediction of the titration conditions, ciz. evaluation of the titration error, choice of a proper indicator, appropriate concentration of the base to be titrated, etc. There is reason enough to expect that the same approch could be extended to other amphiprotic solvents possessing properties similar to those of EG.
Y. %
curve of O.OlM codeine with hydrochloric glycol. -_t theoretical titration curve o experimental data
Acknowledgemenrs-The authors are indebted to the High Medical Scientific Council, Ministry of Health of P. R. Bulgaria for financial support for this work.
SHORT COMMUNICATlONS
590
I.
REFERENCES
1. P. Zikolov, 22, 511. 2. P. Zikolov
A. Astrug
and 0. Budevsky,
and 0. Budevskv.
Talanta,
1975, 8.
ibid.. 1973. 20. 487.
9,
T. A. Khudyakova and A. P: Kreshkov, j. Eiectroanal Chem., 1971. 29, 181. J. -P Escarfad. Untrrsuchungen an Stiure-Basen-Gleichgewrchtrn in organischen Lkungsmitteln, Promotionsarbeit, ETH, Ziirich, 1963. F. Seel, Grundlagen der analytischen Chemie, IV Aufl., Cherme, Wemheim, 1968. 0. Budevsky, Fundamentals of Analyt~al Chemistry, (in Bulgarian), Medicina i Fizkultura, Sofia, 1974.
10. ll. 12. 13.
S. R. Palit, M. N. Das and G. R. Somayajulu, Nonaqueous Titration, (m Russian), Goskhirmzdat, Moscow, 1958. F. J. Rossottl and H. Rossottl, J. Chem. Educ., 1965, 42, 315. L. G. Sill&n, Graphic Presentation of Equilibrium Data, in Treatise on Analytical Chemistry, eds. I. M. Kolthoff and P. Elving, Part I, Vol. I. Interscience, New York, 1959. G. H&g, Die theoretischen Grundlagen der analytwhen Chemie, BirkhPuser, Basel, 1950. J. N. Butler, Ionic Equilibrmm; (in Russian), Khlmla, Leningrad, 1973. N. A. Izmailov, Selected Works, (in Russian), ed. Naukova Dumka, Kiev, 1967. K. K. Kundu and M. N. Das, J. Chem. Eng. Data, 1964, 9, 82.
Summary-Theoretical titration curves are used for the selection of appropriate conditions for the acid-base volumetric determination of weak bases in ethylene glycol medium. The theoretical curves for titration of some alkaloids are deduced graphically on the basis of the logarithmic concentration diagram. The acid-base constants used for the construction of the theoretical titration curves were determined by potentiometric titration in a cell without liquid junction, equipped with a glass and a silver-silver chloride electrode. It is shown that the alkaloids investigated can be determined accurately by visual or potentiometric titration. The same approach for the selection of titration conditions seems to be applicable to other non-aqueous amphiprotic solvents.
Tulamu. ‘401 23. pp, 590-593. Pergamon Press. 1976Prmted m
CONCENTRATION
GreatBntam
AND SEPARATION OF TRACE METALS WITH AN ARSONIC ACID RESIN* JAMES S. FRITZ and
Ames Laboratory-ERDA
ELIZABETH M. MOVERS
and Department of Chemistry, Ames, Iowa 50010, U.S.A.
Iowa State University,
(Received 10 November 1975. Accepted 19 January 1975)
Because of their selectivity for heavy metals, chelating resins have been used for the concentration and separation of trace metals in food extracts,‘.’ oil-field brine,3 industrial waste waters,4 geological samples,’ and sea-water.6-8 Most of this work was performed on Dowex A-l (Chelex IOO), which has an iminodiacetic acid functional group. Dowex A-l is a gel-type resin which undergoes pronounced swelling and shrinking. This resin has rather slow kinetics and a comparatively high affinity for calcium and magnesium.6z9 To meet the growing needs of ecological research in pollution control” and elsewhere, new chelating resins are contmually being developed. Among those showing prormse for trace metals are dithiocarbamate,“,” natural polymer, I3 8-aminoquinoline,‘4 polyamine-polyurea” and lsothiouronium’ exchangers. Hirsch et a1.16 started with a macroporous polystyrene-DVB resin and prepared a resm containing the iminodiacetic acid functional group and another contaming an arsenic acid group. Macroporous resins are much less susceptible to swelling and shrinking and appear to have faster reaction kinetics than gel-type chelating resins. A series of macroporous arsomc acid resins has now been prepared by a synthetic method similar to that used *A non-exclusive royalty-free license in and to copyright is retained by the U.S. Government.
by Hirsch et al. I6 The effect of varying pore diameter and surface area on the properties of the final resins has been studied. These arsomc acid resins are now proposed for the concentration of trace metal ions from hard water and sea-water.
EXPERIMENTAL
Apparatus A Milton Roy Pump No. 19-60029-003 or hehum pressure was used to maintam a constant flow-rate through the resin column. An RIDL AEC 320-3 single-channel analyser was used for counting the activity of the radiotracers, at a wmdow setting of 1.3 MeV. Plasma emission analyses were performed on the ICP-OES system built at Ames LaboratorIes. Reagents XAD-1, -2, and -4 macroporous resins were obtained from Rohm and Haas. The 15&200 mesh resm was prewashed with acetone and concentrated hydrochloric acid For all trace analyses 0.5 g of resin IV was packed m a column measuring 2.8 x 0.6 cm. with 997” radio54Mn, 65Zn, and 59Fe gamma-emitters metric purity were obtained from New England Nuclear