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Argentometric titration of chloride with dichlorofluoroscein as an adsorption indicator: A useful modification
Short communications
617
Talanfa, 1969,Vol. 16,pp. 617to 618. PergamonPress. Printedin NorthernIreland
Argentometric titration of chloride with dichlorofluoroscein as an adsorption indicator: A useful modification (Received 24 October 1968. Accepted 18 December 1968) DICHL~ROFLUOROSCEIN wag
first used as an adsorption indicator in the argentometric titration of chloride ion by Kolthoff.’ Common practices is to watch the coagulated precipitate (floes appear about 0.5% before the equivalence point) for the sharp colour change from yellow-green to pink. The sensitivity of the end-point therefore depends on the amount of precipitate, i.e., on the chloride concentration in the solution being titrated. If the colloidal precipitate can be kept dispersed at the end-point, the colour change is observed throughout the whole volume of suspension rather than on a small amount of flocculated precipitate. This was realized by Kolthoff,s who used “protective colloids” such as dextrin, a degraded form of starch, to maintain stability of the suspension beyond the end-point. According to a recent review by NaDDer4 these “nrotective colloids” stabilize the oarticles bv mechanisms such as steric stabilization. M&h>otra’ also ;sed this technique but with Congo Red as indicator to improve the end-point for the same titration. This investigation stemmed from an attempt to use the adsorption indicator technique for the titration of butylammonium chloride, CH8(CH&NHs+Cl-, solutions; premature flocculation of the silver chloride and deterioration of the end-point colour change occurred. EXPERIMENTAL Reagents Butylammonium chloride. Prepared by passing dry hydrogen chloride through a concentrated solution of butylamine in diethyl ether. The precipitated butylammonium chloride was recrystallized twice from dry acetone. Its melting point was sharp and its NMR spectrum had the expected proton distribution. Indicator solution. A O-1% solution of dichlorofluoroscein (DCF) in 60 % ethyl alcohol. Water was doubly distilled, the second time from alkaline permanganate solution. Procedure It was found that in the presence of the butylammonium cation the simple argentometric titration technique with DCF as indicator* was inadequate because the precipitate flocculated well before the end-point. Consequently detection of an end-point depended solely on a colour change that was at best rather limited in extent and certainly less sharp than in the absence of butylammonium ions. The modified technique consisted simply of diluting the solution with three times its volume of ethyl alcohol before addition of the indicator. DISCUSSION Premature flocculation presumably occurs because of preferential adsorption of the butylammonium ion onto the negative silver chloride particle,@ causing early neutralization of the particle’s charge and concomitant destabilization. This was confirmed by mobility measurements made with microelectrophoresis apparatus of the Alexander and Saggers’ type. At constant ionic strength (ca. O-l&f) replacement of Kf by BuNH,+ caused the particle mobility to be increased from -5.2 x 10-a to -2.4 x 1O-2 mm2.V-1.sec-1. Deterioration or even total absence (depending on electrolyte concentration) of the colour change at the end-point is somewhat more difficult to explain. Mehrotra and Tandon,* in a comprehensive review of the theories of adsorption indicators, favour the existence of a definite compound between the silver cation and the DCF anion. This compound, which they have isolated, has the same colour as is seen in the titrated solution after the end-point and is more soluble than silver chloride. These workers suggest that the compound co-precipitates with the silver chloride after the end-point is reached so that the colour change is confined to the precipitate. To test this, a sample of freshly precipitated and well-washed silver chloride was prepared with a slight excess of Ag+ ions adsorbed on the surface. After centrifugation, a small quantity of DCF was added to the supernatant liquid, whereupon the precipitate rapidly turned pink whilst the supematant liquid remained green, the colour of the simple dye. Addition of more Ag+ ions had no effect on this system, which is
618
Short communications
further evidence for a simple adsorption mechanism and renders the invoking of a co-precipitation mechanism unnecessary. The most likely explanation for the poor colour change is that specific adsorption of the BuNHa+ ion displaces or masks the Agf ion on the surface, preventing formation of the coloured complex. With the addition of ethyl alcohol the precipitate remained stable even after the end-point, which was marked by a colour change from colourless to pink throughout the solution. This colour change was sharper than in the simple case of chloride titration by silver ions in the absence of interfering ions. In the absence of interfering ions such as butylammonium the original technique* can be used reasonably successfully but addition of excess of ethyl alcohol still improves the sharpness of the end-point considerably. It would seem, therefore, that addition of alcohol produces two effects. First, when butylammonium ion is present, reduction of the solution’s dielectric constant reduces the specific adsorption of this ion. Secondly the solubility of silver chloride is less by a factor of about ten in a 1: 3 water-alcohol mixture8 than in water, probably resulting in smaller silver chloride particles with greater total surface area and a greater observable colour change at the end-point. The possibility that the addition of ethyl alcohol increases the solubility of the silver complex causing colour change in the homogeneous phase as well as at the solid-liquid interface can be discarded because centrifugation of the suspension after the end point showed that the colour change was confined to the particles. Dept. of Physical Chemistry University of Sydney AustraIia
JOHN P. FRIEND
Summary-The addition of excess of ethyl alcohol renders more useful the argentometric titration of chloride with dichlorofluoroscein as adsorption indicator by improving the end-point, both when simple counter-ions are present and also when the presence of adsorbable and/ or complexing counter-ions renders the normal technique inadequate.
Zusannnenfassung-Die Zugabe von tiberschiissigem Athanol erhiiht den Nutzen der argentometrischen Titration von Chlorid mit Dichlorfluorescein als Adsorptionsindikator, da sie den Endpunkt verbessert. Dies gilt sowohl, wenn nur einfache Gegenionen anwesend sind, als such, werm die Gegenwart adsorbierbarer und/oder komplexbildender Gegenionen das normale Verfahren unbrauchbar macht.
R&nn&L’addition d’un ex&s d’alcool Cthylique rend plus utile le titrage argentometrique du chlorure avec la dichlorofluoresceine comme indicateur d’adsorption en ameliorant le point de virage, tant lorsque des ions antagonistes simples sont presents que lorsque la presence d’ions antagonistes adsorbables et/au complexants rend la technique normale inadequate. REFERENCES 1. I. M. Kolthoff, W. M. Lauer and C. J. Sunde, J. Am. Chem. Sot., 1929 51,3273. 2. A. I. Vogel, A Text Book of Quantitative Inorganic Analysis, 3rd Ed., p. 60. Longmans, London, 1961. 3. I. M. Kolthoff, 2. Anal. Chem. 1927, 71,235. 4. D. H. Napper, Sci. Progr. (London) 1967, 55, 91. 5. R. C. Mehrotra, Anal. Chim. Acta, 1948, 2, 36. 6. P. Somasundaran, T. Healy and D. Fuerstenau, J. Colloid Sci., 1966, 22, 599. 7. A. E. Alexander and L. Saggers, J. Sci. Znstr., 1948, 25, 374. 8. R. C. Mehrotra and K. Tandon, Talanta, 1964,11,1093. 9. H. Stephen and T. Stephen, Solubilities of Inorganic and Organic Compounds, Vol. 2, Part 1, p. 432. Pergamon, Oxford, 1964.
617
Talanfa, 1969,Vol. 16,pp. 617to 618. PergamonPress. Printedin NorthernIreland
Argentometric titration of chloride with dichlorofluoroscein as an adsorption indicator: A useful modification (Received 24 October 1968. Accepted 18 December 1968) DICHL~ROFLUOROSCEIN wag
first used as an adsorption indicator in the argentometric titration of chloride ion by Kolthoff.’ Common practices is to watch the coagulated precipitate (floes appear about 0.5% before the equivalence point) for the sharp colour change from yellow-green to pink. The sensitivity of the end-point therefore depends on the amount of precipitate, i.e., on the chloride concentration in the solution being titrated. If the colloidal precipitate can be kept dispersed at the end-point, the colour change is observed throughout the whole volume of suspension rather than on a small amount of flocculated precipitate. This was realized by Kolthoff,s who used “protective colloids” such as dextrin, a degraded form of starch, to maintain stability of the suspension beyond the end-point. According to a recent review by NaDDer4 these “nrotective colloids” stabilize the oarticles bv mechanisms such as steric stabilization. M&h>otra’ also ;sed this technique but with Congo Red as indicator to improve the end-point for the same titration. This investigation stemmed from an attempt to use the adsorption indicator technique for the titration of butylammonium chloride, CH8(CH&NHs+Cl-, solutions; premature flocculation of the silver chloride and deterioration of the end-point colour change occurred. EXPERIMENTAL Reagents Butylammonium chloride. Prepared by passing dry hydrogen chloride through a concentrated solution of butylamine in diethyl ether. The precipitated butylammonium chloride was recrystallized twice from dry acetone. Its melting point was sharp and its NMR spectrum had the expected proton distribution. Indicator solution. A O-1% solution of dichlorofluoroscein (DCF) in 60 % ethyl alcohol. Water was doubly distilled, the second time from alkaline permanganate solution. Procedure It was found that in the presence of the butylammonium cation the simple argentometric titration technique with DCF as indicator* was inadequate because the precipitate flocculated well before the end-point. Consequently detection of an end-point depended solely on a colour change that was at best rather limited in extent and certainly less sharp than in the absence of butylammonium ions. The modified technique consisted simply of diluting the solution with three times its volume of ethyl alcohol before addition of the indicator. DISCUSSION Premature flocculation presumably occurs because of preferential adsorption of the butylammonium ion onto the negative silver chloride particle,@ causing early neutralization of the particle’s charge and concomitant destabilization. This was confirmed by mobility measurements made with microelectrophoresis apparatus of the Alexander and Saggers’ type. At constant ionic strength (ca. O-l&f) replacement of Kf by BuNH,+ caused the particle mobility to be increased from -5.2 x 10-a to -2.4 x 1O-2 mm2.V-1.sec-1. Deterioration or even total absence (depending on electrolyte concentration) of the colour change at the end-point is somewhat more difficult to explain. Mehrotra and Tandon,* in a comprehensive review of the theories of adsorption indicators, favour the existence of a definite compound between the silver cation and the DCF anion. This compound, which they have isolated, has the same colour as is seen in the titrated solution after the end-point and is more soluble than silver chloride. These workers suggest that the compound co-precipitates with the silver chloride after the end-point is reached so that the colour change is confined to the precipitate. To test this, a sample of freshly precipitated and well-washed silver chloride was prepared with a slight excess of Ag+ ions adsorbed on the surface. After centrifugation, a small quantity of DCF was added to the supernatant liquid, whereupon the precipitate rapidly turned pink whilst the supematant liquid remained green, the colour of the simple dye. Addition of more Ag+ ions had no effect on this system, which is
618
Short communications
further evidence for a simple adsorption mechanism and renders the invoking of a co-precipitation mechanism unnecessary. The most likely explanation for the poor colour change is that specific adsorption of the BuNHa+ ion displaces or masks the Agf ion on the surface, preventing formation of the coloured complex. With the addition of ethyl alcohol the precipitate remained stable even after the end-point, which was marked by a colour change from colourless to pink throughout the solution. This colour change was sharper than in the simple case of chloride titration by silver ions in the absence of interfering ions. In the absence of interfering ions such as butylammonium the original technique* can be used reasonably successfully but addition of excess of ethyl alcohol still improves the sharpness of the end-point considerably. It would seem, therefore, that addition of alcohol produces two effects. First, when butylammonium ion is present, reduction of the solution’s dielectric constant reduces the specific adsorption of this ion. Secondly the solubility of silver chloride is less by a factor of about ten in a 1: 3 water-alcohol mixture8 than in water, probably resulting in smaller silver chloride particles with greater total surface area and a greater observable colour change at the end-point. The possibility that the addition of ethyl alcohol increases the solubility of the silver complex causing colour change in the homogeneous phase as well as at the solid-liquid interface can be discarded because centrifugation of the suspension after the end point showed that the colour change was confined to the particles. Dept. of Physical Chemistry University of Sydney AustraIia
JOHN P. FRIEND
Summary-The addition of excess of ethyl alcohol renders more useful the argentometric titration of chloride with dichlorofluoroscein as adsorption indicator by improving the end-point, both when simple counter-ions are present and also when the presence of adsorbable and/ or complexing counter-ions renders the normal technique inadequate.
Zusannnenfassung-Die Zugabe von tiberschiissigem Athanol erhiiht den Nutzen der argentometrischen Titration von Chlorid mit Dichlorfluorescein als Adsorptionsindikator, da sie den Endpunkt verbessert. Dies gilt sowohl, wenn nur einfache Gegenionen anwesend sind, als such, werm die Gegenwart adsorbierbarer und/oder komplexbildender Gegenionen das normale Verfahren unbrauchbar macht.
R&nn&L’addition d’un ex&s d’alcool Cthylique rend plus utile le titrage argentometrique du chlorure avec la dichlorofluoresceine comme indicateur d’adsorption en ameliorant le point de virage, tant lorsque des ions antagonistes simples sont presents que lorsque la presence d’ions antagonistes adsorbables et/au complexants rend la technique normale inadequate. REFERENCES 1. I. M. Kolthoff, W. M. Lauer and C. J. Sunde, J. Am. Chem. Sot., 1929 51,3273. 2. A. I. Vogel, A Text Book of Quantitative Inorganic Analysis, 3rd Ed., p. 60. Longmans, London, 1961. 3. I. M. Kolthoff, 2. Anal. Chem. 1927, 71,235. 4. D. H. Napper, Sci. Progr. (London) 1967, 55, 91. 5. R. C. Mehrotra, Anal. Chim. Acta, 1948, 2, 36. 6. P. Somasundaran, T. Healy and D. Fuerstenau, J. Colloid Sci., 1966, 22, 599. 7. A. E. Alexander and L. Saggers, J. Sci. Znstr., 1948, 25, 374. 8. R. C. Mehrotra and K. Tandon, Talanta, 1964,11,1093. 9. H. Stephen and T. Stephen, Solubilities of Inorganic and Organic Compounds, Vol. 2, Part 1, p. 432. Pergamon, Oxford, 1964.