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Elimination of equivalence point errors in the potentiometric titration of chloride, bromide and iodide mixtures with silver nitrate
Microchemical Journal 74 (2003) 187–192
Elimination of equivalence point errors in the potentiometric titration of chloride, bromide and iodide mixtures with silver nitrate Michael J. Ellwooda,*, Keith A. Hunterb, Robert G. Cunninghameb a
National Institute of Water & Atmospheric Research, P.O. Box 11 115, Hamilton, New Zealand b Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand
Received 23 September 2002; received in revised form 22 December 2002; accepted 4 January 2003
Abstract When two or more halides are determined in solution by precipitation titration with silver nitrate as the titrant, significant errors can occur at the first equivalence point as a result of coprecipitation. Errors of up to 33% were found for the first equivalence point for solutions containing mixtures of halides at micromolar levels. The addition of a flocculating agent to the solution reduced coprecipitation by increasing the rate of exchange between the precipitated silver halide and the halide ion remaining in solution. A logarithmic relationship was observed between the charge of the flocculating agent and the logarithmic concentration of the agent needed to minimise coprecipitation. Although flocculating agents reduced coprecipitation, they do not, however, completely eliminate equivalence point errors. Here a new method is presented which effectively eliminates the problem of coprecipitation during precipitation titrations for solutions containing two halides. In order to decrease the possibility of coprecipitation, we used selective complexation of the precipitation ion Agq in order to control the AgX solubility. For example, in the case of CFy plus Xy (XsBry or Iy), we added sufficient NH3 to form Ag(NH3)q so that the free Agq activity was reduced below that required for theoretical AgCl precipitation in the absence of the other halides. Once the titration of the less soluble halide was completed and the first equivalence point determined, the Ag(NH3 )q complex was destroyed by acidification of the solution to a pH less than 6. The titration is then continued and the second equivalence point determined. Equivalence point errors were reduced to less than 1.5% with careful application of the method. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Equivalence point; Errors; Silver nitrate; Halides; Titration
1. Introduction Standard methods for determining halide concentrations in microanalysis usually involve either *Corresponding author. Tel.: q64-7-856-7026; fax: q64-7856-0151. E-mail address: [email protected] (M.J. Ellwood).
gravimetric or potentiometric precipitation with silver nitrate. Gravimetric procedures are adequate provided solutions contain only one halide; however, when two or more halide ions are present in solution gravimetric methods cannot distinguish between individual halides. Thus, only a total halide concentration is obtained. In contrast to
0026-265X/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0026-265X(03)00023-7
188
M.J. Ellwood et al. / Microchemical Journal 74 (2003) 187–192
itation, but only reduces it. In this paper we present a new method for overcoming coprecipitation and we further investigate the use of flocculating agents in reducing coprecipitation errors. 2. Experimental 2.1. Reagents
Fig. 1. Titration of 19 mmol of Iy in the presence (d) and absence (h) of 35 mmol of Bry with a 0.11 M AgNO3 solution. Vertical lines indicate where each equivalence point should theoretically occur.
gravimetric methods, potentiometric methods allow individual halide ions to be determined whilst in a mixed halide ion solution. Such methods are also simple to use, give reproducible results and do not require large volumes of solution w1x. However, several authors have reported errors in the determination of the first end point when determining halides at low concentrations w2–4x. Such errors have been ascribed to the coprecipitation of halides when titrated with silver ions w1,3,5,6x. Coprecipitation occurs when the more soluble halide precipitates before it theoretically should result in a larger first equivalence point volume and smaller second equivalence point volume. This point is illustrated in Fig. 1 where an iodide solution is titrated in the presence and absence of a second halide, bromide. When bromide is present the iodide equivalence point is over estimated by 13%. The addition of flocculating agents to solution has shown to reduce coprecipitation errors w3x. These agents work by reducing the surface charge on colloids and allowing them to flocculate quickly. Bowers et al. w3x have shown when the AgX(s) (XsCl, Br, I) colloids aggregate there is a rapid exchange between coprecipitated silver halide and the free halide with the lower solubility product left in solution. The use of such flocculating agents does not, however, completely eliminate coprecip-
All solutions used were made up to appropriate concentrations using analytical grade reagents and Milli-Q water (Millipore, USA). Sodium chloride and silver nitrate solutions were standardised gravimetrically. The relative concentrations of the sodium bromide and sodium iodide solutions were determined by titrating individually against standardised silver nitrate. A 36Cly solution was made by diluting 0.2 ml of H36Cl (0.88 M) in 100 ml of Milli-Q water. A non-radioactive chloride solution was ‘spiked’ with 60 ml of 36Cly (0.001756 M) and diluted to 1 l with Milli-Q water. This solution was then standardised against silver nitrate. 2.2. Apparatus and procedure All titrations were performed manually by adding the titrant, silver nitrate, to the stirred halide solution. The volume of each sample was between 60 and 70 ml at the start of any one titration. Silver nitrate was added to the titration via a Radiometer autoburette. The concentration of silver nitrate added was approximately 100 times greater than that of the halides being titrated w2x. Each titration took between 15 and 40 min so that the rate of delivery was less than 0.02 mlymin w3x and the pH of each titration was below 4 except when solutions contained ammonia. When ammonia was present, the titration was carried out as normal until after the first equivalence point, after which the titration was stopped and the ammonia neutralised with acetic acid until the pH of the solution fell below 6. The titration was then continued to determine the second equivalence point. Equivalence points were determined using Gran’s method for end point determination w7x. The activity of the silver ions in solution was measured by using an Ag–Ag3PO4 electrode vs. a
M.J. Ellwood et al. / Microchemical Journal 74 (2003) 187–192
189
Table 1 Percentage of coprecipitation for each halide pair determined using 36 Cl and Gran functions along with Dlog Ksp and Dionic radius for each halide pair Components in solution (mmol)
Dlog Kspa
Bry 15 Cly 46
2.55
Iy 19 Bry 35 Iy 20 CIy 48 a
Dionic radiusa
Percentage of coprecipitation of sample at first equivalence point Using 36Cl as a tracer
Using Gran functions
14
15.8
33
3.80
21
–
13.5
6.35
35
5.3
6
Data taken from Aylward and Findlay w9x.
Hg–HgSO4 reference electrode. The Ag–Ag3PO4 electrode was made by electrolysis of a silver electrode immersed in a phosphate solution to form a surface layer of Ag3PO4(s) on the electrode. The electrode was stored in Milli-Q water before use and between titrations. A Hg–HgSO4 electrode was used as the reference electrode to avoid any chance of chloride contamination of the sample. This electrode is essentially the same as a saturated calomel electrode except the Hg2Cl2(s) is replaced with HgSO4(s) and the saturated solution potassium chloride solution is replaced with a saturated potassium sulfate solution. For samples containing 36Cly, the titration was started as normal except it was stopped at where the theoretical equivalence point should have been. For example, for solutions containing two halides the titration was stopped at the theoretical first point. The sample was then flocculated by adding one drop (;0.04 ml) of 1 M aluminium nitrate to the solution. The precipitate was collected on Whatman GFyF filter paper, bagged and sealed. The radioactivity of these samples was then compared to the radioactivity of a ‘standard Ag36Cl(s) sample’ to obtain the percentage of 36Cly coprecipitated. Sample and standard radioactivities were determined by counting until approximately 10 000 disintegrations were obtained using a Geiger Muller counter. From the number of counts and the
time taken to obtain those counts, the activity of the sample was calculated. 3. Results and discussion 3.1. Coprecipitation Presented in Table 1 are the coprecipitation results for the titration of three mixed halide solutions. The results show that there is considerable error in the determination of each halide pair resulting from coprecipitation of the more soluble silver halide with the less soluble silver halide at the first equivalence point. The percentage of coprecipitation for two of the three halide combinations was evaluated using two methods: (a) determining the amount of 36Cl incorporated into the silver bromideyiodide solids; and (b) using Gran functions to evaluate the actual equivalence point and comparing this to the theoretical equivalence point to calculate the percentage of coprecipitation w7x (Table 1). For the Bry yCly pair there is considerable difference between results calculated using the 36Cl tracer and those calculated using Gran functions. This difference between the two methods results from the need to flocculate the silver halide colloids in the radiotracer method before filtration and b counting. Upon flocculation some of the 36Cl is likely to have exchanged back into solution with bromide, which forms a more insoluble silver halide precipitate w3x.
190
M.J. Ellwood et al. / Microchemical Journal 74 (2003) 187–192 Table 2 Percentage of coprecipitation for a solution containing 48 mmol of Cly and 17 mmol of Bry in the presence of three flocculating agents Cationa
Concentration (M)
Percentage of coprecipitation
Naq
0 0.16 0.32 0 0.001 0.01 0 0.0013 0.016 0.032
11.5 5.6 12.6 11.5 5.7 3.1 11.5 4.7 10.5 10.9
Baq
Al3q Fig. 2. Relationship between Dlog Ksp and Dionic radius vs. percentage of coprecipitation for each halide pair. a
There is a general decrease in the percentage of coprecipitation for halide pairs with an increasing difference between solubility product (Ksp) and halide ion size (Table 1). For example, the differences between Ksp and ion size are larger between the chloride and iodide pair hence the percentage of coprecipitation is lower than between the chloride and bromide pair where Ksp and ion size are similar (Fig. 2). Indeed, Kolthoff w8x noted that if the size difference between two halides is less than 15% coprecipitation is enhanced.
Cations were added as nitrate solutions.
3.3. Ammonia complexation of silver Although flocculating agents help to reduce coprecipitation they do not completely eliminate it. To fully overcome coprecipitation we developed a new method involving the addition a complexing ligand, ammonia in this case, to the sample solution so that each halide could be selectively precipitated with silver nitrate. To selectively precipitate, say, iodide from a chloride containing solution, the Ag(NH3)q 2 complex must be ther-
3.2. Flocculating agents The addition of a flocculating agent reduces coprecipitation by making the silver halide colloids flocculate early in the titration thus allowing the exchange rate of halide between the silver halide crystal and the surrounding solution to increase w3x. Therefore the ability of the flocculating agent to flocculate the precipitate early in the titration is critical. Motonaka et al. w2x has noted that the charge on the flocculating cation and its corresponding concentration are important factors in reducing coprecipitation to a minimum. The optimum concentrations for Al3q, Ba2q and Naq to reduce coprecipitation were 0.0013 M, 0.01 M and 0.16 M, respectively (Table 2). Indeed, a plot of cation charge vs. log concentration yields a near logarithmic relationship (Fig. 3).
Fig. 3. Relationship between cation charge and optimum cation concentration for reducing coprecipitation of Bry with AgI(s) (d) and Cly with AgBr(s) (h). Data taken from Table 2 (d) and Bowers et al. (1961) (h).
M.J. Ellwood et al. / Microchemical Journal 74 (2003) 187–192
Fig. 4. Titration of a solution containing 19 mmol of Iy and 35 mmol of Bry with (d) and without (h) added NH3 (5 mmol) with a 0.11 M AgNO3 solution. After the first equivalence point the NH3 containing solution was neutralised with 5 mmol of acetic acid and the titration continued. Vertical lines indicate where each equivalence point should theoretically occur.
modynamically more stable than AgCl(s) but less thermodynamically stable than AgI(s). Once all the iodide has been titrated the ammonia present in solution is then neutralised and the titration continued to obtain the concentration of the second halide, in this case chloride. To neutralise the ammonia after the first equivalence point has been reached, an equimolar amount of acetic acid was added to the sample. The addition of acetic acid simply reduces the ammonia concentration in solu-
191
tion by forming its conjugate acid, NHq 4 , which does not complex with silver. Presented in Fig. 4 is the titration of a solution containing both bromide and iodide with and without added ammonia. Without added ammonia there is a clear offset in the data from where the first equivalence point should theoretically occur indicating coprecipitation of bromide during silver iodide formation. For the titration with added ammonia, the first equivalence points occur where it theoretically should indicate that bromide has not precipitated prior to the first equivalence point (Fig. 4). The method of adding ammonia to the titration solution to reduce coprecipitation worked for two of the three halide pairs (Table 3). However, for the Bry yCly pair the addition of ammonia alone did not allow the accurate determination of the bromide end point prior to ammonia neutralisation. For this halide pair there was a lack of resolution at the first equivalence point making it difficult to use Gran functions to determine the bromide end point. Increased resolution at the bromide end point was obtained by adding 25 mmoles of sodium nitrate to the sample (in addition to the added ammonia), which allowed an accurate end point determination using Gran functions (Table 3, Fig. 5). As the results show, the addition of ammonia, plus sodium chloride for the bromideychloride pair, reduces coprecipitation to less than 1.5% thereby allowing the accurate determination of
Table 3 Percentages of coprecipitation for each halide pair titrated with and without added ammonia Components in solution (mmol)
Percentage of coprecipitation without added NH3
Percentage of coprecipitation with added NH3 (5 mmol)
Cly 20 Iy 48
6
0.03"0.01
Iy 19 Bry 35
12.3
0.30"0.01
Bry 15 Cly 46
33
a
Twenty-five mmol NaNO3 were also added to the solution.
1.3"0.1a
192
M.J. Ellwood et al. / Microchemical Journal 74 (2003) 187–192
Fig. 5. Titration of a solution containing 15 mmol of Bry and 46 mmol Cly in the presence (d) and absence of NH3 and NaNO3 (s). Also presented are the corresponding Gran functions for each titration (j, h), respectively. Vertical lines indicate where each equivalence point should theoretically occur.
each individual halide concentration during potentiometric titration with silver nitrate.
neutralisation the titration is continued to obtain the concentration of the second halide present.
4. Summary
References
The phenomenon of coprecipitation was investigated and found to cause serious errors when determining the first equivalence point for titrations involving two or more halides when titrated with silver nitrate. Coprecipitation can be reduced by the addition of a flocculating agent, such as aluminium nitrate to the solution, however, coprecipitation is not completely eliminated. To eliminate end point errors a new method was developed involving the addition of ammonia to the titration solution thereby allowing the selective titration of each halide. Once the more insoluble silver halide has completely titrated, the titration is stopped and the ammonia is neutralised with acetic acid. After
w1x H. Puxbaum, V. Simeonov, J. Rendle, Mikrochimica Acta 2 (1977) 325–330. w2x J. Motonaka, S. Ikeda, N. Tanaka, Anal. Chim. Acta 105 (1979) 417. w3x R.C. Bowers, L. Hsu, J.A. Goldman, Anal. Chem. 33 (1961) 190. w4x L.S. Jovanovic, J.D. Fisl, F.F. Gaal, J. Serbian Chem. Soc. 59 (1994) 175–183. w5x A. Nara, N. Kobayashi, K. Honba, Microchem. J. 20 (1975) 200–212. w6x I.M. Kolthoff, F.T. Eggerstsen, Am. Chem. Soc. 61 (1939) 1036–1040. w7x G. Gran, A. Johansson, S. Johansson, Analyst 106 (1981) 1109–1118. w8x I.M. Kolthoff, J. Phys. Chem. 36 (1932) 860. w9x G.H. Aylward, T.J.V. Findlay, SI Chemical Data, second ed, Wiley, 1974, 136 pp.
Elimination of equivalence point errors in the potentiometric titration of chloride, bromide and iodide mixtures with silver nitrate Michael J. Ellwooda,*, Keith A. Hunterb, Robert G. Cunninghameb a
National Institute of Water & Atmospheric Research, P.O. Box 11 115, Hamilton, New Zealand b Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand
Received 23 September 2002; received in revised form 22 December 2002; accepted 4 January 2003
Abstract When two or more halides are determined in solution by precipitation titration with silver nitrate as the titrant, significant errors can occur at the first equivalence point as a result of coprecipitation. Errors of up to 33% were found for the first equivalence point for solutions containing mixtures of halides at micromolar levels. The addition of a flocculating agent to the solution reduced coprecipitation by increasing the rate of exchange between the precipitated silver halide and the halide ion remaining in solution. A logarithmic relationship was observed between the charge of the flocculating agent and the logarithmic concentration of the agent needed to minimise coprecipitation. Although flocculating agents reduced coprecipitation, they do not, however, completely eliminate equivalence point errors. Here a new method is presented which effectively eliminates the problem of coprecipitation during precipitation titrations for solutions containing two halides. In order to decrease the possibility of coprecipitation, we used selective complexation of the precipitation ion Agq in order to control the AgX solubility. For example, in the case of CFy plus Xy (XsBry or Iy), we added sufficient NH3 to form Ag(NH3)q so that the free Agq activity was reduced below that required for theoretical AgCl precipitation in the absence of the other halides. Once the titration of the less soluble halide was completed and the first equivalence point determined, the Ag(NH3 )q complex was destroyed by acidification of the solution to a pH less than 6. The titration is then continued and the second equivalence point determined. Equivalence point errors were reduced to less than 1.5% with careful application of the method. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Equivalence point; Errors; Silver nitrate; Halides; Titration
1. Introduction Standard methods for determining halide concentrations in microanalysis usually involve either *Corresponding author. Tel.: q64-7-856-7026; fax: q64-7856-0151. E-mail address: [email protected] (M.J. Ellwood).
gravimetric or potentiometric precipitation with silver nitrate. Gravimetric procedures are adequate provided solutions contain only one halide; however, when two or more halide ions are present in solution gravimetric methods cannot distinguish between individual halides. Thus, only a total halide concentration is obtained. In contrast to
0026-265X/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0026-265X(03)00023-7
188
M.J. Ellwood et al. / Microchemical Journal 74 (2003) 187–192
itation, but only reduces it. In this paper we present a new method for overcoming coprecipitation and we further investigate the use of flocculating agents in reducing coprecipitation errors. 2. Experimental 2.1. Reagents
Fig. 1. Titration of 19 mmol of Iy in the presence (d) and absence (h) of 35 mmol of Bry with a 0.11 M AgNO3 solution. Vertical lines indicate where each equivalence point should theoretically occur.
gravimetric methods, potentiometric methods allow individual halide ions to be determined whilst in a mixed halide ion solution. Such methods are also simple to use, give reproducible results and do not require large volumes of solution w1x. However, several authors have reported errors in the determination of the first end point when determining halides at low concentrations w2–4x. Such errors have been ascribed to the coprecipitation of halides when titrated with silver ions w1,3,5,6x. Coprecipitation occurs when the more soluble halide precipitates before it theoretically should result in a larger first equivalence point volume and smaller second equivalence point volume. This point is illustrated in Fig. 1 where an iodide solution is titrated in the presence and absence of a second halide, bromide. When bromide is present the iodide equivalence point is over estimated by 13%. The addition of flocculating agents to solution has shown to reduce coprecipitation errors w3x. These agents work by reducing the surface charge on colloids and allowing them to flocculate quickly. Bowers et al. w3x have shown when the AgX(s) (XsCl, Br, I) colloids aggregate there is a rapid exchange between coprecipitated silver halide and the free halide with the lower solubility product left in solution. The use of such flocculating agents does not, however, completely eliminate coprecip-
All solutions used were made up to appropriate concentrations using analytical grade reagents and Milli-Q water (Millipore, USA). Sodium chloride and silver nitrate solutions were standardised gravimetrically. The relative concentrations of the sodium bromide and sodium iodide solutions were determined by titrating individually against standardised silver nitrate. A 36Cly solution was made by diluting 0.2 ml of H36Cl (0.88 M) in 100 ml of Milli-Q water. A non-radioactive chloride solution was ‘spiked’ with 60 ml of 36Cly (0.001756 M) and diluted to 1 l with Milli-Q water. This solution was then standardised against silver nitrate. 2.2. Apparatus and procedure All titrations were performed manually by adding the titrant, silver nitrate, to the stirred halide solution. The volume of each sample was between 60 and 70 ml at the start of any one titration. Silver nitrate was added to the titration via a Radiometer autoburette. The concentration of silver nitrate added was approximately 100 times greater than that of the halides being titrated w2x. Each titration took between 15 and 40 min so that the rate of delivery was less than 0.02 mlymin w3x and the pH of each titration was below 4 except when solutions contained ammonia. When ammonia was present, the titration was carried out as normal until after the first equivalence point, after which the titration was stopped and the ammonia neutralised with acetic acid until the pH of the solution fell below 6. The titration was then continued to determine the second equivalence point. Equivalence points were determined using Gran’s method for end point determination w7x. The activity of the silver ions in solution was measured by using an Ag–Ag3PO4 electrode vs. a
M.J. Ellwood et al. / Microchemical Journal 74 (2003) 187–192
189
Table 1 Percentage of coprecipitation for each halide pair determined using 36 Cl and Gran functions along with Dlog Ksp and Dionic radius for each halide pair Components in solution (mmol)
Dlog Kspa
Bry 15 Cly 46
2.55
Iy 19 Bry 35 Iy 20 CIy 48 a
Dionic radiusa
Percentage of coprecipitation of sample at first equivalence point Using 36Cl as a tracer
Using Gran functions
14
15.8
33
3.80
21
–
13.5
6.35
35
5.3
6
Data taken from Aylward and Findlay w9x.
Hg–HgSO4 reference electrode. The Ag–Ag3PO4 electrode was made by electrolysis of a silver electrode immersed in a phosphate solution to form a surface layer of Ag3PO4(s) on the electrode. The electrode was stored in Milli-Q water before use and between titrations. A Hg–HgSO4 electrode was used as the reference electrode to avoid any chance of chloride contamination of the sample. This electrode is essentially the same as a saturated calomel electrode except the Hg2Cl2(s) is replaced with HgSO4(s) and the saturated solution potassium chloride solution is replaced with a saturated potassium sulfate solution. For samples containing 36Cly, the titration was started as normal except it was stopped at where the theoretical equivalence point should have been. For example, for solutions containing two halides the titration was stopped at the theoretical first point. The sample was then flocculated by adding one drop (;0.04 ml) of 1 M aluminium nitrate to the solution. The precipitate was collected on Whatman GFyF filter paper, bagged and sealed. The radioactivity of these samples was then compared to the radioactivity of a ‘standard Ag36Cl(s) sample’ to obtain the percentage of 36Cly coprecipitated. Sample and standard radioactivities were determined by counting until approximately 10 000 disintegrations were obtained using a Geiger Muller counter. From the number of counts and the
time taken to obtain those counts, the activity of the sample was calculated. 3. Results and discussion 3.1. Coprecipitation Presented in Table 1 are the coprecipitation results for the titration of three mixed halide solutions. The results show that there is considerable error in the determination of each halide pair resulting from coprecipitation of the more soluble silver halide with the less soluble silver halide at the first equivalence point. The percentage of coprecipitation for two of the three halide combinations was evaluated using two methods: (a) determining the amount of 36Cl incorporated into the silver bromideyiodide solids; and (b) using Gran functions to evaluate the actual equivalence point and comparing this to the theoretical equivalence point to calculate the percentage of coprecipitation w7x (Table 1). For the Bry yCly pair there is considerable difference between results calculated using the 36Cl tracer and those calculated using Gran functions. This difference between the two methods results from the need to flocculate the silver halide colloids in the radiotracer method before filtration and b counting. Upon flocculation some of the 36Cl is likely to have exchanged back into solution with bromide, which forms a more insoluble silver halide precipitate w3x.
190
M.J. Ellwood et al. / Microchemical Journal 74 (2003) 187–192 Table 2 Percentage of coprecipitation for a solution containing 48 mmol of Cly and 17 mmol of Bry in the presence of three flocculating agents Cationa
Concentration (M)
Percentage of coprecipitation
Naq
0 0.16 0.32 0 0.001 0.01 0 0.0013 0.016 0.032
11.5 5.6 12.6 11.5 5.7 3.1 11.5 4.7 10.5 10.9
Baq
Al3q Fig. 2. Relationship between Dlog Ksp and Dionic radius vs. percentage of coprecipitation for each halide pair. a
There is a general decrease in the percentage of coprecipitation for halide pairs with an increasing difference between solubility product (Ksp) and halide ion size (Table 1). For example, the differences between Ksp and ion size are larger between the chloride and iodide pair hence the percentage of coprecipitation is lower than between the chloride and bromide pair where Ksp and ion size are similar (Fig. 2). Indeed, Kolthoff w8x noted that if the size difference between two halides is less than 15% coprecipitation is enhanced.
Cations were added as nitrate solutions.
3.3. Ammonia complexation of silver Although flocculating agents help to reduce coprecipitation they do not completely eliminate it. To fully overcome coprecipitation we developed a new method involving the addition a complexing ligand, ammonia in this case, to the sample solution so that each halide could be selectively precipitated with silver nitrate. To selectively precipitate, say, iodide from a chloride containing solution, the Ag(NH3)q 2 complex must be ther-
3.2. Flocculating agents The addition of a flocculating agent reduces coprecipitation by making the silver halide colloids flocculate early in the titration thus allowing the exchange rate of halide between the silver halide crystal and the surrounding solution to increase w3x. Therefore the ability of the flocculating agent to flocculate the precipitate early in the titration is critical. Motonaka et al. w2x has noted that the charge on the flocculating cation and its corresponding concentration are important factors in reducing coprecipitation to a minimum. The optimum concentrations for Al3q, Ba2q and Naq to reduce coprecipitation were 0.0013 M, 0.01 M and 0.16 M, respectively (Table 2). Indeed, a plot of cation charge vs. log concentration yields a near logarithmic relationship (Fig. 3).
Fig. 3. Relationship between cation charge and optimum cation concentration for reducing coprecipitation of Bry with AgI(s) (d) and Cly with AgBr(s) (h). Data taken from Table 2 (d) and Bowers et al. (1961) (h).
M.J. Ellwood et al. / Microchemical Journal 74 (2003) 187–192
Fig. 4. Titration of a solution containing 19 mmol of Iy and 35 mmol of Bry with (d) and without (h) added NH3 (5 mmol) with a 0.11 M AgNO3 solution. After the first equivalence point the NH3 containing solution was neutralised with 5 mmol of acetic acid and the titration continued. Vertical lines indicate where each equivalence point should theoretically occur.
modynamically more stable than AgCl(s) but less thermodynamically stable than AgI(s). Once all the iodide has been titrated the ammonia present in solution is then neutralised and the titration continued to obtain the concentration of the second halide, in this case chloride. To neutralise the ammonia after the first equivalence point has been reached, an equimolar amount of acetic acid was added to the sample. The addition of acetic acid simply reduces the ammonia concentration in solu-
191
tion by forming its conjugate acid, NHq 4 , which does not complex with silver. Presented in Fig. 4 is the titration of a solution containing both bromide and iodide with and without added ammonia. Without added ammonia there is a clear offset in the data from where the first equivalence point should theoretically occur indicating coprecipitation of bromide during silver iodide formation. For the titration with added ammonia, the first equivalence points occur where it theoretically should indicate that bromide has not precipitated prior to the first equivalence point (Fig. 4). The method of adding ammonia to the titration solution to reduce coprecipitation worked for two of the three halide pairs (Table 3). However, for the Bry yCly pair the addition of ammonia alone did not allow the accurate determination of the bromide end point prior to ammonia neutralisation. For this halide pair there was a lack of resolution at the first equivalence point making it difficult to use Gran functions to determine the bromide end point. Increased resolution at the bromide end point was obtained by adding 25 mmoles of sodium nitrate to the sample (in addition to the added ammonia), which allowed an accurate end point determination using Gran functions (Table 3, Fig. 5). As the results show, the addition of ammonia, plus sodium chloride for the bromideychloride pair, reduces coprecipitation to less than 1.5% thereby allowing the accurate determination of
Table 3 Percentages of coprecipitation for each halide pair titrated with and without added ammonia Components in solution (mmol)
Percentage of coprecipitation without added NH3
Percentage of coprecipitation with added NH3 (5 mmol)
Cly 20 Iy 48
6
0.03"0.01
Iy 19 Bry 35
12.3
0.30"0.01
Bry 15 Cly 46
33
a
Twenty-five mmol NaNO3 were also added to the solution.
1.3"0.1a
192
M.J. Ellwood et al. / Microchemical Journal 74 (2003) 187–192
Fig. 5. Titration of a solution containing 15 mmol of Bry and 46 mmol Cly in the presence (d) and absence of NH3 and NaNO3 (s). Also presented are the corresponding Gran functions for each titration (j, h), respectively. Vertical lines indicate where each equivalence point should theoretically occur.
each individual halide concentration during potentiometric titration with silver nitrate.
neutralisation the titration is continued to obtain the concentration of the second halide present.
4. Summary
References
The phenomenon of coprecipitation was investigated and found to cause serious errors when determining the first equivalence point for titrations involving two or more halides when titrated with silver nitrate. Coprecipitation can be reduced by the addition of a flocculating agent, such as aluminium nitrate to the solution, however, coprecipitation is not completely eliminated. To eliminate end point errors a new method was developed involving the addition of ammonia to the titration solution thereby allowing the selective titration of each halide. Once the more insoluble silver halide has completely titrated, the titration is stopped and the ammonia is neutralised with acetic acid. After
w1x H. Puxbaum, V. Simeonov, J. Rendle, Mikrochimica Acta 2 (1977) 325–330. w2x J. Motonaka, S. Ikeda, N. Tanaka, Anal. Chim. Acta 105 (1979) 417. w3x R.C. Bowers, L. Hsu, J.A. Goldman, Anal. Chem. 33 (1961) 190. w4x L.S. Jovanovic, J.D. Fisl, F.F. Gaal, J. Serbian Chem. Soc. 59 (1994) 175–183. w5x A. Nara, N. Kobayashi, K. Honba, Microchem. J. 20 (1975) 200–212. w6x I.M. Kolthoff, F.T. Eggerstsen, Am. Chem. Soc. 61 (1939) 1036–1040. w7x G. Gran, A. Johansson, S. Johansson, Analyst 106 (1981) 1109–1118. w8x I.M. Kolthoff, J. Phys. Chem. 36 (1932) 860. w9x G.H. Aylward, T.J.V. Findlay, SI Chemical Data, second ed, Wiley, 1974, 136 pp.