Diese Präsentation wurde erfolgreich gemeldet.
Die SlideShare-Präsentation wird heruntergeladen. ×

# 4_5780566401735462527.pdf

Anzeige
Anzeige
Anzeige
Anzeige
Anzeige
Anzeige
Anzeige
Anzeige
Anzeige
Anzeige
Anzeige
Nächste SlideShare
DONDUCTOMETRIC TITRATION.
Wird geladen in …3
×

1 von 24 Anzeige

# 4_5780566401735462527.pdf

Its important

Its important

Anzeige
Anzeige

## Weitere Verwandte Inhalte

Anzeige

### 4_5780566401735462527.pdf

1. 1. CHAPTER NINE 9. CONDUCTOMETRY Conductometry measures electrolytic conductivity or resistance of the solution located between two electrodes. This property of the solutes is the result of the ion movement under the influence of the electric field applied. Only ionizable molecules (electrolytes) conduct electric current. The magnitude of the conductivity depends on the amount of ions presented in the solution. The electrolyte kind, its concentration and temperature affects both conductivity and specific conductance. 1 instrumental analysislecturenote
2. 2. Conductance is the extrinsic property while conductivity is the intrinsic property. This means that conductance is the property of an object dependent of its amount/mass or physical shape and size, while conductivity is the inherent property of the material that makes up the object. No matter how the object changes in terms of shape/size/mass, as long as it is made of the same material and the temperature remains the same, its conductivity does not change. 2 instrumental analysislecturenote
3. 3. Conductance (L) is the reciprocal of the electric resistance R: 𝐿 = 1 𝑅 (1) Conductance unit is Ω-1 (also called Siemens, S). Conductance of aqueous electrolyte solutions is, as rule in the range of mS or µS. it is the ability of the solution to conduct electricity. The conductance of the solution is the sum of the conductances of all of the ions that are in the solution. 𝐿 = 𝐿𝑖 (2) 3 instrumental analysislecturenote
4. 4. In order to do conductance measurements, two metal electrodes (Pt) should be immersed in the solution and connected to an alternating current (AC) voltage source (Figure 9.1). Under the effect of the potential difference between electrodes, each ion in solution moves to the electrode of opposite charge. According to Ohm’s law, the electric current in this circuit (including the solution) is: 𝐼 = 𝐸 𝑅 = 𝐿𝐸 (3) L depends on both solution properties (i.e. conductivity, k) and geometrical parameter of the cell (i.e. electrode area, A, and distance between electrodes, l, Figure 9.1) according to the following equation: 𝐿 = 𝑘 𝐴 𝑙 (4) 4 instrumental analysislecturenote
5. 5. Theratio(A/l) (incm)istermedasthe cell constant andcan bedeterminedbymeasuringthe conductance(L)ofasolutionwithaknownconductivity.Conductivity(inΩ−1 cm−1 orderivate units)dependsonsolutionproperties,suchasionconcentrations,ionchargesandionmobilities. ItisworthnotingthatH+ andOH- haveamuchhighermobilitycomparedtootherions(Table1). 5 instrumental analysislecturenote
6. 6. Figure 9.1. (A) Schematics of conductance measurement setup. (B) The conductometric probe. 1, 2, and 3 are platinized platinum sheets; 1 and 3 are electrically connected and form the two parts of a split electrode. 6 instrumental analysislecturenote
7. 7. Table 1. Mobility of ions in water at 25 oC. Ion Mobility (m2/(s.V))o Ion Mobility (m2/(s.V))o H+ 36.30 x10-8 OH- 20.50 x10-8 K+ 7.62 x10-8 SO4 2- 8.27 x10-8 NH4 + 7.61 x10-8 Br- 8.13 x10-8 La3+ 7.21 x10-8 I- 7.96 x10-8 Ba2+ 6.59 x10-8 Cl- 7.91 x10-8 Ag+ 6.42 x10-8 NO3 - 7.40 x10-8 Ca2+ 6.12 x10-8 ClO4 - 7.05 x10-8 Cu2+ 5.56 x10-8 F- 5.70 x10-8 Na+ 5.19 x10-8 CH3CO2 - 4.24 x10-8 Li+ 4.01 x10-8 7 instrumental analysislecturenote
8. 8. Conductivity is widely used for estimating the overall ion content in various sample of practical interest. But conductivity values cannot indicate the concentration of a specific ion in the sample. Ion concentration can be determined by means of conductometric titration. The reaction between the ion of interest and the added reagent (i.e. neutralization, precipitation, or formation of complex compound) brings about a strong modification of solution conductivity. The reagent should be added in the form of a standard solution and the conductance (or conductivity) is measured as a function of the added volume. This procedure is a conductometric titration. With a properly selected reaction, the end point of the titration is indicated by a particular point on the titration curve (i.e. conductance vs. added reagent volume). 8 instrumental analysislecturenote
9. 9. Conductometric Titrations The principle of conductometric titration is based on the fact that during the titration, one of the ions is replaced by the other and invariably these two ions differ in the ionic conductivity with the result that conductivity of the solution varies during the course of titration. The equivalence point may be located graphically by plotting the change in conductance as a function of the volume of titrant added. The main advantages to the conductometric titrationare its applicability to very dilute, and coloured solutions and to system that involve relative incomplete reactions.  For example, which neither a potentiometric, nor indicator method can be used for the neutralization titration of phenol (Ka = 10–10) a conductometric endpoint can be successfully applied. 9 instrumental analysislecturenote
10. 10. Application: Acid-base titration, especially at trace levels. Relative precision better than 1% at all levels. There are also few disadvantages with this technique. As you know the conductance is a non-specific property, concentration of other electrolyte can be troublesome. The electrical conductance of a solution is a measure of its currents carrying capacity and therefore determined by the total ionic strength. It is a non-specific property and for this reason direct conductance measurement are of little use unless the solution contains only the electrolyte to be determined or the concentrations of other ionic species in the solution are known.  Conductometric titrations, in which the species in the solution are converted to non-ionic for by neutralization, precipitation, etc. are of more value. 10 instrumental analysislecturenote
11. 11. Some Typical Conductometric Titration Curves are: 1. Strong Acid with a Strong Base, e.g. HCl with NaOH: Before NaOH is added, the conductance is high due to the presence of highly mobile hydrogen ions. When the base is added, the conductance falls due to the replacement of hydrogen ions by the added cation as H+ ions react with OH− ions to form undissociated water. This decrease in the conductance continues till the equivalence point. At the equivalence point, the solution contains only NaCl. After the equivalence point, the conductance increases due to the large conductivity of OH- ions (Figure 9.2). 11 instrumental analysislecturenote
12. 12. Figure 9.2. Conductometric titration of a strong acid (HCl) vs. a strong base (NaOH) 12 instrumental analysislecturenote
13. 13. • 2. Weak Acid with a Strong Base, e.g. acetic acid with NaOH: Initially the conductance is low due to the feeble ionization of acetic acid. • On the addition of base, there is decrease in conductance not only due to the replacement of H+ by Na+ but also suppresses the dissociation of acetic acid due to common ion acetate. • But very soon, the conductance increases on adding NaOH as NaOH neutralizes the un-dissociated CH3COOH to CH3COONa which is the strong electrolyte. • This increase in conductance continues raise up to the equivalence point. • The graph near the equivalence point is curved due the hydrolysis of salt CH3COONa. • Beyond the equivalence point, conductance increases more rapidly with the addition of NaOH due to the highly conducting OH− ions (Figure 9.3). 13 instrumental analysislecturenote
14. 14. Figure 9.3. Conductometric titration of a weak acid (acetic acid) vs. a strong base (NaOH) 14 instrumental analysislecturenote
15. 15. 3. Strong Acid with a Weak Base, e.g. sulphuric acid with dilute ammonia: Initially the conductance is high and then it decreases due to the replacement of H+. But after the endpoint has been reached the graph becomes almost horizontal, since the excess aqueous ammonia is not appreciably ionized in the presence of ammonium sulphate (Figure 9.5). 15 instrumental analysislecturenote
16. 16. Figure 9.5. Conductometric titration of a strong acid (H2SO4) vs. a weak base (NH4OH) 16 instrumental analysislecturenote
17. 17. 4. Weak Acid with a Weak Base: The nature of curve before the equivalence point is similar to the curve obtained by titrating weak acid against strong base. After the equivalence point, conductance virtually remains same as the weak base which is being added is feebly ionized and, therefore, is not much conducting (Figure 9.6). • Figure 9.6. Conductometric titration of a weak acid (acetic acid) vs. a weak base (NH4OH) 17 instrumental analysislecturenote
18. 18. 5. Mixture of a Strong Acid and a Weak Acid vs. a Strong Base or a Weak Base: In this curve there are two break points.  The first break point corresponds to the neutralization of strong acid. When the strong acid has been completely neutralized only then the weak acid starts neutralizing. The second break point corresponds to the neutralization of weak acid and after that the conductance increases due to the excess of OH− ions in case of a strong base as the titrant. However, when the titrant is a weak base, it remains almost constant after the end point similar to Figure 9.6 (Figure 9.7). 18 instrumental analysislecturenote
19. 19. Figure 9.7. Conductometric titration of a mixture of a strong acid (HCl) and a weak acid (CH3COOH) vs. a strong base (NaOH) or a weak base (NH4OH) 19 instrumental analysislecturenote
20. 20. Calculations Corrections During the titration process, the sample gets diluted by added titrant solution. A correction should be therefore applied to each conductance reading. If it assumed that the conductance is proportional to the total ion concentration, C (k = a proportionality constant): 𝐿 = 𝑘𝐶 = 𝑘 𝑛 𝑉 (5) Here, n is the total number of ion moles and V is solution volume. The conductance at a given moment of the titration is: 𝐿𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 = 𝑘𝑛/(𝑉0 + 𝑉 𝑎 ) (6) where 𝑉0 is the initial volume of the titrated sample and 𝑉 𝑎 stands for the added volume of titrant. In the absence of dilution, the corrected conductance assumes the corrected value which is: 𝐿𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑 = 𝑘𝑛/𝑉0 (7) 20 instrumental analysislecturenote
21. 21. From Equations (6) and (7) it results: 𝐿𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑 = 𝐿𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 (𝑉0+𝑉𝑎 ) 𝑉0 (8) This correction should be applied to each measured values and corrected conductance should be plotted against 𝑉 𝑎 in order to get the titration curve. Calculating analysis result The amount of analyte in the whole sample can be calculated by means of the well-known equation: 𝑚𝑋 = 𝑉 𝑒 𝐶𝑀𝑋 𝑉𝑠 𝑉0 (9) Here, 𝑉 𝑒 is the equivalence volume (in liter), C is the molar concentration of titrant and 𝑀𝑋 is the mol weight of the analyte (g/mol), 𝑉0 is the volume of the titrated aliquot (sample) and 𝑉 𝑠 is the overall volume of the sample. 21 instrumental analysislecturenote
22. 22. Applications of Conductometric Titrations Acid-base titrations redox titrations are known to us in which commonly indicators are used to locate the end point e.g., methyl orange, phenolphlthalene for acid base titrations and starch solutions for iodemetry type redox process. However, electrical conductance measurement can be used as a tool to locate the end point.  This method can be used with much diluted solutions.  This method can be used with colored or turbid solutions in which end point cannot be seen by eye.  This method can be used in which there is no suitable indicator. Has many applications, i.e. it can be used for acid base, redox, precipitation, or complex titrations  Determination of sulphur dioxide in air pollution studies  Determination of soap in oil  Determination of accelerators in rubber  Determination of total soap in latex  Specific conductance of water 22 instrumental analysislecturenote
23. 23. 23 instrumental analysislecturenote
24. 24. 24 instrumental analysislecturenote