INORGANIC CHEMISTRY (LAB MANUAL)

Transcription

1 Acid Base Titrations In acid base titrations the OH - ions of the base combine with H + ions of the acid as H + + OH - H 2 O If an acid is titrated with a base, the addition of the base decreases concentration of H + and increases the ph. However, if a base is titrated with an acid as the titration progresses the concentration of OH - decreases and the ph falls. At a certain definite ph value the equivalence point is reached. The ph at equivalence point depends on the nature of the acid or base. If both of them are strong, the solution is neutral at the equivalence point and ph is 7. If a weak acid is titrated with a strong base (eg. CH 3 COOH vs. NaOH), the salt formed (in this case CH 3 COONa) hydrolyses and ph at equivalence point is more than 7. Similarly when we titrate a weak base with a strong acid the ph at equivalence point is lower than 7. The exact ph at equivalence point in such cases may be calculated from the dissociation constant of the weak acid (K a ), or the dissociation constant of the weak base (K b ) and the concentration of the salt (c) formed during the titration. Weak acid and strong base 1 1 ph = pkw + pka + log c ----(5) 2 2 Weak base and strong acid ph = 2 1 pkw pkb log c ----(6) Weak acid and weak base ph = 2 1 pkw pka pkb ----(7) When we choose an indicator, we want the colour change of the indicator to take place at a ph as close as possible to the ph at the equivalence point. 1

2 Acid Base Indicators Acid base indicators are organic dyes which change colour with change of ph. The indicator exists in two forms acidic and basic. Two common indicators used in acid base titration are methyl orange and phenolphthalein. Methyl orange is an example of a weak base in basic medium it is yellow / orange in colour and in acidic medium it is red. + Na - O 3 S N N N(CH 3 ) 2 H + OH - + Na - O 3 S H N N N(CH 3 ) 2 (yellow) methyl orange (red) The ph range of colour change is 3.1 to 4.4. Phenolphthalein is a week acid. In acid solution it is colourless and in basic solution it is pink. HO OH O O - C C O OH - H + C C O - O O (colourless) phenolphthalein (pink) The ph range of colour change is 8.3 to The details of some useful acid-base indicators, together with the ph range of colour change are shown in Table 1. 2

3 Table 1. Common Acid-Base Indicators with their colour changes and ph ranges. Colour at Indicator ph range Lower ph Higher ph Cresol red Red Yellow Phenol blue Red Yellow Bromophenol Yellow Blue blue Methyl orange Red Orange / Yellow Methyl red Red Yellow Bromothymol Yellow Blue blue Phenol red Yellow Red Thymol blue Yellow Blue Phenolphthalein Colourless Pink Thymolphtholein Colourless Blue Titration Curves The mechanism of a neutralization process can be understood by studying the change in ph during the course of the titration. The change in ph at the vicinity of the equivalence point is very important as it enables the correct choice of the indicator. The plot of ph against the volume of the titrant added is called the neutralization curve or titration curve. This may be evaluated experimentally by determining the ph at different stages of the titration using a ph meter or it may be calculated from theoretical principles. The titration curves differ in shape depending upon the nature of acid and base. Some representative curves are shown below. 3

4 Strong acid Strong base (HCl vs NaOH) Sharp change in ph at equivalence point ph at equivalence point is 7 Suitable indicators methyl orange, methyl red, bromothymol blue Figure 1 shows a representative ph titration curve when a strong base is added to a strong acid and figure 2 shows a representative ph titration curve when a strong acid is added to a strong base. Figure 1. Titration curve for the addition of a strong base to a strong acid 4

5 Figure 2. Titration curve for the addition of a strong acid to a strong base Weak acid strong base (CH 3 COOH vs NaOH) Gradual change in ph ph at equivalence point is 8.7 Suitable indicator phenolphthalein Figure 3 shows a representative ph titration curve when a strong base is added to a weak acid and figure 4 shows a representative ph titration curve when a weak acid is added to a strong base. 5

6 Figure 3. Titration curve for the addition of a strong base to a weak acid Figure 4. Titration curve for the addition of a weak acid to a strong base 6

7 Weak base strong acid (aqueous NH 3 vs HCl) Gradual change in ph ph at equivalence point is 5.28 Suitable indicator methyl orange, methyl red. Figure 5 shows a representative ph titration curve when a weak base is added to a strong acid and figure 6 shows a representative ph titration curve when a strong acid is added to a weak base. Figure 5. Titration curve for the addition of a weak base to a strong acid 7

8 Figure 6. Titration curve for the addition of a strong acid to a weak base Weak acid weak base (CH 3 COOH vs aqueous NH 3 ) Very gradual change in ph ph at equivalence point is about 7 No sharp change in ph at equivalence point No suitable indicator. 8

9 Figure 7 shows a representative ph titration curve when a weak acid is added to a weak base. Figure 7. Titration curve for the addition of a weak acid to a weak base 9

10 EXPERIMENT 6 AIM To determine the amount of sodium carbonate and sodium hydrogen carbonate present together in the given solution by titration with hydrochloric acid. To prepare standard sodium carbonate solution (M/40). Learning Objectives Necessity of a standard solution in titrimetry Difference between AR and LR grades of reagents Handling of the analytical balance Calculation of amount of reagent needed to prepare standard solutions of different molarities Concept of weighing by difference Accurate measurement of volumes Concept of completion of stoichiometric reactions Choice of indicator Visual detection of end point Concept and need of concordant readings How to standardize an acid To carry out molarity based calculations Determination of two components in a mixture Use of successive indicators Applications in determination of alkalinity of water samples 10

11 Theory Preparation of a standard solution of sodium carbonate (M/40) A standard solution is one, which contains a known weight of the reagent in a definite volume of solution, i.e., a solution of accurately known concentration. The concentration of a standard solution is generally expressed in molarity, though in some cases the older concept of normality is still used. Sodium carbonate is a primary standard. Its molecular mass (106) corresponds to the formula Na 2 CO 3. The amount of salt needed to prepare 1000 cm 3 of M/40 standard solution is 106/40 i.e g. Generally we prepare 250 cm 3 or 100 cm 3 of standard solution. If we wish to prepare 100 cm 3 of M/40 standard solution, then the amount of salt needed is g. By the method of weighing by difference we transfer exactly an amount of the solute close to g and calculate the molarity of the solution. Standardization of HCl solution by titration with standard Na 2 CO 3 solution. Sodium carbonate is the salt of a weak acid, carbonic acid (H 2 CO 3 ). Its reaction with HCl is a two step process and may be represented as, Na 2 CO 3 + HCl NaHCO 3 + NaCl NaHCO 3 + HCl NaCl + H 2 O + CO 2 The overall reaction may be represented as Na 2 CO 3 + 2HCl 2 NaCl + H 2 O + CO 2 It is clear that 1 mol of Na 2 CO 3 reacts with 2 mol of HCl and the molarity expression becomes M Na2 CO 3 V Na2 CO 3 M HCl V HCl = 1 2 or, 2 ( M Na2 CO 3 V Na2 CO 3 ) = M HCl x V HCl 11

12 The titration curve (Figure 8.) in this case will indicate 2 equivalence points, the first corresponding to the formation of NaHCO 3 (half-neutralization of Na 2 CO 3 ) and the second to the complete neutralization of Na 2 CO 3.The ph at this stage is ~ 3.7 and a suitable indicator is methyl orange and the colour change is yellow to red. Figure 8. Titration curve for the titration of Na 2 CO 3 solution with HCl solution Estimation of sodium carbonate and sodium hydrogen carbonate present in a mixture by titrating it with HCl solution. This titration involves the successive use of two indicators phenolphthalein and methyl orange. Na 2 CO 3 reacts with 1 mol of HCl to give NaHCO 3. Na 2 CO 3 + HCl NaHCO 3 + NaCl 12

13 The ph at this stage is ~ 8.5 and the end point corresponding to the half neutralization Na 2 CO 3 can be visualized using phenolphthalein. The volume of acid consumed corresponds to the half neutralization of Na 2 CO 3. Hence, twice the amount of acid will correspond to the total amount of Na 2 CO 3 present. At this stage the solution contains NaHCO 3 and if methyl orange is added the solution turns yellow. When this solution is titrated with HCl, the total amount of NaHCO 3 i.e. amount of NaHCO 3 originally present and the amount of NaHCO 3 produced by half neutralization of Na 2 CO 3 is neutralized. NaHCO 3 + HCl NaCl + H 2 O + CO 2 At this stage the ph is ~ 4.0 and the solution acquires a red colour. The total volume of acid consumed corresponds to the total alkali content. Requirements Apparatus Quantity Chemicals Weighing bottle 1 Na 2 CO 3 (AR) It should be dried in an oven at Volumetric flask 120 o C, and cooled in a desiccator to remove (100 cm 3 ) 1 any moisture. (You will be provided with the Funnel 1 dried sample). Burette (50 cm 3 ) 1 HCl solution (provided by your teacher) Pipette (10 cm 3 ) 1 Solution of Na 2 CO 3 and NaHCO 3 (provided by your teacher) Conical flasks 2 (100 cm 3 ) Methyl orange Beaker (250 cm 3 ) 1 Phenolphthalein Procedure (I) Preparation of standard Na 2 CO 3 solution Weigh a clean and dry weighing bottle on an analytical balance. 13

14 (II) Take the required amount of sodium carbonate in the weighing bottle and weigh it accurately. Transfer the salt through a funnel into a volumetric flask, add a little distilled water and swirl the flask gently to dissolve the contents. Wash the funnel thoroughly with distilled water from a wash bottle so that the solution sticking to the funnel is washed down to the volumetric flask. Add enough distilled water carefully just below the etched mark. Add the last few drops of distilled water using a pipette or a dropper so that the lower meniscus of the solution just touches the etched mark. Stopper the flask and shake to get a homogenous solution. Weigh the weighing bottle on the same balance after transference. The mass of salt transferred is the difference between the second and third weighing. Calculate the molarity of the solution up to 4 decimal places. Standardization of HCl Rinse the burette with distilled water followed by the given HCl solution. Fill the burette with the given HCl solution, remove the funnel and mount it on a stand. Adjust the level of HCl to eye level, note the reading and record it in the observation Table 1. Rinse the pipette with distilled water, followed by standard Na 2 CO 3 solution. Pipette out 10 cm 3 of standard solution into a 100 cm 3 conical flask, previously rinsed with distilled water. Add 2 drops of methyl orange indicator. The solution will be yellow in colour. Titrate with constant swirling against a white background till the colour changes to red. Note the final reading of the burette in the observation Table 1. Repeat to get 3 concordant readings. Note: It will be helpful if you go through the section on volumetric apparatus prior to doing the experiment. 14

15 (III) Titration of mixture Pipette out 10 cm 3 of the mixture into a 100 cm 3 conical flask, previously rinsed with distilled water. Add 1 drop of phenolphthalein indicator. The solution will turn pink. Note the burette reading in Table 2. Titrate with HCl till the solution is colourless; note the reading in Table 2. Add 2 drops of methyl orange indicator. The solution will be yellow in colour. Continue the titration till the colour changes to red/ pink. Note the burette reading in Table 2. Observations and Calculations (I) Preparation of standard Na 2 CO 3 solution Mass of weighing bottle Mass of weighing bottle+ Na 2 CO 3 = m 1 = g = m 2 = g Mass of weighing bottle after transfer = m 3 = g Mass of Na 2 CO 3 transferred = (m 2 m 3 ) = m g Molar mass of Na 2 CO 3 = 106 g mol -1 Molarity of the standard solution = Mass of the salt Molar Mass x 1 3 Volume(in dm ) m 1 = mol dm -3 M V = mol dm -3 (M 1 ) 15

16 (II) Standardization of HCL Table 1 Titration of standard Na 2 CO 3 solution vs HCl Indicator End point Volume of standard Na S.No. 2 CO 3 solution pipetted out V 1 /cm Methyl orange Yellow to Red Burette Readings Initial Final Volume of HCl used V 2 /cm 3 Molarity of standard Na 2 CO 3 solution = M 1 = mol dm -3 Volume of standard Na 2 CO 3 solution = V 1 = 10 cm 3 Molarity of HCl solution = M 2 = (?) Volume of HCl solution = V 2 =. cm 3 (from Table 1) Using Molarity equation 2 M 1 V 1 = M 2 V 2 Molarity of HCl solution, M 2 = (2 M 1 V 1 )/ V 2 16

17 (III) Titration of mixture of sodium carbonate and sodium hydrogen carbonate Table 2 Titration of mixture vs HCl Indicators used Phenolphthalein and Methyl Orange in succession End point Pink to Colourless (Stage 1) Yellow to Red (Stage 2) S.No. Volume of mixture pipetted out V 3 /cm 3 (cm 3 ) Initial Burette Readings Final (Phenolphthalein) Final (Methyl Orange) A B C Volume of HCl (Phenolphthalein) Volume of HCl (Methyl Orange) B - A = C - A = V B /cm 3 V C /cm 3 Amount of Na 2 CO 3 Volume of HCl needed for the half neutralization Na 2 CO 3 = V B cm 3 Volume of HCl needed for complete neutralization Na 2 CO 3 = 2V B cm 3 (V 2 *) Molarity of HCl solution = M 2 mol dm -3 Volume of mixture = 10 cm 3 (V 3 ) Molarity of Na 2 CO 3 in mixture = M 3 (?) Using Molarity equation 2 M 3 V 3 = M 2 V 2 * 2 x 10 x M 3 = M 2 x 2 V B M 3 = [(M 2 x 2 V B )/ 20] mol dm -3 Strength of Na 2 CO 3 = Molarity x Molar mass = (M 3 x 106 g mol -1 ) =. g dm -3 17

18 Amount of NaHCO 3 Volume of HCl needed to neutralize total alkali (Na 2 CO 3 + NaHCO 3 ) = V C cm 3 Volume of HCl needed to neutralize Na 2 CO 3 =2V B cm 3 Volume of HCl needed to neutralize NaHCO 3 = (V C 2V B ) cm 3 = (V 2 ** cm 3 ) Molarity of HCl = M 2 Volume of mixture = 10 cm 3 (V 3 ) Molarity of NaHCO 3 in mixture # = M 3 NaHCO 3 and HCl react in 1:1 molar ratio and thus the molarity equation is M # 3 V 3 = M 2 V 2 ** M # 3 x 10 = M 2 x (V C 2V B ) M 3 # = [{M 2 x (V C 2V B )}/10] mol dm -3 Molar mass of NaHCO 3 = 84 g mol -1 Strength of NaHCO 3 = Molarity x Molar mass = (M 3 # x 84 g mol -1 ) = g dm -3 Result: The amount of Na 2 CO 3 and NaHCO 3 present in the given mixture was found to be g dm -3 and g dm -3 respectively. Precautions Weigh the salt accurately. Do all the weighings on the same balance. Make the solution up to the mark very carefully. Shake the solution thoroughly to ensure uniform concentration. All apparatus should be clean and rinsed with the appropriate solutions. 18

19 The burette readings should be noted carefully by reading the lower meniscus. Add HCl drop wise near the end point. Trouble shooting question How do I know that the end point is correct? Answer: Add a drop of sodium carbonate solution to the titrated solution in the conical flask. If the color turns yellow then the end point is correct, if it is still red then you have crossed the end point. Do another titration, you now have an idea of the end point. Extension of the experiment Determination of the purity of the given washing soda sample. Determination of the number of molecules of water of crystallization of hydrated sodium carbonate. Determination of the carbonate hardness of water. Determination of the total hardness of water. (Hardness is due to Ca 2+ and Mg 2+ ions. If a known volume of excess standard Na 2 CO 3 solution is added, a precipitate of CaCO 3 and MgCO 3 is obtained. This is filtered off and the unreacted Na 2 CO 3 is estimated by titration with HCl). Determination of the alkali content in soaps and detergents. 19