Experiment 2 Adsorption Isotherm Aim: To determine the adsorption isotherm of acetic acid by activated charcoal. Apparatus/Reagents: Six stoppered bottles (or 100 ml conical flasks). burette with stands (two sets). pipette, measuring cylinder, funnel, standard solutions of acetic acid (0.6 M) and sodium hydroxide (0.1 M), powdered activated charcoal and phenolphthalein indícator. Theory Introduction The molecules or ions present at the surface of a solid do not have all their forces satisfied by union with other particles. Due to this instauration, solid surfaces tend to satisfy their residual forces by attracting and retaining on them the molecules of other species which are brought into contact with them. As the molecules remain only at the surface and do not go deeper into the bulk, their concentration is more at the surface than in the bulk of the solid. This phenomenon of higher concentration of molecular species on the surface than in the bulk of a solid is known as adsorption. Adsorption is a phenomenon in which particles adhere to the surface of a solid. The solid on which adsorption takes place is called the adsorbent and the material adsorbed is called adsorbate. For a given weight of adsorbent, its adsorptive capacity is directly proportional to the surface area since the surface area presented by a given weight of a solid is very large when it is in colloidal state, colloids possess very good adsorptive properties. The reverse process of removal of an adsorbed substance from the surface of a solid is known as desorption. Procedure (i) Take 6 cleaned and dry stoppered bottles (or 100 ml conical flasks) and label them from 1 to 6. (ii) Set up two burettes. Fill one with 0.6 M acetic acid and the other with distilled water. (iii) Add different volumes of acetic acid and water in the labelled bottles as given below. Bottle No. Vol. of 0.6 M Vol. of Water (ml) Total Volume (mL) CH3COOH (ml) 1 60 0 60 2 50 10 60 3 40 20 60 4 30 30 60 5 20 40 60 6 10 50 60 (iv) Shake the solutions well and add 3.0 g of charcoal in each bottle. (v) Stopper the bottles and let them stand for about an hour with intermittent shaking. (v) Now, filter the contents of bottle 1 into a beaker through a filter paper. Pipette out 10 ml of the filtrate in to a 100ml conical flask and titrate it against N/10 sodium hydroxide solution using phenolphthalein as indicator. Take three reading in each case. Observations Room temp. = t °C. Weight of activated charcoal in each bottle = 2g Vol. of acetic acid solution for titration in each case = 10 ml Bottle No. 1 Initial (i) Final Vol. of 0.1 M Mean NaOH used Value V1 (ii) 2 (i) V2 Calculations: () Bottle No. I Initial conc. of acetic acid =0.6 M 10 ml of M1, acetic acid=V1, ml of 0.1 M NaOH. M1 = π1 ×0.1 10 = 0.01 V1=Conc. after adsorption . Thus change in concentration after adsorption (0,6-0.01 ×V1)M Also if x1 = Change in conc. in grams. per 60 ml of the solution in Bottle No, 1. Than X1 = = πΆβππππ ππ πΆπππ.ππ πππππ ×ππππ’ππ π‘ππππ 1000 (0.6−0.01π1)×60 1000 × Mol.wt. × 60g (ii) Bottle No.2 Initial Conc. of Acetic acid = 06×50 60 M = 0.5 M As 10 ml of M2 acetic acid = V2 ml of π 10 NaOH 0.1 M2 = V1 × 10 = 0.01 V2 = conc. after adsorption Change in conc. in mples/litre = (0.5-0.01 V2)× M And x2 = change in conc. in grams per 60 ml of the solution (0.50−0.01π2) = 1000 ×60×60g Similarly calculate the change in conc. in gm per 60 ml of the volume of the solution of acetic acid taken for bottle no. 3, 4, 5 and 6 and let these values be as x3, X4, X5, and x6, respectively. Now tabulate the values of log C and log π₯ π₯ π as given in the table l and plot graph by taking log C along X-axis (abscissa) and log π along Y-axis (ordinate). Table-1 Bottle Initial No.1 conc. of Conc. acetic acetic acid after solution adsorption 1. 0.6 M 2. 0.5 M 3. 0.4M Equilibrium Amount(moles) of Of acetic acid adsorbed (x) Weight of acid Adsorbent (m) π₯ π Log π₯ π Result A straight line obtained in the plot shows the validity of the Freundlich isotherm over the concentration range studied. Log C Quiz: 1. What is adsorption isotherm? (Chemical Engineering Kinetics, n.d.; robert_treybal, n.d.) 2. Discuss difference between physical and chemical adsorption. Suggested Reference: Smith J. M. (1981). Chemical Engineering Kinetics (3rd ed.). McGraw-Hill chemical engineering series. robert_treybal. (1980). mass_transfer_operations. McGraw-Hill chemical engineering series. References used by the students: Rubric wise marks obtained: Rubrics Marks 1 2 3 4 5 Total Experiment 3 Adsorption Isotherm Aim: Study the adsorption of oxalic acid aqueous solution on charcoal and prove the validity of Freundlich's adsorption isotherm and Langmuir adsorption isotherm. Chemicals Standard NaOH π 10 ,standard oxalic acid solution π 4 , Powdered activated charcoal., Thermostat 25°C and Sìx stoppered bottles. Indicator Phenolphthalein. End Point Appearance of permanent light pink color (NaOH in burette) Theory We Know that the amount of adsorbate on the adsorbed is dependent on pressure as well as temperature. So, we can say that the amount (x) adsorbed is a function of pressure (P) ín the case of gas or concentration in case of a solution and temperature (T) i.e. x= f(P. T), A plot of P and x. keeping temperature constant is known as adsorption isotherm. Freundlich adsorption isotherm: Freundlich proposed an empirical equation to represent, in general, the adsorption relationship and which is known as Freundlich adsorption isotherm. According to it, π₯ π = kc1/n ……..(i) where, x is the amount of solute adsorbed, m is the amount of adsorbent, c is equilibrium concentration of adsorbate in the solution, k is a constant depending upon the nature of both adsorbent and adsorbate, while n is another constant which is dependent on the nature of the adsorbate. The value of 1/n is generally less than unity. On taking logarithms of equation (i), we get, π₯ 1 log π = log k π logc …….(ii) If the values of log x/m are plotted as ordinate against log c as abscissa, we get a straight line, with a slope and 1/n is an intercept on the ordinate log k. Freundlich adsorption isotherm At a given temperature, the mass of solute (or gas) adsorbed by a solid adsorbent at various concentration (pressure in case of gases) is given by the following empirical relation: π₯ π = k. C1/n (1) Where x is the mass of solute9 (or gas) adsorbed by m gram of the solid at various concentration, k and n are constant at a given temperature for the given solid adsorbent and solute. Equation(1) can also be written in the following manner: π₯ 1 Log π = log k + π log C π₯ A plot of logπ versus log C (concentration) gives a straight line with slope equal to 1 π Langmuir adsorption isotherm: Langmuir gave a relation between the amount adsorbed and the con-centration for a unimolecular layer and is known as Langmuir adsorption isotherm. According to it π₯ π π1π2π =1+π1 π …...(iii) where, k1 and k2, are constants. Equation (iii) can also be written as πΆ π₯/π 1 = π1π2 π + π2 Let K1, K2, be taken as another constant α and k is taken as ß. The equation (iv) becomes: π₯ πΌπ = 1+π½π π π π₯/π 1 π½ = πΌ + πΌc Where a & B are constants. Their equation may be verified by plotting which should result in a straight line. The slope π π₯/π of the straight line will give against, C π½ πΌ and the intercept along y-axis will give Hence the value of the constants a& B can be evaluated experimentally Procedure (i) Standardize NaOH with oxalic acid by titration using phenolphthalein as indicator. (ii) Put exactly 2 g of finely powdered charcoal into six well cleaned and dried stoppered reagent bottle and label them from 1 to 6. (iii) with the help of burette and 50, 40,30, 20, 10 and 5 ml of standard Oxalic acid solution and 0, 10, 20,30 ,40 and 45 ml. of distilled water in bottle nos. I,2, 3,4,5 and 6 respectively. Stopper the bottles and shake them vigorously for about 20 minutes. Place the bottles in a thermostat for about 30 minutes with intermittent shaking. (iv) Filter the supernatent liquid of each of the bottles through filter paper. (v) Titrate the each filtrate by pipetting out 10 ml of each with standard NaOH by using Phenolphthalein as indicator till the appearance of permanent light pink colour. Observations Room Temperature = .... t °C Amount of charcoal added into each bottle (m) = 2 g Volume of filtrate take for each titration = 10 ml 10 ml of original acid required v ml of π 10 NaOH. Table Initial conc. Vol. of π 10 of Oxalic Used (ml) acid Bottle No.1 Equilibrium Oxalic π₯ π acid Conc. of acid a Adsorbent Ce (mol/l) π₯ Log π xg solution(C0) (mol/l) 1. 2. 3. And so on Calculation First calculate the initial conc. (C0) of oxalic acid in moles/ litre for each flask. From the titration data, calculate the value of equilibrium concentration (Ce) for each flask. The value of oxalic and adsorbed (x) for each flaskcan be calculated from the change in concentration ie (C0-Ce) and the initial volume of the solution (50 ml) as under: X= (πΆπ−πΆπ)×50 1000 (πΆπ−πΆπ) = 20 moles From this, calculate the amount of acid adsorbed per gram of the adsorbent π₯ π in moles or grams. 1. Plot a graph between log π₯ π values (ordinate) vs. log Ce, (abscissa). If the plot obtained is straight line, this means Freundlich isotherm is valid here. 1 Evaluate the value ofπ from the slope of the line and log K from the intercept on the ordinate by the straight line for a value of Log Ce equal to zero. 2. Similarly test Langmuir's adsorption isotherm by plotting the contant α and β. Precautions πΆπ π₯/π Vs. Ce Obtain the values of Log C 1. A wet filter paper is avoided so that the filtrate is not diluted by the wetting solvent. 2. To avoid change in concentration of filtrate due to adsorption by the filter paper, a small initial volume of the filtrate is rejected. 3.Conical flasks should b thoroughly cleaned and dried. Quiz: 1. What is Langmuir adsorption isotherm? 2. Discuss Freundlich’s adsorption in brief. Suggested Reference: Smith J. M. (1981). Chemical Engineering Kinetics (3rd ed.). McGraw-Hill chemical engineering series. robert_treybal. (1980). mass_transfer_operations. McGraw-Hill chemical engineering series. References used by the students: Rubric wise marks obtained: Rubrics Marks 1 2 3 4 5 Total