EXPT. 4 DETERMINATION OF pka OF ORTHOPHOSPHORIC ACID

Transcription

1 EXPT. DETERMINATION OF pka OF ORTHOPHOSPHORIC ACID Structure.1 Introduction Objectives.2 Principle.3 Requirements. Solutions Provided.5 Procedure.6 Observations and Calculations.7 Result.1 INTRODUCTION You would recall from your earlier knowledge that gravimetric and volumetric analysis form two common methodologies of quantitative chemical analysis. In the previous two experiments of this course you have learnt about and have performed quantitative determinations based on gravimetric analysis. In this and the subsequent experiments you would learn about and perform a wide variety of quantitative determinations using different types of volumetric methods. The determination of the mass of a suitable precipitate formed in the quantitative reaction between the analyte and the reagent formed the basis of gravimetric method. In volumetric or titrimetric determinations we are concerned with the determinations of the volume of the suitable reagent that reacts with the analyte solution. We are sure that you are familiar with and have performed simple acid base titrations in your earlier classes. In this first experiment of this type in the present course we would take up the determination of pk a of a triprotic acid, orthophosphoric acid using acid base titration. In the next experiment you would learn about and perform the determination of different types of alkalinities of a water sample. Objectives After studying and performing this experiment you should be able to: state and explain the principle of acid-base titration with special reference to the determination of the pk a of a triprotic acid, prepare a standard solution of oxalic acid, standardise the given solution of sodium hydroxide and use it in the pk a determination of orthophosphoric acid, state the indicators employed in the titration of triprotic orthophosphoric acid with sodium hydroxide, explain the meaning and importance of a mixed indicator, explain the reason for non determinability of the third equivalence point in the titration of orthophosphoric acid, and determine the pk a s of orthophosphoric acid. 28

2 .2 PRINCIPLE Orthophosphoric (phosphoric (V)) acid is a triprotic acid; the different stages of ionization of Orthophosphoric acid can be given as shown below: H 3 PO H + + H 2 PO H 2 PO H HPO H HPO 3 PO On the basis of simple arguments we know that the removal of each successive proton - is more and more difficult. Thus, H 3 PO is a stronger acid than HPO 2 which in turn is stronger than HPO. Due to large difference in the values of the successive acid 2- dissociation constants a solution of phosphoric acid acts as a mixture of three monoprotic acids. A titration with monoacidic base like sodium hydroxide or potassium hydroxide can be conveniently performed because the neutralisation of the first stage is almost complete before the second stage is appreciably affected, and similarly the second stage neutralisation is almost complete before the third stage is apparent. A calculated titration curve for the titration of 50 cm 3 of 0.1 M-H 3 PO with 0.1 M-KOH is given in Fig..1. Fig..1: Titration curve for the titration of 50 cm 3 of 0.1 M-H 3 PO with 0.1 M-KOH The first stage of ionization of phosphoric acid is like that of a strong acid and its titration with KOH is quite straight forward. The equivalence point can be determined by using either methyl orange, bromocresol green, or Congo red as the indicator. The second stage of ionization of phosphoric acid is like that of a weak acid and its titration with KOH is also simple. The second equivalence point can be determined reasonably by using thymolphthalein. However, a mixed indicator composed of 3 parts 29

3 Sometimes we need to have a sharp colour change over a narrow range of ph and a single indicator may not be appropriate as it changes colour over about two units of ph. In such cases we use a suitable mixture of indicators. These indicators are so chosen that their pk In values are close together and the overlapping colours are complementary at an intermediate ph value. of phenolphthalein and 1 part of 1-naphtholphthalein is a better choice for the determination of second equivalence point. The colour changes from pale rose to violet. While the first two equivalence points can be detected by suitable indicators as mentioned above, the titration curve is flat for the third stage of neutralization and there is no suitable indicator available for the same. We can perform two titrations of the given solution of orthophosphoric acid using bromocresol green and the mixed indicator respectively as the indicators. In the titration using bromocresol green as the indicator, the end point is obtained when the first stage of the neutralisation of phosphoric acid is complete. On the other hand in the titration using mixed indicator, the end point is obtained when the second stage of the neutralisation of phosphoric acid is complete. Thus, the titre value at the bromocresol green end point, B E equals the volume of sodium hydroxide required for the neutralisation of phosphoric acid upto stage 1. In other words, the end point will be observed when one mole of sodium hydroxide is added to phosphoric acid as per the following reaction H 3 PO + NaOH NaH 2 PO + H 2 O Similarly, the titre value at the mixed indicator end point, M E equals the volume of sodium hydroxide required for the neutralisation of phosphoric acid upto stage 2. In other words, the end point will be observed when two moles of sodium hydroxide is added to phosphoric acid as per the following reaction: H 3 PO + 2NaOH Na 2 HPO + 2H 2 O You would recall from your earlier knowledge that Henderson Hasselbach equation provides the relationship between ph and pka value for a weak acid, HA. ph = pk a + [ A ] log [ HA ] Further when the concentration of the acid and the anion becomes equal ph = pka; in other words, at the ph equal to the pka the concentrations of the acid and the conjugate base form are equal. We would use this fact to determine the pka. We would first determine the bromocresol green end point, B E and the mixed indicator end point, M E for a given volume of the acid solution. Then we ensure half neutralization of different stages of neutralization by adding appropriate volumes of alkali to a fresh lot of the acid solution and determine the ph of the solutions. These would provide the pka values. Let us take the example of bromocresol green end point, the end point for the first stage of neutralization to understand it. H 3 PO H + + H 2 PO Let us assume that for 10 cm 3 of the acid solution the end point is at V cm 3. Now if we take 10 cm 3 of the acid solution and to this add V/2 cm 3 of the base, this would cause half neutralization i.e., the solution would have equal concentrations of the two species viz., H 3 PO and H 2 PO. As discussed above, the ph of the solution at this stage would be equal to the pka of the first acid dissociation of orthophosphoric acid. Similarly, we can do for the second acid dissociation. The third pka can not be determined in this fashion in the absence of a suitable indicator. However, an estimate for the same can be made..3 REQUIREMENTS 30

4 Apparatus Volumetric flask (100 cm 3 ) 1 Burette (50 cm 3 ) 1 Pipette (10 cm 3 ) 1 Weighing bottle 1 Burette stand with clamp 1 Conical flasks 2 Funnel 1 ph meter with electrode Chemicals Phosphoric acid Sodium hydroxide Bromocresol green indicator Phenolphthalein Naphtholphthalein Buffer solutions for ph meter caliberation. SOLUTIONS PROVIDED Phenolphthalein indicator: It is prepared by dissolving 1g of the reagent in 100 cm 3 of ethanol and adding 100 cm 3 of water. If a precipitate is formed, it is filtered. Bromocresol green indicator: It is prepared by dissolving 100 mg of bromocresol green in 1.3 cm 3 of 0.01 M NaOH and diluting to 250 cm 3 with distilled water. (In case bromocresol green is not available you may use methyl orange though it would have a slightly higher indicator error) Mixed indicator: It is prepared by mixing 3 parts of a 0.1 per cent solution of phenolphthalein in ethanol with 1 part of 0.1 per cent solution of naphtholphthalein in ethanol (In case mixed indicator is not available phenolphthalein may be used though it would have a slightly higher indicator error) Sodium hydroxide solution (~ 0.1 M): It is prepared by dissolving about g of sodium hydroxide and dissolving it in about cm 3 of conductivity water and diluting it to 1 litre..5 PROCEDURE The procedure for the determination of the pk a s for orthophosphoric acid consists of the following steps: a) Preparation of standard solution of oxalic acid b) Standardisation of sodium hydroxide solution by titrating against standard oxalic acid solution c) Determination of bromocresol green end point, B E by titrating against standardised sodium hydroxide solution d) Determination of mixed indicator end point, M E by titrating against standardised sodium hydroxide solution e) Determination of pka values Follow the procedure given below in a step wise manner to determine the pka values for orthophosphoric acid. a) Preparation of 100 cm 3 of standard solution of 0.05 M oxalic acid i) As you know, to prepare a standard solution we need to first calculate the amount of the substance required. The amount of oxalic acid (M m = 126 g mol -1 ) required to prepare 100 cm 3 of 0.05 M solution would be: m ( in g) = = 0.63g 1000 Weigh about 0.65 g of oxalic acid on a rough weighing balance and transfer it to a clean dry weighing bottle and accurately weigh the weighing bottle with oxalic acid. 31

5 i Transfer the oxalic acid to a clean volumetric flask of 100 cm 3 capacity through a glass funnel. iv) Weigh the weighing bottle containing oxalic acid left (if any) and find the exact mass of oxalic acid transferred by subtracting this mass from the mass of the weighing bottle plus oxalic acid. v) Dissolve oxalic acid in about 30-0 cm 3 of distilled water added to the volumetric flask. Once dissolved, make up the volume up to the mark with distilled water. b) Standardisation of sodium hydroxide solution by titrating against standard oxalic acid solution i) Fill up the burette with sodium hydroxide solution with the help of a funnel and mount it on the burette stand. Note the reading on the burette and record it in the Observation Table.1 under the initial reading column. i iv) Carefully pipette out 10 cm 3 of the standard oxalic acid solution and transfer to a clean 100 cm 3 conical flask. Add two to three drops of phenolphthalein indicator. Titrate the solution with constant swirling till a persistent pink colour is obtained. Record the reading in the Observation Table.1 under the final reading column. Repeat the titration to get at least two concordant readings and record the same in the Observation Table.1 c) Determination of bromocresol green end point, B E by titrating against standardised sodium hydroxide solution i) Transfer 10 cm 3 of the given orthophosphoric acid solution into a conical flask and dilute with about 0 cm 3 of distilled water. Add 1 drop of bromocresol green indicator, and titrate the sample with the standardised NaOH solution till a faint blue colour persists. Record the initial and final burette readings in Observation Table.2. Repeat the titration to get at least two concordant readings and record the same in the Observation Table.2. d) Determination of mixed indicator end point, M E by titrating against standardised sodium hydroxide solution i) Transfer 10 cm 3 of the given Orthophosphoric acid solution into a conical flask; add 2-3 drops of mixed indicator, and titrate the sample with the standardised NaOH solution till the colour changes from pale rose to violet. Record the initial and final burette readings in Observation Table.3. Repeat the titration to get at least two concordant readings and record the same in the Observation Table.3 e) Determination of pk a values As explained in the section.2, the bromocresol green end point and mixed indicator end point titre values refer to the volumes of sodium hydroxide required for neutralising the phosphoric acid up to the first stage and the second stage respectively. From Observation Table.1 and.2 32

6 Volume of orthophosphoric acid solution taken, V P =.. cm 3 Volume of sodium hydroxide solution used for the neutralisation of orthophosphoric acid up to first stage, B E =.. cm 3 Volum`e of sodium hydroxide solution used for the neutralisation of orthophosphoric acid up to second stage, M E =. cm 3 To determine the pk a for the first stage of neutralization, take Vp cm 3 of orthophosphoric acid in a 50 cm 3 beaker and to this add B E /2 cm 3 of NaOH solution used for the titration and determine the ph of the solution with the help of a suitably caliberated ph meter ( If you do not know how to use the ph meter, take the help of your counselor for the purpose). Record your observation in Observation Table.. The ph of the solution would be equal to the pka for the first stage of neutralization To determine the pk a for the second stage of neutralization, take Vp cm 3 of orthophosphoric acid in a 50 cm 3 beaker and to this add [B E + (M E - B E )/2] cm 3 NaOH solution used for the titration and determine the ph of the solution with the help of a suitably caliberated ph meter. Record your observation in Observation Table.. The ph of the solution would be equal to the pk a for the second stage of neutralization.6 OBSERVATIONS AND CALCULATIONS a) Preparation of standard solution of oxalic acid Mass of weighing bottle + oxalic acid = m 1 g =...g Mass of weighing bottle (after transferring oxalic acid) = m 2 g =... g Amount of oxalic acid transferred = m 1 m 2 = m g =... g Molar mass (M m ) of oxalic acid = 126 g mol -1 Volume of oxalic acid prepared = 100 cm 3 m m Molarity of oxalic acid solution = M Oxalicacid = = =... M b) Standardisation of sodium hydroxide solution by titrating against standard oxalic acid solution Volume of standard oxalic acid solution taken in conical flask, V Ox = cm 3 Solution in the burette: Sodium hydroxide Indicator used: Phenolphthalein Observation Table.1: Standardisation of sodium hydroxide S.No. Volume of oxalic acid (in cm 3 ) Burette reading Initial Final Titre value (in cm 3 ) (Final-initial reading) Concordant reading c) Determination of bromocresol green end point, B E by titrating against standardised sodium hydroxide Volume of given sample of orthophosphoric acid taken, V P = 10 cm 3 Solution in the burette: NaOH 33

7 Indicator used: Bromocresol green Observation Table.2: Determination of bromocresol green end point, B E S.No. Volume of orthophosphoric acid (in cm 3 ) Concordant reading Burette reading Initial Final Titre value (in cm 3 ) (Final-initial reading) d) Determination of mixed indicator end point, M E by titrating against sodium hydroxide Volume of given sample of orthophosphoric acid taken, V P = 10 cm 3 Solution in the burette: NaOH Indicator used: mixed indicator Observation Table.3: Determination of mixed indicator end point, M E S.No. Volume of orthophosphoric acid (in cm 3 ) Burette reading Initial Final Titre value (in cm 3 ) (Final-initial reading) Concordant reading e) Observation Table.: Determination of pk a values S.No. volume of orthophosphoric acid solution (in cm 3 ) 1 Vp 2 Vp 3 Vp volume of NaOH solution used in the titration (in cm 3 ) ph [B E /2] = pk a 1 [B E + (M E -B E )/2] = pk a 2 [2B E + (M E -B E )/2] = pk a 3.7 RESULT The pka values of orthophosphoric acid are determined to be: pk a 1 = pk a 2= pk a 3=