EXPT. 3 DETERMINATION OF CHROMIUM AND MANGANESE IN A MIXTURE

Transcription

1 Spetrosopi Methods EXPT. 3 DETERMINTION OF CHROMIUM ND MNGNESE IN MIXTURE Struture 3. Introdution Objetives 3. Priniple 3.3 Requirements 3.4 Solutions Provided 3.5 Proedure 3.6 Observations and Calulations 3. Result 3.8 Preautions 3. INTRODUCTION In the previous two experiments you have learnt about the spetrophotometri determination of the onentration of iron and nikel ions respetively by alibration plot method. In these experiments the metal ions were onverted into suitable omplexes that were oloured and formed the basis for their determination. In this experiment you would learn about the simultaneous determination of two speies whose UV-VIS spetra have overlapping regions. You would reall that you have learnt about this aspet of UV-VIS spetrophotometry in subsetion.6.4 of Unit of MCH-003 ourse. It would be worthwhile to have a relook at the same. In this experiment you would determine the amount of hromium and manganese in a given mixture. These are present as dihromate and manganate ions respetively. In fat this determination mimis the determination of the amount of hromium and manganese in alloy steels wherein these are oxidised to dihromate and permanganate ions respetively. The spetra of these ions overlap to ertain extent. In the next experiment you would learn about the determination of methylethyl ketone an important industrial solvent. Objetives fter studying and performing this experiment you should be able to: explain the priniple underlying simultaneous spetrophotometri determination of speies with overlapping spetrum, arry out absorbane measurements for solutions on a spetrophotometer and draw its UV-VIS spetrum, determine the wavelength of maximum absorption in the spetrum and ompute the orresponding molar absorption oeffiient, alulate the onentrations of hromium and manganese in a mixture from the absorption measurements of the mixture at two different wavelengths, apply and adapt the method of simultaneous determination for other similar ombination of ions. 3. PRINCIPLE On dissolution of alloy steel ontaining hromium and manganese, we get these elements as their ions; Cr 3+ and Mn + ions respetively. In the determination, these are oxidised respetively to dihromate and permanganate ions using potassium persulphate and potassium periodate, respetively. The orange red oloured dihromate shows 0

2 maximum absorption ( max at nm while for the pink oloured permanganate the max is at nm. However, permanganate also absorbs at nm to a smaller extent. Similarly, dihromate ions also have small absorption at nm (Fig.3.. You would reall from subsetion.6.4 of Unit of MCH-003 ourse that in order to determine these speies we need to find two wavelengths where molar absorption oeffiient of two speies is different and it is desirable that the wavelengths are suh where the ratio of molar absorptivities is largest. Fig. 3.: Shemati diagrams of the visible spetra of potassium dihromate and potassium permanganate in M sulphuri aid, indiating the overlapping of the spetra and the relative values of molar absorption oeffiients In simple words, both the speies absorb at the wavelengths of maximum absorptions mentioned above. Thus, the absorption of the solution ontaining a mixture of dihromate and permanganate ions at these wavelengths would be the sum of the absorbanes of the two speies. It an be shown that the absorbanes of these two ions obey Beer-Lambert s law when measured individually and also in a mixture in presene of 0.5 M sulphuri aid. Under these onditions we an write the following expression assuming a unit path length. ε Cr, [Cr O ] + ε Mn, [Mn O 4 ] (3. ε Cr, [Cr O ] + ε Mn, [Mn O 4 ] (3. Here, the meaning of the terms used is self evident. For example, ε Cr, refers to the molar absorption oeffiient at nm for hromium as dihromate ion; refers to the absorbane of the mixture at nm and [Cr O ] represents the onentration of dihromate ions and so on. We need to solve these simultaneous equations to determine both the speies without their separation. The general expressions for the simultaneous equations are: + C ( ( ε C ε + C ( ( ε C ε Solving Eq. (3. and (3. we get the following expressions for the onentrations of dihromate and permanganate ions. [ Cr O ] [ MnO ] 4 CrO CrO [ Cr O ] CrO (3.3 (3.4

3 Spetrosopi Methods Thus, the onentrations of the two ions an be obtained from the absorbane measurements at and nm, if we know the molar absorption oeffiients for both the ions at these wavelengths. The values of the molar absorption oeffiients are obtained from absorbane measurements of pure solutions of the two substanes at the designated wavelengths. Fe 3+, Ni +, Co + and V + also absorb in these regions, therefore these should be absent from the analyte solution. 3.3 REQUIREMENTS pparatus Spetrophotometer/ Filter photometer Mathed uvette Volumetri flasks ( litre Volumetri flasks (50 m 3 Graduated pipette (0 m 3 Beakers (00 m 3 Weighing bottle Burettes 0 Chemials Potassium dihromate Potassium permanganate Sulphuri aid 3.4 SOLUTIONS PROVIDED i M sulphuri aid (Referene blank: It is prepared by adding (with onstant swirling 53.3 m 3 of analytial grade onentrated sulphuri aid into a one litre flask ontaining about 500 m 3 of distilled water, allowing the flask to ool and making up the volume using distilled water. ii iii iv Potassium dihromate (0.00 M in M sulphuri aid: It is prepared by aurately weighing 0.4 g of analytial grade potassium dihromate and quantitatively transferring to a 50 m 3 volumetri flask and adding suffiient amount of M sulphuri aid to dissolve it. One the ompound dissolves the volume is made upto the mark with M sulphuri aid. Oxali aid (0.005 M: It is prepared by aurately weighing g of oxali aid and quantitatively transferring to a 00 m 3 volumetri flask and adding suffiient amount of water to dissolve it. One dissolved the volume is made upto the mark with distilled water. Potassium permanganate (0.00 M in M sulphuri aid: It is prepared as follows. Weigh about.8 g.r. potassium permanganate and dissolve in 50 m 3 of distilled water taken in a 00 m 3 beaker. Heat the solution to boiling and gently ontinue for about fifteen minutes. Cool the solution and filter through a plug of glass wool in the funnel. Take 0 m 3 of the filtrate in a 00 m 3 beaker and dilute to 00 m 3 with M sulphuri aid. Standardise the solution by titrating with a standard solution of oxali aid in aidi medium. Calulate the volume of the solution required to prepare 50 m 3 of 0.00 M solution.

4 Take the requited volume of the standardised solution of potassium permanganate in a 50 m 3 volumetri flask. Make up the volume with M sulphuri aid. v Sample solution: prepared by suitably mixing the 0.00 M solutions of potassium dihromate and potassium permanganate to make 0 m 3 of the mixture and adding 0 m 3 of M sulphuri aid to it. 3.5 PROCEDURE The proedure for the experiment an be divided into three omponents. These are given below. a Determination of molar absorption oeffiients of dihromate and permanganate ions b Establishing the additivity of absorbane values of dihromate and permanganate ions Determination of the onentrations of dihromate and permanganate ions in the mixture. Follow the instrutions given below in the sequential order to aomplish these tasks. a Determination of molar absorption oeffiients for dihromate and permanganate ions. Pipette out 0 and 40 m 3 respetively of the 0.00 M stok solution of potassium dihromate into two 00 m 3 standard flasks and make up the volume with M sulphuri aid. This would give M and M solutions respetively of potassium dihromate. Mark the flasks aordingly.. Pipette out 0 and 40 m 3 respetively of the 0.00 M stok solution of potassium permanganate into two 00 m 3 standard flasks and make up the volume with M sulphuri aid. This would give M and M solutions respetively of potassium permanganate. Mark the flasks aordingly. 3. Measure the absorbane values at nm and nm for the stok solutions (0.00 M of potassium dihromate and potassium permanganate and the dilutions prepared in step and. Make measurements in uvettes of m path length and use M sulphuri aid as the referene. Reord the values in Observation Table Calulate the molar absorption oeffiients for potassium dihromate and potassium permanganate at nm and nm as per the expressions given under olumn 4 and 6 and reord appropriately in the Observation Table 3.. b Establishing the additivity of absorbane values of dihromate and permanganate ions at and nm. Take two lean burettes and fill them with M potassium dihromate and M potassium permanganate respetively.. Take beakers of 00 m 3 apaity labelled from to and transfer the solutions of M potassium dihromate, M potassium permanganate and M sulphuri aid as per the details given in olumn, 3 and 4 of the Observation Table 3.. 3

5 Spetrosopi Methods 3. Measure the absorbane values of the resulting solutions using M sulphuri aid as the referene and reord your observations in the olumn 5 and of Observation Table 3.. You must use uvettes having a path length of m. 4. Calulate the absorbane values for the mixtures of potassium dihromate and potassium permanganate at nm and nm by using Eq. (3. and Eq. (3. respetively and reord them in the olumn 6 and 8 of Observation Table Compare the observed and alulated absorbane values for the validation of additivity. Reord your observations in the spae provided after Observation Table 3.. Determination of the onentrations of dihromate and permanganate ions in the mixture.. Measure the absorbane values of the given sample solution taken in a uvette of path length equal to m. Use M sulphuri aid as the referene and reord your observations in the olumn and of Observation Table Calulate the onentration of potassium dihromate and potassium permanganate in the mixture by using Eq.(3.3 and Eq. (3.4 respetively and reord your results in the olumn 3 and 4 of Observation Table OBSERVTIONS ND CLCULTIONS. Determination of molar absorption oeffiients for dihromate and permanganate ions Observation Table-3.: Determination of molar absorption oeffiients for K Cr O and KMnO 4 at and nm : Potassium dihromate Column S. No. Conentration bsorban ε mm # ε mm # of K Cr O e at at nm nm nm KCrO nm nm K CrO KCrO nm B: Potassium permanganate S. No. Conentration of KMNO 4 KMnO at nm nm verage verage ε mm # nm KMnO 4 at nm nm ε mm # nm KMnO 4 verage verage 4

6 # the expression is valid only if the absorbane measure is made in a uvette of path lengthm. The molar absorption oeffiients for the [Cr O ] and [MnO 4 ] ions are found to be: ε nm [Cr O ] ε nm [Cr O ] ε nm [Mn O 4 ] ε nm [Mn O 4 ] B. Establishing the additivity of absorbane values of dihromate and permanganate ions at and nm Observation Table 3.: The observed and alulated absorbane values for the mixtures of potassium dihromate and potassium permanganate. Column S. No. Volume of M K Cr O (m 3 Volume of M KMnO 4 (m Volume of M H SO 4 (m 3 at nm nm at nm nm Obsd. Cal. * Obsd. Cal. ** * The absorbane value is alulated as per Eq. (3.. ** The absorbane value is alulated as per Eq. ( 3.. Write your onlusions about the additivity of the absorbane values here. 5

7 Spetrosopi Methods C. Determination of the onentrations of dihromate and permanganate ions in the mixture. Observation Table 3.3: Determination of the onentrations of dihromate and permanganate ions in the mixture at nm nm at nm nm K CrO CrO CrO KMnO 4 CrO [ Cr O ] K CrO 3. RESULT KMnO 4 i The onentrations of potassium dihromate in the given mixture is found to be...m ii The onentrations of potassium permanganate in the given mixture is found to be...m 3.8 PRECUTIONS ll the reagents used must be of R grade. ll the glassware used must be quite lean. The measurements should preferably be made in uvettes of m path length. (In the absene of these the expressions given for alulation of molar absorption oeffiients and onentrations would need to be modified. 6