First week Experiment No.1 / /2013. Spectrophotometry. 1. Determination of copper via ammine complex formation using standard series method

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1 First week Experiment No.1 / /2013 Spectrophotometry 1. Determination of copper via ammine complex formation using standard series method Principal In the standard series method the teat and standard solution are placed in two similar test tubes. The colour intensity of the unknown solution is compared with the colours of a series of standard solution. Since the concentration of the standard solution is known, that the test solution can be compared. The pale blue colour of copper sulphate solution is of intensity insufficient for colorimetric determination. On adding ammonia, The intensely azure coloured complex is formed Cu NH 3 Cu [(NH 3 ) 4 ] 2+ Procedure 1- Into five clean test tube, transfer 0.5, 1, 2, 3 and 5 ml of standard copper solution. 2- To each tube, add 5 ml of 2 N ammonia solution. 3- Complete to 10 ml with distilled water (using a graduated pipette) 4- Repeat the above procedure using 1 ml of your unknown copper solution 5- Select the standard solution which is matched in colour with your unknown solution and determine the unknown concentration of cu using the formula under calculation 1

2 Calculation: Concentration of Cu 2+ in unknown solution can be calculated from the formula: Cu unkn = Cu Stand V stand V unkn C unkn, C stand are the concentration of the test and standard solutions. V unkn V stand and are the volumes of the test and standard solutions. Results 2

3 Quiz No 1 1- Define the following: Frequency, v. velocity, c. wave-length, l. wave number, v. Absorption. Emission :. Beer s Law. 2- Write on the type of electronic transitions:.... 3

4 Second week Experiment No.2 / / Absorption spectrum and calibration curve of potassium permanganate Principle: Transition elements of the lanthanide group contain partially filled d" orbitals. Manganese is one of the transition elements. Electronic transition between the "d" orbitals are possible leading to highly coloured solutions and high intensity of absorption. Potassium permanganate has two absorption maxima at wavelength range nm. Solutions: a- Stock M potassium permanganate solution. b- Unknown; potassium permanganate solution (s). A- Determination of absorption spectrum of potassium permanganate solution in water at the wavelength range of nm: Procedure: 1. Pipet 50.0 ml of the provided stock solution into 250- ml standard flask then dilute with distilled water to the mark. This stock solution contains (0.005x 50)/250 = 0.00l M solution. 2. Pipet 20.0 ml of stock l in 100- ml standard flask, dilute to l00 ml with distilled water and mix well. 3. Fill the sample cell with the solution prepared in 2, then scan the spectrum in the indicated wavelength range and notice the two X max. 4. Calculate ε max for the two λ max using Beer s- lambert s law formula: Where, C = concentration in moles/l. ε = molar absorptivity b = path length (1 cm) A = absorbance. A = εbc 4

5 Procedure: B- Verification of Beer s-lambert s law (Standard curve): 1. Pipet 5.0, 10.0, 15.0, 20.0 mls in five (100-ml) flasks, dilute to the mark with distilled water and mix well. 2. Select one of the two λ max, and then use it for obtaining the absorbance (A) of the solutions against water as a blank. 3. Plot the standard curve (concentration vs Absorbance) on a sheet of graph paper label the axes (X and Y). C- Determination of the concentration of the provided unknown potassium permanganate solution. Procedure: 1. Dilute the unknown solution provided to the mark with distilled water. 2. Measure the absorbance of the diluted unknown at the same X max used for the standard curve. 5

6 3. Find out the concentration of the unknown in moles/l. from the standard curve. Calculation The molecular weight of potassium permanganate = 158 λ max 1 λ max 2 ε max 1 ε max 2 a max 1 a max 2 Where a is the absorptivity E 1% 1 cm (λ max 1 ) = E 1% 1 cm (λ max 2 ) = Unknown concentration= moles/l Results 6

7 Quiz No 2: Define the following Chromophore: Auxchrome: Bathochromic shift (red shift): Hypsochromic shift (blue shift): Hyperchromic effect: Hypochromic effect: 7

8 Third week Experiment No.3 / /2013 Determination of the composition of a binary mixture (potassium dichromate and potassium permanganate), spectrophotometrically. Purpose The objective of this experiment is to gain hands-on experience with the UV- VIS instrument. Theory: This experiment is concerned with the simultaneous spectrophotometric determination of two solutes in a solution. The absorbances are additive, provided there is no reaction between the two solutes. Hence we may write Αλ1 = λ1a1 + λ1a2 (1) and Αλ2 = λ2a1 + λ2a2 (2) Where A 1 and A 2 are the measured absorbance s at the two wavelengths λ 1 and λ 2 respectively. The subscripts 1and 2 refer to the two different substances, and the subscripts λ 1 and λ 2 refer to the different wavelengths. The wavelengths are selected to coincide with the absorption maxima of the two solutes; the absorption spectra of the two solutes should not overlap appreciably, so that substance 1 absorbs strongly at wavelength λ 1 and weakly at wavelength λ 2, and substance 2 absorbs strongly at λ 2 and weakly at λ 1. Now, A= ε c l, where ε is the molar absorption coefficient at any particular wavelength, c is the concentration (mol L -1 ) and l is the thickness, or length, of the absorbing solution (cm). If we set l = 1 cm then Αλ1 = λ1ε1c1 + λ1ε2c2 (3) and Aλ2= λ2ε1c1 + λ2ε2c2 (4) Solutions of the these simultaneous equations gives C1 = (λ2 ε2α λ1 λ1ε2 Αλ2) / (λ1ε1 λ2ε2 λ1ε2 λ2ε1) (5) C2 = (λ1 1Αλ2 λ2 ε1α λ1) / ( λ1ε1 λ2 ε2 λ1ε2 λ2ε1) (6) The values of molar absorption coefficients ε 1 and ε 2 can be deduced from measurements of the absorbances of pure solutions of substances 1and 2. By measuring absorbance of the mixture at wavelengths λ 1 & λ 2, the concentrations of the two components can be calculated. 8

9 Requirements: M, M and M solutions of potassium dichromate (in 1 M sulphuric acid and 0.7 M phosphoric acid) M, M and M solutions of potassium permanganate (in 1 M sulphuric acid and 0.7 M phosphoric acid) Procedure: 1. Prepare a series of solutions: Potassium dichromate M, M and M in mixed solvent of sulphuric acid (1 M) and phosphoric acid (0.7 M). 2. Prepare a series of solutions: Potassium permanganate 0.001M, M and M in sulphuric acid (1M) and phosphoric acid (0.7 M). 3. Measure the absorbance A for each of the three solutions of potassium dichromate and also each of the three solutions of potassium permanganate at both 440 nm and 545 nm by taking 1ml solution each. Calculate ε in each case by A= ε.c.l and record the mean values for dichromate (2) and permanganate (1) at the two wavelengths. 4. Mix potassium dichromate (0.001 M) and potassium permanganate ( M) in the following amounts shown in Table1 in 100 ml beakers. In each case, total volume of solution should be 25 ml. To each of these solutions add 0.5 ml of concentrated sulphuric 3acid, (a set of typical results included in Table I for guidance). Measure the absorbance of each of the mixtures at 440 nm. Calculate the absorbance of the mixtures from A440 = 440 εcr c Cr+ 440 εμn c Mn A545 = 545 ε Crc Cr+ 545 εμn c Mn 5. Record the absorbance of the unknown solution (supplied) at 545nm and 440 nm. Calculate the concentrations of permanganate and dichromate in this solution. 9

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11 Quiz No 3 Write on the following 1. Monochronators: 2. Draw the main components of the Single beam spectrophotometer 11

12 Fourth week Experiment No.4 / /2013. Effect of ph on absorption curve of sulphanilamide Principle: In alkaline solution the primary amino group is retained as auxochrome. In acid solution quaternisation occur to a co-ordinatively auxochrome. (Hypsochromic shift and hypochromic effect resulting from the removal of the non-bonded electrons of the NH 2 auxochrome from the conjugated system) Solutions: 0.0l g % w/v aqueous solution of sulphanilamide. 1 N hydrochloric acid. 1 N sodium hydroxide. Procedure: 1. Pipet l0.0 ml of sulphanilamide solution into l00- ml standard flask, dilute to the mark with ln hydrochloric acid solution and mix. Determine the absorbance of this solution in l cm cells at l0 nm interval from nm, using 1 N hydrochloric acid as the blank (use 5 nm interval around λ max ) 2. Repeat step (1) but using 1N sodium hydroxide instead of 1N hydrochloric acid, 3. Plot the absorption curves of the acid and alkaline solutions on the same graph paper. 4. Determine λ max and calculate E 1% 1 cm for each solution. 12

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14 Quiz No 4 1. Write on the following Effect of ph on absorption spectrum of phenol and aniline Phenol: Aniline 14

15 Fifth week Experiment No.5 / /2013. Principle: 5. Effect of ph on absorption curve of thymol blue Thymol blue is an acid base indicator, it has phenolic hydroxyl group in its structure. In alkaline solution it changes into phenolate anion and the interaction of the negative charge on the oxygen atom with the ring leads to different resonating structures. As a result, a bathochromic shift and a hyperchromic effect occur and the colour changes from red (acid medium) into blue (alkaline medium). Solutions: 0.02 g % w/v thymol aqueous solution.. 1 N hydrochloric acid. 1 N sodium hydroxide. Procedure: 1. Pipet 10.0 ml of thymol blue solution into 25- ml standard flask. Dilute to the mark with 1 N hydrochloric acid solution and mix. Determine the absorbance of this solution in l cm cell at l0 nm interval from nm using 1 N hydrochloric acid as a blank (use 5 nm interval around the λ max ) 2. Repeat step l using 1N sodium hydroxide instead of l N hydrochloric acid. 3. Plot the absorption curves of the acid and alkaline solutions on the same graph paper. 4. Determine λ max and calculate E 1% 1 cm for each solution. 15

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17 Write on the following Quiz 5 General requirements of the coloured product Photocell 17

18 Sixth week Experiment No.6 / / Determination of the Molar Ratio of Iron-Thiocyanate Complex. Principle: At 500 nm, neither ferric ions in nitric acid solution nor the thiocyanate ions absorb strongly. When the two solutions are combined together, they react to form a blood red complex that has a high absorptivity value at 500 nm. m Fe 3+ + n SCN - [Fem (SCN)n] (3m-n) Solutions: Equimolar solutions of ferric ammonium sulphate solution (A) and ammonium thiocyanate solution (B), 3xl0-3 M, are prepared. Procedure: 1. Prepare a series of solutions by mixing the following indicated amounts of solution A and B: l ml solution A + 9 ml solution B... to 9 ml solution A + l ml solution B 2. Measure the absorbances at 500 nm against blanks prepared similarly. 3. Plot the absorbances versus the corresponding mole fractions of Fe The resultant plot is round shaped curve. When the straight line portions of the curve are extrapolated, the point on the abscissa corresponding to their intersection represents the molar ratio of the complex. 18

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20 Quiz 6 General requirement of an ideal chromogen: Photomultiplier 20

21 Seventh Week Experiment No.7 / / Determination of pka of Methyl Orange. Principle: Many acid-base indicators are Tweak acids and can be represented simply as HIn Hi + + ln - Acidic form (red) basic form (yellow) In case of methyl orange both forms have strong absorption peaks in the visible region. The pka value of an indicator is given from the equation: ph = PK a + log [In ] [HIn] The absorption spectrum of an indicator dye shows some variation with ph changes. A number of curves showing absorption spectra could be plotted as we vary the ph. The point of intersection of all these curves is known as the isosbestic point. At high ph values [ln - ] > [HIn] and basic form predominates. While at low ph values [HIn] > [In - ] and acid form predominates. So, in order to determine pka value of an indicator we select a wavelength at which either the acidic or basic form absorbs most strongly. In the case of methyl orange, at 510 nm thentrizlic formhln predominates. Then we vary the ph and observe the value of absorbance of each solution as a function of ph. At the midpoint of` the curve, we register the ph and that is equal to the pka value of the indicator. At this point [HIn] = [ln - ] and ph = pka. Solutions: Methyl orange aqueous solution : g/l Buffer solutions: ph N Hydrochloric acid solution. 1 N Sodium hydroxide solution. 21

22 A- Determination of absorption spectra: Procedure: 1. Pipet 2.0 ml methyl orange solution into a 10-ml standard flask, complete to volume with IN hydrochloric acid solution. 2. Repeat the same experiment using IN sodium hydroxide instead of hydrochloric acid. 3. Determine the absorption spectra of` methyl orange in IN hydrochloric acid and in I N sodium hydroxide in the range of nm against a blank of distilled water. B- Determination of pka of an indicator: Procedure: 1. Pipet 2.0 ml methyl orange solution into I0- ml standard flask, add 2 ml buffer solution (ph 2), complete to volume with distilled water. 2. Measure the absorbance at 5I0 nm vs. blank treated similarly. 3. Repeat the experiment with other buffer solutions (2.5, 3,... 8). 4. Plot the obtained absorbance values versus their respective ph values and determine the pka value of methyl orange indicator. 22

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24 Write on the following Quiz 7 The measurement of complexation (ligand/metal ratio in a complex): Deviation from Beer s law 24

25 Eighth week Experiment No. 8 / /2013. Conductometry 8. Couductomctric Titration of Strong Acid with Strong Base Principle: H + + Cl - + Na + + OH - H2O + Na + + Cl - When sodium hydroxide is added to hydrochloric acid, the highly conducting hydrogen ions initially present in the solution are replaced by sodium ions having a much lower conductance. The addition of the alkali to the solution will thus be accomplished by a decrease in conductance. When neutralization is complete the further addition of alkali results in an increase of conductance. At the neutral point the conductance of the solution will have a minimum value, from which the equivalence point of the reaction can be estimated. Procedure: 1. Pipet 10.0 ml of hydrochloric acid into 250 ml beaker and dilute with distilled water to l00 ml (a dip cell is supported in the solution and connected to a conductometer). 2. Add from a burette 0.1 ml of 0.01 M sodium hydroxide into the solution, mix, and after 2 or 3 minutes determine its conductance. 3. Add another 0.1 ml portion of 0.01 M sodium hydroxide and determine the conductance again 4. Continue the addition of sodium hydroxide solution until the conductance increases considerably with each addition.. 5. Plot the titration curve (The conductance is plotted as ordinate and the volume of sodium hydroxide as abscissa). 6. Determine the equivalence point graphically.(the two straight portions of the curve are extrapolated until they intersect. The point of intersection is taken as the equivalence point) 25

26 7. Calculate the concentration of hydrochloric acid. 26

27 Quiz 8 Write on the following The conductance of the solution depends on: Conductivity Bridge: 27

28 Ninth week Experiment No. 9 / / Conductometric titration of mixture of weak acid and strong acid with weak base A sample of vinegar (CH 3 COOH) has been adulterated with HCl, it is titrated with O.5 N NH 4 OH and the following reading representing relative conductances are obtained Volume of 0.5 N NH 4 OH (ml) Relative conductance (G) 1. Plot the conductimetric titration curve 2. State the value of the two points. 3. Calculate the number of grams of : a- HCl and b- CH 3 COOH N (Cl=35.5 C=l2 H=l O:16) 28

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30 Quiz 9 Write on the following Determination of mixture of hydrochloric acid (strong acid) and acetic acid (weak acid) with sodium hydroxide (strong base) 30

31 Tenth week Experiment No. 10 / /2013. Potentiometery 10. Potentiometric Titration of Weak Acid with Strong Base Principle: In this experiment the titration of acetic acid is followed by (measuring the mv of the solution after each addition of NaOH (titrant). From the data the end point is determined. Apparatus: Potentiometer (commercial ph/m.v meter). ph electrode (glass and reference electrode, SCE or Ag/AgCl ) or Combination ph electrode. Magnetic stirrer or glass rod. Beaker 300 ml (tall form ) Burette. Pipet Procedure. 1. Fit the apparatus with the electrode assembly (or Combination electrode) supplied with ph meter supported inside the beaker. The beaker contains 50 ml of 0.l acetic acid. 2. Fill the burette with 0.2 N NaOH. 3. Add 2-3 ml of 0.2N NaOH from the burette, stir for about 30 seconds and after waiting for 30 seconds measure the e.m.f of the cell (mv). Record also reading of alkali in the burette. 4. Repeat the addition of the same portion of the NaOH solution, stirring and measuring the mv after each addition until a point is reached within about l ml of the expected end point. Henceforth add the alkali solution in portion of 0.2 ml; and record the potentiometric reading (mv) after 31

32 each addition. Continue the addition until the equivalent point has been passed by Plot potential against volumes of reagent added (0.2N NaOH). Draw a curve through the points. The equivalence point is the volume corresponding to the steep portion of the curve. V 2 (ml) E (mv) E V 1 E/ V V 2 for 1 st derive. plot 32

33 Quiz 10 Write on the following Glass membrane electrode Calomel electrode 33

CHEMISTRY LABORATORY - I

The Great Chemist ALFRED NOBEL CHEMISTRY LABORATORY - I -1- WORK SHEET Titration 1 : Standardization of AgNO 3 Standard Sodium chloride Vs AgNO 3 Sl.No Vol.of Sodium chloride V 1 (ml) Burette reading (ml)