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ANALYTICAL CHEMISTRY, , Ix, , Spectrophotometry, , , , Experiment No. 1 (3 and 6 Units), , Estimation of Cr (VI) using diphenylcarbazide, , Aim: Estimation of chromium (VI) in water sample spectrophotometrically/spectrocscopically, by using diphenylcarbazide., , Theory: Amount of light absorbed by a solution is dependent upon the thickness of the light, , absorbing material through which the light has to pass and concentration of absorbing solution., The Beer-Lamberts law is the linear relationship of absorbance with thickness and concentration., , The mathematical statement of the law is, A=ecl, , Where,, , 4— Absorbance measured, , &- Molar extinction coefficient, , ¢ ~ concentration of the solution, , ! ~ thickness of the light absorbing material, ___ Ina system under the given experimental conditions, if ¢ and / is, 'S directly proportion to the concentration., , Calibration Curve Method:, , A series of standard solutions are prepared in exactly the, developed by adding reagents. Unknown (sample) is treated in exactly the, - 161, kept constant, absorbance, , i or is, d of different known concentrations. Color is, same way. Absorbance

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162, , College Practical Chemistry (TY. B.Sc ), , of the standard and the sample solutions are noted and hence, the concentration of unknown is, determined graphically., , A, , small quantity of Cr(VI) may be determined spectrophotometrically using 0.25%, , diphenylcarbazide reagent in acidic medium. Cr(VI) gives a soluble violet coloured compound with, this reagent. The absorbance is measured at the wavelength maxima of the complex (A max)., , Requirements: 1000 ppm K,Cr,O, solution, IM H,SO, solution, 0.25% diphenylcarbazide, reagent solution, (filter 540 im), 350 cm} and 500 em? beakers, 1000 cm} and 100 cm} standard, measuring flasks, 10 cm} and 25 cm} pipettes etc., , Procedure:, (1) Prepare 1000 ppm chromium solution using K,Cr,0, by dissolving 283 mg of K, 2Cr,0,, , GB), , (4), (5), , (6), , in 100 cm} of distilled water in a 100 cm? standard measuring flask. This is called stock, solution. From this solution take 1.0 cm? of solution and dilute it to 100 cm? this is working, standard. (10 ppm), , From the working standard solution, pipette out 2, 4, 6, 8 and 10 cm? solution and transfer, to 01 to 05 serially numbered 100 cm? standard measuring flasks., , Add 10 cm} of 1M H,SO, solution and 2 cm} of 0.25% diphenylcarbazide reagent solution, to each of the standard measuring flask containing Cr ion solution., , Dilute each solution to 100 cm} by adding distilled water., , Take the given sample solution and dilute it to 100 cm} and use 10 cm} for colour, development., , Measure absorbance of each solution at 540 nm against the reagent blank as given in the, Table., , Observations and Calculations:, , , , , , , , , , Sr. | Volume of 10 | Concentration| Volume of Volume of | Final volume | Absorbance, No. | ppm of Cr(VI) of Cr(VI), 1M H,SO, 0.25% with distilled A, solution in ions in solution in diphenyl- water, cm ppm cm carbazide, solution in, cm, , ol 2.0 0.2 10.0 2.0 100, 02 4.0 0.4 10.0 2.0 100, 03 6.0 06 10.0 2.0 100, 04 8.0 08 10.0 2.0 100, 05 10.0 1.0 10.0 2.0 100, 06 Unknown _- 10.0 2.0 100 Ax

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specirophotomeny 163, , Graph: :, Plot a graph of absorbance (A) against concentration (c) of chromium ions in ppm., , A, , , , A, = Absorbance of, unknown, , C, = Concentration of, chromium (VJ) in, unknown, , Absorbance (A), , , , Cy, Concentration in ppm, , Fig. 9.1, From the graph, concentration of unknown chromium solution (C) = ppm., , So, the concentration of chromium in the given sample is (10 « C)= ppm., For calculation of percentage error, prefer page No. 84., , Results:, , (1) The amount of Cr(VI) present in the water sample = ppm., (2) Percentage error = %,, , oO a0

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ANALYTICAL CHEMISTRY, , x, , Analysis of Commercial Samples, , , , Experiment No. 1 (6 Units Only), Estimation of Glucose, , Aim: Estimation of reducing sugar (glucose) in honey by Wilstatter’s method., , Theory: Honey contains glucose, fructose, maltose and sucrose. Glucose contains easily oxidisable, aldehyde group. Glucose is treated with known excess of, solution in the presence of NaOH which, oxidises glucose to gluconic acid. The unreacted I, is back titrated against standard Na,S,0,, solution using freshly prepared starch indicator. Redox reaction is invoved in estimation., Requirements: Honey sample, 0.1N Na,S,0, solution, 0.1N I, solution, 0.5N NaOH solution, 2N, H,SO, solution, freshly prepared starch indicator, 100 cm} standard measuring flask, 10 cm? pipette,, 150 cm} conical flask, burette, conc. HCI, 10% KI solution, 100 cm} glass shoppered bottles etc., Procedure:, , Part I: Standardisation of Na,S,O, solution:, , (1) Prepare 100 cm? of 0.1N K,Cr,O, solution (0.490 g of K,Cr,O, dissolved in 100 cm? of, distilled water in a standard measuring flask)., , (2) Pipette out 10 cm} of the diluted solution in a 150 cm} conical flask. Add 5 cm} of conc., HCI to it., , (3) Add 10 cm} of 10% KI solution to it and shake well., , (4) Titrate the liberated I, against 0.1N (approximate) Na,S,O, solution from the burette using, 1-2 cm} of freshly prepared starch indicator. End point will be from blue to faint green, colour (due to formation of Cr salt solution) (x cm)., , - 164

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sie Wy, , Analysis of Commercial Samples, part II: Estimation of glucose: 165, , (1) Dissolve accurately about 2 g or the Supplied quantity of, and dilute it to 100 cm} in a standard measuring flask., , (2) Pipette out 10 em? of the diluted solution in a 100 cm}, , (3) Add 5 cm} of 0.5N NaOH solution and 25 cm}, shaking. Cork the bottle tightly., , (4) Moisten the stoppers with 5% KI Solution to prevent loss of iodine., , (5) Keep the glass stoppered bottle in dark for 30 minutes. Shake the bottle from time to time., , (6) After 30 minutes, add 5 cm} of 2N H,SO, solution to bottle, Shake well for Proper mixing., , (7) Titrate the liberated iodine against standardised Na,S,0, solution from the burette till, the light brown colour appears., , (8) Add 2 cm} of freshly Prepared starch indicator to it and continue to titrate. End point will, be from blue to colourless. (y cm'),, , Part III: Blank Titration;, , (1) Perform blank titration with the same procedure given in Part II by taking 10 cm? of the, distilled water instead of honey solution. (z cm})., , honey in warm distilled water, , glass stoppered bottle., of 0.IN 1, solution to it with constant, , Reactions:, (1) K,Cr,0, + 14HCl —> 2KCI + 2CrCl, + 7H,O + 3Cl,t, (2) Cl, + 2KI ——> 2KCI + 1,, (3) 1, + 2Na,S,0, —-> Na,S,O, + 2Nal, (4) 2NaOH + 1, —-> Nal + NaOI + H,O, (5) C,H,,0, + NaOl —-» C,H,,0, + Nal, Glucose Gluconic acid, , Calculations:, , Part I: Normality of Na,S,O, solution:, 10 cm of 0.1N K,Cr,O, solution required x cm? of Na,S,O, solution., 10x 0.1, , “. Normality of Na,S,O, solution = a, , = AN., , Part Il: Estimation of glucose in honey:, Volume of 0.1N I, solution added to honey solution in terms of 4 N Na,S,O, solution, , =z cm},, Volume of 0.1N 1, solution remained unused in terms of A N Na,S,O, solution, . 3, =ycm’., , “. Volume of 0.1N I, sotution used up for the oxidation of 10 cm? of honey solution., . =z-y=Cecm’., , a