The Great Chemist ALFRED NOBEL CHEMISTRY LABORATORY - I
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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) Initial Final Volume of AgNO 3 V 2 (ml) Concordant value Calculation Volume of Sodium chloride Strength of Sodium chloride Volume of AgNO 3 Strength of AgNO 3 V 1 = ------- ml N 1 = -------N V 2 = ml N 2 =?.N V 1 N 1 = V 2 N 2 N 2 = V 1 N 1 / V 2 = [------- (V 1 ) X ------ (N 1 )] / ------(V 2 ) Strength of AgNO 3 N 2 = ----------- N -2-
Ex.No. Date: DETERMINATION OF CHLORIDE CONTENT OF WATER SAMPLE BY ARGENTOMETRIC METHOD Aim To determine the chloride content of water sample by argentometric method (Mohr s method). Principle Chloride is present in water usually as NaCl, MgCl 2 and CaCl 2. Although chlorides are not harmful as such, their concentrations beyond 250 ppm impart a peculiar taste of the water, thus rendering the water unacceptable for drinking purposes. Further, existence of unusually high concentration of chloride in a water sample indicates pollution from domestic sewage or industrial waste water. Salts like MgCl 2 may undergo hydrolysis under the high pressure and temperature prevailing in the boiler, generating hydrochloric acid which causes corrosion in boiler parts. Chlorides in the form of MgCl 2 and CaCl 2 cause various disadvantages in household and industrial use of water. By argentometric method chloride ions in water sample, which is neutral or slightly alkaline can be determined by titrating it against standard silver nitrate solution using potassium chromate as an indicator. The ph should be between 7 and 8. At higher ph Ag + + OH - Ag OH and at lower ph K 2 CrO 4 indicator is converted to K 2 Cr 2 O 7 2 K 2 CrO 4 + 2 HCl 2 KCl + K 2 Cr 2 O 7 +H 2 O -3-
WORK SHEET Titration 2 : Standard AgNO 3 Vs Distilled water Sl.No Vol.of Distilled Water (ml) Burette reading (ml) Initial Final Volume of AgNO 3 V 1 (ml) Concordant value Titration 3: Standard AgNO 3 Vs given Water Sample Sl.No Vol.of Sample Water (ml) Burette reading (ml) Initial Final Volume of AgNO 3 V 2 (ml) Concordant value -4-
As KHCrO 4 is weak in nature, concentration of CrO 2-4 decreases and therefore, higher concentration of Ag + is needed for the solubility product of Ag 2 CrO 4 to be exceeded. Ag + reacts with Cl - and CrO 2-4, when AgNO 3 solution is released from the burette sample solution which has Cl - Ag + + Cl - AgCl (K sp = 3X10-10 ) White ppt 2Ag + + CrO 2-4, Ag 2 CrO 4 (K sp = 5X10-12 ) Red ppt Red colour formed because of formation of silver chromate, disappears as the solution contains high concentration of Cl - Ag 2 CrO 4 +2Cl - 2AgCl +CrO 4 2- As the concentration Cl - ion decreases, the red colour disappears more slowly and when all the chloride is precipitated, a faint reddish or pinkish tinge persisting the white precipitate even after brisk shaking. Procedure : Titration 1 : Standardization of AgNO 3 Transfer 20 ml of standard NaCl in a conical flask and add about 3-4 drops of potassium chromate indicator. Then slowly add standard solution of AgNO 3 solution from the burette and the value of titrant is noted as end point, when yellow colour changes to red colour. The titration is repeated until a concordant volume V 2 is obtained. -5-
WORK SHEET Calculation : Volume of AgNO 3 consumed by given sample V 2 =.. ml Volume of AgNO 3 consumed by dis.water V 1 =.. ml Net volume (V 2 V 1 ) =. ml (V 2 ) Volume of given sample Strength of given sample Volume of Net AgNO 3 consumed Strength of AgNO 3 consumed V 1 = 50mL N 1 =?.N V 2 =...ml N 2 = N V 1 N 1 = V 2 N 2 N 1 = V 2 N 2 / V 1 = [------- (V 2 ) X ------ (N 2 )] / ------(V 1 ) N 1 = ---------- N The amount of chloride in the given sample = (N 1 ) X 35.5 X 1000 = ------------ ppm Precautions 1. The whole apparatus must be washed with distilled water. 2. The volume of the indicator should be same in all titrations. 3. The reaction mixture should be briskly shaken during the titration. -6-
Titration 2 : Distilled water Vs AgNO 3 Transfer 50 ml of standard distilled water in a conical flask and add about 3-4 drops of potassium chromate indicator. Then slowly add standard solution of AgNO 3 solution from the burette and the value of titrant is noted as end point, when yellow colour changes to red colour. The titration is repeated until a concordant volume V 1 is obtained. Titration 3: Estimation of chloride present in the given sample. Transfer 50 ml of given water sample in a conical flask and add about 3-4 drops of potassium chromate indicator. Then slowly add standard solution of AgNO 3 solution from the burette and the value of titrant is noted as end point, when yellow colour changes to red colour. The titration is repeated until a concordant volume V 2 is obtained. Result The amount of Chloride present in the given water sample = -------------- ppm. Viva questions 1. Name the sources of chlorides in water 2. Why does AgNO 3 combine first with chloride ions in the water and not with K 2 CrO 4? 3. What is the action of chloride on pathogens? 4. How is the ph adjusted when the alkalinity is not being measured? 5. What is the effect of temperature on the determination of chloride content in water? 6. What is the principle of Mohr s method? -7-
Observation Sl. No Volume of NaOH added (ml) WORK SHEET Observed Conductance (ohm -1 ) -8-
Ex.No. Date: CONDUCTOMETRIC TITRATION (STRONG ACID Vs STRONG BASE) Aim To find out the strength of acid (HCl) (approximately N/10), by titrating it against sodium hydroxide solution conductometrically. Principle When a strong acid like HCl is titrated against a strong base like NaOH, the neutralization reaction takes place. HCl +NaOH NaCl +H 2 O This neutralization reaction can be followed by conductometric method. During the course of titration, the conductivity decreases due to the replacement of H + ions by Na + ions. After the neutralization, the conductivity increases steeply due to the excessive presence of OH - ions. Procedure 0.5 N NaOH solution is taken in the burette. 50 ml of the given HCl is taken in a 100 ml beaker. Conductivity cell containing two platinum electrodes, is placed in HCl. The cell is connected to the conductivity meter. Now add 1mL, 2mL, 3mL and 4mL of NaOH from the burette and stir the contents of the beaker thoroughly. Note the conductance readings after each addition. -9-
WORK SHEET A graph is plotted with observed conductivity against volume of NaOH added. The shape of the curve will be as shown in the figure. Calculation : V 1 N 1 = V 2 N 2 Volume of NaOH V 1 = ---------- ml (from graph) Strength of NaOH N 1 = ---------- N Volume of HCl V 2 = --------- ml Strength of HCl N 2 = -----?----- N N 2 = (V 1 X N 1 ) / V 2 = [..( V 1 ) X..( N 1 )] /.( V 2 ) Strength of unknown HCl (N 2 ) = ------------- N Amount of HCl present in the whole of the given solution = [---------------- (N 2 ) x ------------- (Eq.wt) x 50] / 1000 = ------------------- g (Equivalent Weight of HCl = 36.5) -10-
Result (i) (ii) The strength of the given acid = ----------------- N Amount of HCl present in the whole of the given solution = ------------g -11-
Observation Volume of NaOH solution added (ml) WORK SHEET ph ph V ph/ V Average volume of NaOH (ml) Where, V = difference in two consecutive volumes of titrant added in ml ph = difference in two consecutive reading -12-
EX.NO. Date: DETERMINATION OF STRENGTH OF HYDROCHLORIC ACID BY ph TITRATION Aim To find out the strength of given hydrochloric acid solution by titrating it against sodium hydroxide (0.1N) using ph meter. Principle When an alkali is added to an acid solution, the ph of the solution increases slowly, but at vicinity of the end point, the rate of change of ph of the solution is very rapid. From the sharp break in the curve, we can find out the end point, from which the strength of HCl can be calculated. Procedure First standardize the ph meter using different buffers of known ph, then wash the glass electrode and reference electrode with distilled water and then with the acid solution. Take 20 ml of HCl solution in a 250 ml beaker. Add sufficient distilled water (100mL) so that the glass electrode as well as the reference electrode is completely dipped. Note the ph of the pure acid solution. Now add 1 ml of 0.1 N NaOH from the burette in the beaker. Stir the contents well. Note the ph of the solution. Now keep on adding NaOH solution from the burette and the note the ph of the solution, up to 9-10 ml of the NaOH. Near the end point add very small amount of sodium hydroxide, because change in ph will be very much appreciable when the acid is neutralized, further addition of such a small amount as 0.01 ml raises the ph about 9 to 10. -13-
Graph WORK SHEET Calculation: Volume of NaOH required for complete neutralization of HCl = --------- ml Volume of NaOH Strength of NaOH Volume of HCl Strength of HCl V 1 = ------------- (ml) from graph N 1 = ------------ N V 2 = ------------- (ml) N 2 = ------?------- N Therefore Strength of HCl N 2 = [-------- (V 1 ) x -------- (N 1 )] / ---------- (V 2 ) = ------------N Amount = Normality x Equivalent Weight (HCl) Therefore Amount of HCl present in the whole of the given solution = [---------------- (N 2 ) x ------------- (Eq.wt) x 20] / 1000 = ------------------- g (Equivalent Weight of HCl = 36.5) -14-
Plot a graph between ph and Volume of NaOH added. From this graph, determine the volume of NaOH required for the complete neutralization of HCl. Result (i) Strength of the given hydrochloric acid solution = ---------- N (ii) The amount of hydrochloric acid present in the whole of the given solution = --------- g Viva questions 1. What do you mean by ph? 2. What is the effect of temperature on ph? 3. What are the composition of glass electrode and calomel electrode? 4. What is the effect of dilution on ph of an acidic solution? 5. What is the poh of pure water at 25 o C? 6. What chemicals would you use to make a buffer of ph 10? 7. What is a combined electrode? 8. Why is hydrogen not generally used in ph measurements? 9. What is the significance of ph titration? 10. What are the industrial applications of ph metric measurements? 11. What is the ph value of lime juice, blood and de-ionized water? 12. What is function of calomel electrode? -15-
Observation WORK SHEET Sl.No Volume of NaOH added (ml) Observed Conductance (ohm -1 ) -16-
Ex.No. Date: CONDUCTOMETRIC TITRATION OF MIXTURE OF ACIDS Aim To find out the strength of mixture of acids (hydrochloric acid and acetic acid) of approximate strength N/10 each, by titrating it against sodium hydroxide solution conductometrically. Principle The type of titration is just a combination of two separate titrations, viz. HCl against NaOH and CH 3 COOH against NaOH. By adding to the mixture, the conductivity of sodium ion decreases due to the replacement of H + ions forms the strong acid. The conductivity of the solution increases as the weak acid is converted into salt and finally rises more steeply as excess of alkali is added. A graph is plotted with conductivity against volume of alkali added. It is observed that there is a rounding off at both the end points. Usually, extrapolation of the straight lines of the three branches would lead to a definite location of the end points. It must be noted that the first end point will be that of hydrochloric acid (strong) while the second will be that of acetic acid (weak). -17-
WORK SHEET Calculation : For HCl: (Strength of unknown HCl) V 1 N 1 = V 2 N 2 Volume of NaOH V 1 = --------- ml ( from graph ) Strength of NaOH N 1 = ---------- N Volume of HCl V 2 = ---------- ml Strength of HCl N 2 = [--------( V 1 ) X--------(N 1 )] / --------(V 2 ) Strength of unknown HCl N 2 = --------- N Amount of HCl present in the whole of the given solution = [---------------- (N 2 ) x ------------- (Eq.wt) x 50] / 1000 = ------------------- g (Equivalent Weight of HCl = 36.5) For CH 3 COOH: (Strength of unknown CH 3 COOH) Volume of NaOH V 3 = -------- ml (from graph) Strength of NaOH N 3 = ---------- N Volume of CH 3 COOH V 4 = --------- ml Strength of CH 3 COOH N 4 = [--------(V 3 ) X-------- (N 3 )] / --------(V 4 ) Strength of unknown CH 3 COOH N 4 = --------- N Amount of CH 3 COOH present in the whole of the given solution = [---------------- (N 4 ) x ------------- (Eq.wt) x 50] / 1000 = ------------------- g (Equivalent Weight of CH 3 COOH = 60.05) -18-
Procedure Prepare an exact N/10 Solution of sodium hydroxide. Rinse and fill the burette with the alkali solution. Now take 10 ml of the mixture of HCl and CH 3 COOH in a 200 ml beaker. Add about 100 ml of distilled water. Immerse the cell in this solution and determine the conductance of the solution. (The first end point will lie near about 5 ml, while the second will be near about 10 ml of sodium hydroxide solution). Now add 1mL, 2mL, 3mL and 4mL of NaOH from the burette and stir the contents of the beaker thoroughly. Note the conductance readings after each addition. Now, add NaOH at an increment of 0.2 ml up to 6 ml and then again between 9 ml and 11 ml. After adding 11 ml of NaOH solution, add 1mL in each addition till you have added 18 ml, note the conductance of the solution after each addition thoroughly stir the contents of the beaker. Result The strength of each acid in the given mixture is a. (i) Strength of hydrochloric acid = ---------------- N (ii) Amount of HCl present in the whole of the given solution = ---------------g b. (i) Strength of acetic acid = ---------------- N (ii) Amount of CH 3 COOH present in the whole of the given solution = ---------------g -19-
Observation WORK SHEET Sl.No. Vol.of Stock Solution Vol.of HNO 3 (ml ) Vol.of Potassium Thiocyanate (ml) Vol.of Distilled water (ml) Total Volume (ml) Sl.No. Concentration in ppm Absorbance at 480 nm -20-
Ex.No. Date: ESTIMATION OF FERRIC ION BY SPECTROMETRY Aim To estimate the amount of ferric ion present in the given sample using spectrophotometer. Principle When a monochromatic light passes through a homogeneous coloured solution, a portion of incident light is reflected, a portion is absorbed and the remaining is transmitted. I o = I r + I a + I t Where I o, I r, I a, It are the intensities of the incident, reflected, absorbed and transmitted light, respectively, I r is usually eliminated and hence, I o = I a + I t The mathematical statement of beer- Lambert s law is given by Log I o / I = ε Ct A = ε Ct Where, log I o /I = A I o = intensity of incident light I t = intensity of transmitted light C = Concentration of the solution in moles /L t = thickness of cell in cm ε = molar absorption coefficient A = absorbance or optical of solution density -21-
Graph WORK SHEET -22-
When a ray of monochromatic light passes through an absorbing medium, its intensity decreases exponentially as the concentration of absorbing substance and the length or thickness increase independently. Keeping the path length constant (t=1 cm), the variation is with reference to only concentration, C. Fe 3+ ion do not give any colour. However, it develops a red colour when it reacts with potassium thiocyanate solution. Fe 3+ + 6 KSCN [Fe (SCN) 6] 3- + 6K + Further, this colour is in the blue region (λ- 480 nm). Spectrophotometer has a wide range of adaptability that allows selection of monochromatic light of any wavelength in the visible spectrum. The light source is an ordinary light bulb and monochromatic light is obtained by using either a prism or a diffraction grating. The monochromatic light is then passed through the filter and is directed through a cell containing the sample. The light that penetrates hits the photoelectric cell and the output of this can be seen in the display. -23-
WORK SHEET -24-
Estimation of Ferric ion in the given sample: The given sample solution is taken in 100mL SMF and 3 ml of strong HNO 3 (1:1 V) and 5mL of potassium thiocyanate solution are added and made up to the mark using distilled water. The absorbance of the test solution can be calculated by performing the same procedure as standard. Result The amount of iron present in the given test sample = ------------ ppm -25-