B. Tech Sem 1 Gravimetric Chemical Analysis - Copy.pptx

Gravimetric Analysis, pH Metry, Single Beam Spectrophotometer, Potentiometer

1. GRAVIMETRIC CHEMICAL ANALYSIS By Dr. Rai D hirendra Prasad Bihar Veterinary College, Patna

2. Contents Introduction Basic Principle Types of Gravimetric Methods Steps in Precipitation Gravimetric Analysis Requirement of Good Gravimetric Method Sources of Error Advantage of Gravimetric Analysis Limitations Applications 2 27-10-2025

3. Introduction Gravimetric analysis is one of the oldest and most fundamental techniques in analytical chemistry. It is based on the principle of measuring the mass of a substance to determine the amount of a specific analyte present in a sample . Despite the development of advanced instrumental methods, gravimetric techniques remain essential because of their accuracy, simplicity, and independence from calibration standards. The term gravimetric comes from the Latin word gravitas , meaning " weight.’’ The quantity of an analyte is determined by converting it into a pure, stable, and weighable form , and then measuring its mass with a high-precision balance. 3 27-10-2025

4. Introduction… * In gravimetric methods, the analyte is separated from a complex mixture through a series of chemical transformations , ultimately converted into a pure, stable, and well-defined compound suitable for accurate weighing . As the final result is derived from direct mass measurement , gravimetric analysis does not depend on calibration standards or sophisticated instrumentation, making it a primary or absolute method of analysis. When carefully executed, the method can provide accuracy up to 0.1% or better , making it invaluable for preparing primary standard substances and establishing reference values in analytical chemistry. 4 27-10-2025

5. Basic Principle of Gravimetric Analysis The principle of gravimetric analysis is straightforward : The mass of a known compound containing the analyte in a known stoichiometric ratio is measured, and from this, the amount of the analyte can be calculated . The general equation for gravimetric determination is : Mass of  analyte = Mass  of compound  obtained × Molar  mass of  analyte / Molar mass of  compound. ​ For example, if chloride ions (Cl⁻) are determined as silver chloride ( AgCl ), the mass of AgCl is measured and used to calculate the amount of chloride in the sample. 27-10-2025 5

6. Types of Gravimetric Methods Gravimetric analysis can be broadly divided into four categories : (1) Precipitation Gravimetry : The analyte is precipitated as an insoluble compound, filtered, washed, dried or ignited, and weighed. Example: Determination of sulfate as barium sulfate ( BaSO ₄ ). (2) Volatilization Gravimetry : The analyte or its decomposition product is volatilized and collected or lost, and the change in mass is measured. Example: Determination of water by drying or carbon dioxide by decomposition of calcium carbonate. 27-10-2025 6

7. Types of Gravimetric Methods (3) Electrogravimetry : The analyte is deposited electrolytically on an electrode, which is then weighed. Example: Determination of copper by deposition on a platinum electrode. ( 4) Thermogravimetry (TG ): The mass change of a substance is measured continuously as a function of temperature or time under controlled heating conditions. Example: Thermal decomposition studies of metal oxides or hydrates. 27-10-2025 7

8. Steps in Precipitation Gravimetric Analysis Precipitation gravimetry, the most common type. It involves the conversion of an analyte into an insoluble precipitate, which is then isolated, purified, and weighed to determine the analyte quantity. It involves the following steps : (1) Precipitation: This is the first and most crucial step in gravimetric analysis . Process: The analyte in solution is converted into a sparingly soluble compound by adding a suitable precipitating reagent. 27-10-2025 8

9. Steps in Precipitation Gravimetric Analysis The reaction should proceed completely and under conditions that favor the formation of large, pure, crystalline particles instead of fine colloidal ones . The process is often controlled by adjusting pH, temperature, concentration, and rate of reagent addition . Example: To determine chloride ions: Ag++Cl−→ AgCl (s) The silver chloride precipitate forms as fine white crystals . Important Considerations The precipitating reagent should react selectively with the analyte , producing a compound that is insoluble, pure, and stable. 27-10-2025 9

10. Steps in Precipitation Gravimetric Analysis The solution should be dilute to prevent local super saturation and formation of colloidal precipitates . The reagent should be added slowly with continuous stirring to allow even distribution and controlled crystal growth . Temperature should often be slightly elevated to increase particle size and promote equilibrium precipitation . The pH of the solution must be carefully adjusted since solubility often depends on hydrogen ion concentration . Objectives of Precipitation Step: Achieve complete reaction between analyte and reagent. 27-10-2025 10

11. Steps in Precipitation Gravimetric Analysis Obtain a precipitate that is easily filterable . Minimize co-precipitation and adsorption of impurities . 2. Digestion: After precipitation, the suspension is allowed to stand — usually in a hot mother liquor — for a period known as digestion . Purpose: To improve the crystal size, density, and purity of the precipitate . During digestion, small particles dissolve and re-deposit onto larger ones (a process known as Ostwald ripening). 27-10-2025 11

12. Steps in Precipitation Gravimetric Analysis Mechanism: Small particles, having higher solubility due to surface energy, dissolve slightly . The dissolved ions reprecipitate on larger crystals . This results in fewer, larger, and more uniform particles that can be filtered and washed easily . Conditions for Effective Digestion: Maintain the mixture at a temperature just below boiling . Allow sufficient time (30 minutes to several hours) for recrystallization . Avoid strong agitation, which can break crystals. 27-10-2025 12

13. Steps in Precipitation Gravimetric Analysis Advantages: Reduces the adsorption of impurities . Improves filterability and washing efficiency . Produces purer and denser crystals . 3. Filtration: Once the precipitate has matured, it is separated from the liquid phase . Purpose To isolate the solid product from the surrounding solution without loss of material. Methods: 27-10-2025 13

14. Steps in Precipitation Gravimetric Analysis Filter paper ( ashless or quantitative grade) or Sintered-glass crucibles (porosity grades 1–5 depending on particle size ). Procedure: Decant the supernatant liquid carefully through the filter before transferring the main precipitate . Use a wash bottle to transfer all particles quantitatively . Rinse the beaker and stirring rod to ensure complete transfer . Precautions: Never overfill the filter — filtration should be slow and controlled. 27-10-2025 14

15. Steps in Precipitation Gravimetric Analysis Ensure that no particles escape; otherwise, analytical results will be inaccurate . If filter paper is used, it should be ashless so it leaves no residue on ignition . 4. Washing of the Precipitate: The freshly filtered precipitate often contains impurities trapped or adsorbed on its surface, such as electrolytes or mother liquor residues . Purpose To remove adsorbed and occluded impurities without causing loss of the precipitate. Washing Medium: 27-10-2025 15

16. Steps in Precipitation Gravimetric Analysis Usually distilled water or a dilute electrolyte (like 0.1% HNO₃ or NH₄NO₃) to prevent peptization . In some cases, volatile solvents (like ethanol or acetone) are used to facilitate drying . Technique: Add small portions of washing liquid to the precipitate in the filter . Allow each portion to drain completely before adding the next . Continue washing until tests (e.g., for Cl⁻ using AgNO ₃) show that filtrate is free of impurities . Precaution: 27-10-2025 16

17. Steps in Precipitation Gravimetric Analysis Avoid excessive washing, which can dissolve part of the precipitate . Use minimal but sufficient washing volume . 5. Drying or Ignition: After washing, the precipitate must be converted into a pure, stable, and weighable form . Drying: Carried out at 100–150°C in an oven to remove moisture and volatile impurities . Suitable for compounds that are thermally stable at moderate temperatures. 27-10-2025 17

18. Steps in Precipitation Gravimetric Analysis Ignition: Used when the precipitate must be converted into an oxide or other stable compound by strong heating . Conducted in a muffle furnace at 500–1000°C, depending on the substance . Example: Calcium is precipitated as calcium oxalate, which upon ignition gives calcium oxide : CaC2​O4​⋅H2​ O ―― CaO+CO+CO2​ Precautions: Avoid excessive heating that might decompose the compound or cause volatilization. 27-10-2025 18

19. Steps in Precipitation Gravimetric Analysis Cool the crucible in a desiccator before weighing to prevent moisture absorption . 6. Weighing: The final step involves determining the mass of the pure, dried or ignited precipitate . Procedure: The cooled crucible (with precipitate) is weighed on an analytical balance . Reheat, cool, and reweigh until constant mass is obtained — confirming that drying/ignition is complete. 27-10-2025 19

20. Steps in Precipitation Gravimetric Analysis Step Process Main Objective 1. Precipitation Formation of insoluble compound Convert analyte into weighable form 2. Digestion Standing in hot solution Improve particle size and purity 3. Filtration Separation of solid Isolate precipitate 4. Washing Rinse precipitate Remove impurities 5. Drying/Ignition Remove water and convert to stable form Obtain constant composition 6. Weighing Accurate mass measurement Quantitative determination 27-10-2025 20

21. Steps in Precipitation Gravimetric Analysis Precipitation gravimetry is a precision-based technique . The reliability of the results depends largely on the careful execution of each step — especially the control of conditions during precipitation and digestion. A well-crystallized, pure, and stable precipitate ensures accurate gravimetric determination and high analytical confidence. 27-10-2025 21

22. pH Metry pH metry is an analytical technique used to measure the hydrogen ion concentration (H⁺) of a solution, expressed as pH . It provides a quantitative measure of acidity or alkalinity of a solution using a pH meter, which detects the potential difference between a reference electrode and a glass electrode sensitive to hydrogen ions . Principle of pH Metry : pH metry is based on the Nernst equation, which relates the electrode potential to the hydrogen ion activity in a solution . E=E0 −2.303RT​/ F log[H+] 27-10-2025 22

23. pH Metry Where : E = measured electrode potential E0 = standard electrode potential R= Universal Gas Constant 8.314 J mol⁻¹ K⁻ ¹ T= Temperature in K F= Faraday Constant 96485 C mol⁻ ¹ [H+] = hydrogen ion concentration Since pH = –log [H⁺ ] The potential difference corresponds directly to the pH of solution. 27-10-2025 23

24. pH Metry Instrumentation: It consists of : (1) Measuring electrode: Sensitive to H ions (2) Reference Electrode: Ag/ AgCl electrode (3) pH meter/ Voltmeter Measures potential difference a And converts into pH 27-10-2025 24

25. pH Metry Working: (1) The glass electrode and reference electrodes are dipped into the test solution. (2) The potential difference between the two electrode is measured. (3) The pH meter translates the potential into pH value using Nernst equation. (4) Calibration is done using standard buffer solution. (usually pH 4.00. 7.00, 9.4) Applications: (1) Laboratory Analysis: Determination of pH of solution acids and base. 27-10-2025 25

26. pH Metry Monitoring reaction progress and equilibrium. (2) Industrial Applications: Quality control in food, beverages, pharmaceuticals and cosmetics. Water treatment and waste water monitoring. (3) Environmental Studies: Measurement of soil and water pH. Monitoring pollution level. (4) Medical Applications: Gastric pH meter, oesophegal pH meter. 27-10-2025 26

27. pH Metry Blood and urine analysis. Advantage: Highly accurate and precise. Quick and simple method. Can measure pH continuously. Requires only small sample volume. Limitations: Requires frequent calibration. Electrode is sensitive to contamination and temperature. 27-10-2025 27

28. pH Metry Glass electrode can not be used in non-aqueous or strongly dehydrating solvent. Avoid touching the membrane bulb of glass electrode. Regularly check and recalibrate the meter. 27-10-2025 28

29. Single Beam spectrophotometer A single beam spectrophotometer is an optical instrument used to measure the absorbance or transmittance of light by a sample at a specific wavelength . It operates by passing a beam of light through a sample and measuring how much light is absorbed compared to the incident light . It is called single beam because the same beam of light is used for both the blank (reference) and the sample measurements — one after the other, not simultaneously . Principle: The instrument works on Beer–Lambert’s Law, which relates absorbance to the concentration of the absorbing species in the solution. 27-10-2025 29

30. Single Beam spectrophotometer When a beam of monochromatic light is passed through homogeneous absorbing medium , then the rate of decrease in intensity of light with the thickness of absorbing medium is directly proportional to the intensity of light as well as concentration of solution. A= ε cl Where, A= Absorbance Ε = molar absorptivity in L mol⁻¹ cm⁻ ¹ C = concentration of solution L = path length of cuvette 27-10-2025 30

31. Single Beam spectrophotometer As the concentration increases absorbance increases linearly. Construction and Components: 27-10-2025 31

32. Single Beam spectrophotometer (1) Light Source: Provides continuous radiation Tungsten lamp → visible region (350–800 nm ) Deuterium or hydrogen lamp → UV region (190–350 nm ) (2) Monochromator : Separates polychromatic light into monochromatic (single wavelength) light . Usually a prism or diffraction grating . (3) Wavelength Selector: Selects the desired wavelength and pass through the sample. 27-10-2025 32

33. Single Beam spectrophotometer (4) Sample Holder: Holds the sample solution. Made of quartz or glass. (5) Detector: Converts the transmitted light into electrical signal. Common Detector: Photodiode, Photomultiplier tube (PMT ). (6) Read out Device: Displays absorbance or transmittance digitally or on a meter scale. 27-10-2025 33

34. Single Beam spectrophotometer Working: The instrument is first zeroed using blank solution. Light passes through the monochromator , selecting the desired wavelength. The beam passes through the sample solution in covette . The detector measures the intensity of transmitted light. Absorbance is calculated using formula: A=log10​( Io/I​​ ) 27-10-2025 34

35. Single Beam spectrophotometer Where, Io = Intensity of incident light. I = Intensity of transmitted light. Advantage: Simple design and easy to operate. Lower cost compared to double beam instrument. Suitable for routine absorbance measurements. Compact and portable. 27-10-2025 35

36. Single Beam spectrophotometer Disadvantages: Blank and sample can not be measured simultaneously leading to possible drift error. Less stable over time due to lamp fluctuation. Requires recalibration frequently. Not suitable for kinetic studies or long term monitoring. Applications: Quantitative Analysis: Determination of concentration of colored solution. Enzyme Kinetics: Measurements of reaction rate. 27-10-2025 36

37. Single Beam spectrophotometer Quality Control: Analysis of dyes, drugs, chemicals. Environmental Studies: Water and soil analysis. Biochemical Assay: Protein and nucleic acid quantification. Precautions: Always use clean, scratch free cuvette. Handle cuvette only by frosted side. Avoid bubbles in the solution. Warm up the instrument before use. Regularly check lamp alignment and wavelength calibration. 27-10-2025 37

38. Potentiometer A potentiometer is an instrument used to measure the potential difference (voltage) between two points in an electrical circuit accurately, without drawing any current from the circuit . It is based on the principle of null deflection, meaning that measurement is made when no current flows through the galvanometer . In chemistry and physics laboratories, potentiometers are also used for electromotive force (EMF) measurement, pH determination, titrations, and standardization of cells . Principle : The potentiometer works on the principle that the potential drop across a uniform wire is directly proportional to its length, provided the current through the wire is constant. 27-10-2025 38

39. Potentiometer V∝ L V= kL Where, V= potential difference L= Length of potentiometer wire. K = Potential gradient. At balance point ( Null point): E1​= kL And for another cell, 27-10-2025 39

40. Potentiometer E2​=kL2 ​ So, E1/​E2​​ = L1/L1​2 This allows accurate comparison of EMFs or measurement of an unknown voltage . Construction: (1) Uniform Resistive Wire: Made of manganin or constantan , usually 1–10 m long . Mounted on a wooden or metallic base. 27-10-2025 40

41. Potentiometer (2) Power Supply/ Driving Cell: Provides steady current through the wire. (3) Rheostat: Controls and maintains constant current in the potentiometer wire. (4) Galvanometer: Sensitive current detecting instrument to identify the null point. (5) Jockey (Sliding contact): A moveable contact that slides along the wire to find the balance point. (6) Key and Connecting Wire: 27-10-2025 41

42. Potentiometer Used for completing or breaking the circuit connection. Working: A constant current is passed through the potentiometer wire using the driving cell and rheostat. The positive terminal of the standard cell is connected to the same end of the potentiometer wire as the positive of the driving cell. The Galvanometer and Jockey is used to locate the point along the wire 27-10-2025 42

43. Potentiometer 27-10-2025 43

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