Chapter 24. General Chemistry Experiment
Recommended Post: 【Chemistry】 Chemistry Index
1. Experimental Apparatus
2. Experimental Equipment
⑴ Atomic Force Microscope (AFM)
① Capable of measuring down to about 1/10–1/100 of an atomic diameter.
② Usable in air (operable under ambient conditions).
⑵ Elemental Analyzer
① Determines the percentage amounts of C, H, N, S, and O.
② An accurately weighed sample is combusted in the presence of O₂, ionizing the constituent elements (C, H, N, S, O).
③ In the oxidation reactor, products are oxidized to H2O, CO, CO2, N2, NO, NO2, SO2, and SO3.
④ In the reduction reactor, NO and NO2 are reduced to N2, SO3 to SO2, and CO is re-oxidized to CO2.
⑤ For oxygen determination, the sample undergoes pyrolysis under oxygen-free conditions, and the amount of CO2 ultimately produced is measured.
⑥ Other generated gases are separated using an adsorption trap.
⑦ The final gases are quantified by gas chromatography (GC) with helium as the carrier gas to calculate each content.
⑶ Energy Dispersive X-ray Spectroscopy (EDS)
① Utilizes characteristic x-rays
② Characteristic X-ray: X-rays are generated when an electron from the K-shell is ejected by ionizing radiation, and an electron from the L-shell moves to the K-shell.
Figure 1. The Principle of Characteristic X-rays
○ K: Electron shell with n = 1.
○ L: Electron shell with n = 2.
○ M: Electron shell with n = 3.
③ The term “characteristic” is used to indicate that a common peak is observed regardless of the type of scintillating material.
④ These X-rays can be applied to Single Photon Emission Computed Tomography (SPECT).
⑷ Gas Chromatography (GC)
① Definition: The method in which the evaporated sample components are separated by being distributed between the stationary phase in the column and the mobile gas phase.
② Inert gases like helium, hydrogen, nitrogen, argon are commonly used as carrier gases. Helium is mainly used as the gas phase.
○ Reason 1: Lighter gas molecules like H2, He have higher diffusion coefficients, leading to faster separation
○ Reason 2: Lighter gas molecules like H2, He have higher thermal conductivity, reducing preheating time
○ Reason 3: Gases like H2, O2 can react with the sample
③ Advantages: High resolution
④ Disadvantages: Analyzing non-volatile compounds is challenging.
○ Suitable for small molecular weights (typically 100 - 103.
○ In general, gas chromatography becomes difficult to perform for compounds with molecular weights above 500.
⑤ Application 1: Gas-Liquid Chromatography (GLC) and Gas-Solid Chromatography (GSC)
⑥ Application 2: Measuring molecular weights through gas chromatography: using correlation between retention time and molecular weight
⑦ Application 3: Gas Chromatography Mass Spectrography (GC-MS)
○ 1st Step: Separated substances from gas chromatography are electronically ionized or chemically ionized for mass-dependent classification
○ 2nd Step: Separated substances form unique mass spectra
○ 3rd Step: Structure information is obtained by comparing with accumulated library data or quantitative analysis
○ Can also be used for quantifying trace components by selecting 1st ions and performing 2nd ionization
⑸ High-Performance Liquid Chromatography (HPLC)
① Measures presence and molecular weight of trace substances using the difference in movement distance on a stationary phase in chromatography
② Molecular weight conditions: 6 × 101 ~ 104
③ Comprises solvent, pump, injector, column, detector, recorder
④ Most common detector in HPLC is UV detector
⑤ Refractive Index (RI) detector is sensitive to environmental changes like pressure and temperature, but not very sensitive to solutes
⑹ Inductively Coupled Plasma (ICP)
① ICP: Introduces the sample into argon plasma formed by high-frequency induction coil, measures emitted spectral lines and intensity at 6000 ~ 8000 K when atoms in excited state move to ground state
② Excited electrons move to ground state within 10-7 ~ 10-8 seconds
③ The sample emits light corresponding to the energy difference, which is then spectrally processed using a diffraction grating spectrometer, and the intensity of light is measured for each wavelength.
④ Application 1: ICP-MS
⑺ Scanning Electron Microscope (SEM)
① Mainly obtains surface information of samples
② Depth of focus is over twice that of optical microscopes
⑻ Vibrating Sample Magnetometer (VSM)
① Principle: Magnetic Hysteresis
⑼ X-ray Diffraction (XRD)
① Principle: Bragg Diffraction
⑽ X-ray Photoelectron Spectroscopy (XPS)
① Principle: Photoelectric Effect
3. Experimental Reagents
4. Experimental Techniques
⑴ Fundamentals of Chemical Experiments
① Experiments with toxic gases are conducted in a hood
② Dilute sulfuric acid is prepared by adding water to a beaker before adding concentrated sulfuric acid
③ When transferring concentrated hydrochloric acid, use a glass rod to guide the acid along the beaker wall
④ Due to sodium hydroxide’s high moisture absorption, accurate weighing is challenging
⑤ The produced NaOH solution is not stored in glass containers for long periods
⑥ If a mercury thermometer breaks, sprinkle sufficient sulfur to absorb the mercury vapor and collect it after a day
⑵ Titration and Standardization of Solutions
① Titration: an experimental method for determining the amount of an unknown analyte
○ Buret: an apparatus for delivering/measuring volume
○ Using a buret, add a solution of known concentration until the sample is completely consumed.
○ Use reactions that proceed rapidly and go to completion upon addition.
② Standardization
○ Primary standard: a substance that can be used to prepare a solution of accurately known concentration
○ Condition 1. Must be obtainable in a pure form (definite composition).
○ Condition 2. Should be easy to purify.
○ Condition 3. Should have a high molar mass to minimize relative weighing error.
○ Standard solution: a solution, standardized with a primary standard, suitable for use in titrations.
○ Standardization: the process of accurately determining the concentration of a titrant using an aqueous solution of a primary standard
○ Example: Use an aqueous solution of potassium hydrogen phthalate (KHP) as a primary standard to determine the concentration of NaOH(aq); in this case, NaOH is the standard solution.
⑶ Direct and Indirect Titration
① Direct Titration: Method of directly determining the amount of analyte
○ Condition 1: Rapid reaction between analyte and standard solution
○ Condition 2: Complete reaction
○ Condition 3: Suitable indicator available at equivalence point
○ Example 1: Measuring the concentration of acetic acid using NaOH standard solution
CH3COOH + OH- → CH3COO- + H2O
○ Example 2: Measuring the concentration of Cl- using AgNO3 standard solution
Ag+ + Cl- → AgCl
② Indirect Titration: Used when direct titration conditions are not met
○ Substitution titration: a method in which a solution that reacts completely with the analyte is added in excess, and the resulting product is then titrated.
○ Example: Quantification of metal cations using EDTA
○ Step 1: Add magnesium’s EDTA chelate compound (MgY2-) to a solution containing metal cation Mn+
MgY2- + Mn+ → MYn-4 + Mg2+
○ Step 2: Titrate the resulting excess Mg2+ with EDTA
○ Eriochrome Black T or Calmagite can be used as indicators
○ The molar amount of Mg2+ measured in Step 2 equals the molar amount of Mn+ present in the original solution
○ Back Titration: Adding excess standard solution and titrating the remaining substance
○ Used when direct reaction between analyte and standard solution is slow
○ Example 1: Quantification of Ni2+ using back titration
○ Step 1: Add excess EDTA standard solution to the sample solution containing Ni2+ ions
Ni2+ + Y4- → NiY2-
○ Step 2: After the reaction is complete, titrate the remaining EDTA with Mg2+ to determine its concentration.
Mg2+ + Y4- → MgY2-
○ Step 3: Mg2+ forms a complex with Eriochrome Black T indicator, changing color from blue to violet
○ The moles of Ni2+ in the initial solution are given by the moles of EDTA added in Step 1 minus the moles of EDTA measured in Step 2 (i.e., Step 1 − Step 2).
○ Example 2: DNP Method (2,4-dinitrophenyl hydrazine colorimetric method)
○ Step 1: Under sulfuric acid, oxidation substance and DNP undergo dehydration reaction to form bi-2,4-dinitrophenyl hydrazone
○ Step 2: By colorimetrically quantifying the orange-red bis(2,4-dinitrophenyl)hydrazone, the oxidized form of the substance can be determined.
⑷ Removal of Interfering Substances
① Masking Agent: Substance that prevents unwanted reactions (side reactions) from occurring
② Enable a reaction that reacts with—and removes—the species participating in side reactions.
○ Example: Quantification of copper in steel samples
○ Main reaction for quantification: Cu2+ + 2I- → Cu + I2
○ Undesirable reaction: 2Fe3+ + 6I- → 2Fe + 3I2
○ Masking agent: Adding F- forms FeF3(s) via Fe3+ + 3 F- → FeF3(s), thereby preventing side reactions.
③ Separation by precipitation
○ Precipitate the species involved in side reactions and remove it by filtration.
④ Separation by acidity (pH) control
○ The solubility of hydroxides and acids depends on the solution pH.
○ By adjusting pH, dissolve (or keep dissolved) specific hydroxides to achieve separation.
⑤ Separation by solvent extraction
○ Chemical species distribute differently between two immiscible solvents.
○ The target substance can be extracted into one of the solvents.
Input: 2019-09-08 23:51
Modified: 2022-07-06 17:46