wiki / chemistry

analytical

MS, IR & NMR, uncertainty & data, separation techniques, qualitative tests, green chemistry.

spectroscopy · uncertainty · graphs & data · separation · qualitative tests · safety & green chemistry

Spectroscopy

Mass spectrometry. Molecular ion \(M^{+\bullet}\) gives \(M_r\); base peak = most intense; fragments identify groups. Isotope patterns: Cl gives M : M+2 ≈ 3:1; Br gives 1:1.

IR stretches.

¹H NMR. Number of signals = number of proton environments; integration = ratio of protons; TMS at \(\delta=0\). Splitting: \(n+1\) from adjacent equivalent H; singlet, doublet, triplet, quartet; equivalent H do not split each other. Traps: read the integration ratio first; splitting comes from adjacent non-equivalent H; OH/NH are often broad and may not split.

Structure determination. Combine MS + IR + NMR.

Uncertainty

Errors. Random error affects precision; systematic error affects accuracy.

Quoting. Analogue: ± half the smallest division; digital: ± the smallest increment unless specified; repeated readings: range/2 or SD/SEM as appropriate.

Propagation. Addition/subtraction: absolute uncertainties add — \(y=a\pm b\Rightarrow\Delta y=\Delta a+\Delta b\). Multiplication/division: fractional (percentage) uncertainties add — \(y=ab/c\Rightarrow\frac{\Delta y}{y}=\frac{\Delta a}{a}+\frac{\Delta b}{b}+\frac{\Delta c}{c}\); powers: \(y=a^n\Rightarrow\frac{\Delta y}{y}=|n|\frac{\Delta a}{a}\). Percentage uncertainty \(=\frac{\Delta x}{x}\times100\).

Burette trap. A titre uses initial + final readings, so their uncertainties add: ±0.05 cm³ each ⇒ ±0.10 cm³ per titre.

Significant figures. Final answer to the least precise input; keep guard digits during calculation.

Graphs & data

Axes. Independent variable on x, dependent on y; include units and uncertainty bars when significant; gradient units = y/x; intercept carries y units.

Calibration. Use a line of best fit; read concentration by interpolation. Outliers require justification, not deletion by preference.

Linearization. Beer–Lambert \(A=\varepsilon lc\); first-order kinetics \(\ln[A]\) vs \(t\); Arrhenius \(\ln k\) vs \(1/T\).

Separation & analysis

Chromatography. Separates by different attractions to the mobile/stationary phases; \(R_f=\dfrac{\text{distance moved by spot}}{\text{distance moved by solvent front}}\); higher affinity for the stationary phase ⇒ lower \(R_f\); TLC visualized by UV/iodine/stain.

Distillation & purification. Distillation separates by boiling point; fractional distillation for close boiling points; reflux heats without loss; recrystallization purifies solids; vacuum filtration isolates crystals; a drying agent removes water.

Titration technique. Rinse the pipette with the analyte and the burette with the titrant; the conical flask can be rinsed with water; concordant titres typically within 0.10 cm³.

Qualitative tests

Unsaturation. Bromine water orange → colorless; cold dilute alkaline \(\ce{KMnO4}\) purple → brown/colorless.

Carbonyl. 2,4-DNPH orange precipitate for aldehyde/ketone; Tollens/Fehling positive for aldehyde only.

Carboxylic acid. Effervescence with carbonate; the \(\ce{CO2}\) turns limewater milky.

Halides. \(\ce{AgNO3}\) after acidifying with \(\ce{HNO3}\): AgCl white, dissolves in dilute \(\ce{NH3}\); AgBr cream, dissolves in concentrated \(\ce{NH3}\); AgI yellow, insoluble.

Other ions. Sulfate: \(\ce{Ba^2+}\) gives white \(\ce{BaSO4}\). Carbonate: acid gives \(\ce{CO2}\). Ammonium: warming with NaOH gives \(\ce{NH3}\); damp red litmus turns blue.

Safety & green chemistry

Green principles. Minimize toxicity, waste, energy; maximize atom economy, yield, selectivity; renewable feedstocks; catalysis over stoichiometric reagents; safer solvents; degradable products.

Safety. Risk = hazard × exposure; use PPE, fume hood, small scale; add acid to water; dispose of heavy metals and halogenated organics separately.