Week 4 Chem

From Gravimetry to Precipitation Titrimetry

• Precipitation gravimetry: form a precipitate and weigh the solid to determine the analyte.

• Precipitation titrimetry: measure the volume of titrant needed to react with the analyte and form a precipitate.

• Same precipitation chemistry, but a different measurement:

  • Gravimetry → mass of precipitate

  • Precipitation titration → volume of titrant

• A standard solution (titrant) of known concentration is added from a burette to the unknown solution.

• Titrant is added until the endpoint is reached.

• Endpoint can be detected by an indicator or coloured precipitate.

• Precipitation titrations need a precipitate that forms quickly enough for practical analysis.

• Precipitation titrimetry is faster and better for routine analysis, while gravimetry is very accurate but more time-consuming. Week 5 Precipitation Titrimetry…

Example: chloride + AgNO₃

\[ AgNO_3 \rightarrow Ag^+ + NO_3^- \]\[ \boxed{Ag^+ + Cl^- \rightarrow AgCl(s)} \]

• Ag⁺ reacts with Cl⁻ to make the AgCl precipitate.

• NO₃⁻ is a spectator ion.

• Ag⁺ : Cl⁻ is 1:1, so moles Ag⁺ needed to react with all Cl⁻ = original moles Cl⁻.

🧠 Memory line

Gravimetry = weigh the precipitate ⚖
Precipitation titrimetry = measure titrant volume 🧪

Effects of Acidity on the Solubility of Precipitates

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Alpha Fractions (\(\alpha\))

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Effects of Acidity on the Solubility of Precipitates — Notes

Effects of Acidity on the Solubility of Precipitates — Notes

• Calcium oxalate (CaC₂O₄) is sparingly soluble, meaning only a small amount dissolves in water.

• When it dissolves, it separates into calcium ions (Ca²⁺) and oxalate ions (C₂O₄²⁻).

• Ksp = 2.6 × 10⁻⁹ describes the solubility equilibrium.

• Adding a strong acid increases H⁺ concentration and lowers the pH.

• H⁺ attaches to free oxalate, converting it first into HC₂O₄⁻ and then H₂C₂O₄.

• This reduces the amount of free oxalate in solution, causing more calcium oxalate solid to dissolve to restore equilibrium.

Three Forms of Oxalate

• H₂C₂O₄: Has two H⁺ attached (α₀).

• HC₂O₄⁻: Has one H⁺ attached (α₁).

• C₂O₄²⁻: Has no H⁺ attached (α₂).

• OxT represents the total concentration of all three oxalate forms.

• Alpha (α) represents the fraction of total oxalate present in each form.

• All three fractions add up to 1.

Effect of pH

• Low pH: More H⁺, less free oxalate, and greater calcium oxalate solubility.

• High pH: Less H⁺, more free oxalate, and lower calcium oxalate solubility.

Remember: Lower pH → More H⁺ → Less free oxalate → More solid dissolves.

Effects of Acidity on the Solubility of Precipitates — Notes

• Solubility (s) is the total amount of calcium oxalate that dissolves in water.

• When CaC₂O₄ dissolves, it releases equal amounts of calcium and oxalate, so s = [Ca²⁺] = OxT.

• OxT represents the total concentration of all three dissolved oxalate forms (H₂C₂O₄, HC₂O₄⁻, and C₂O₄²⁻).

• α₂ represents the fraction of total oxalate that is free C₂O₄²⁻.

• To find the concentration of free oxalate, multiply OxT by α₂.

• The Ksp expression only includes free calcium and free oxalate ions, so we replace the free oxalate concentration with OxT × α₂.

• Ka₁ and Ka₂ are acid dissociation constants that help determine how much oxalate is in its free form at a certain pH.

• The conditional solubility product (K′sp) accounts for how acidity affects solubility.

• When pH decreases, more H⁺ attaches to oxalate, reducing the amount of free oxalate and making α₂ smaller.

• As α₂ decreases, K′sp increases, meaning more calcium oxalate can dissolve.

Effects of Complexation on the Solubility of Precipitates.

• Complexation occurs when a ligand attaches to a metal ion, forming a complex.

• Complexing agents compete for free metal ions, while acids can react with anions.

• AgBr dissolves slightly, releasing Ag⁺ and Br⁻.

• When NH₃ is added, it binds to Ag⁺ and forms silver-ammonia complexes.

• This decreases free Ag⁺, causing more AgBr to dissolve to restore equilibrium.

• AgT represents the total concentration of dissolved silver in all forms.

• αM represents the fraction of total silver present as free Ag⁺.

• The Ksp expression can be rewritten using AgT and αM to account for complexation.

Remember: More ligand → Less free metal ion → More precipitate dissolves.

Conditional Solubility Product (AgBr + NH₃)

• Solubility (s) = the amount of AgBr solid that dissolves in water.

• When AgBr dissolves, it releases equal amounts of Ag⁺ and Br⁻.

• AgT = total dissolved silver, including free Ag⁺ and silver attached to NH₃.

• Therefore, s = [Br⁻] = AgT.

• αM = the fraction of total dissolved silver that is free Ag⁺.

• Formula: αM = [Ag⁺] / AgT.

• Ksp only includes free Ag⁺, not silver attached to NH₃.

• Original formula: Ksp = [Ag⁺][Br⁻].

• With complexation: Ksp = AgT × αM × [Br⁻].

• Conditional Ksp (K′sp) accounts for the effect of ammonia on solubility.

• Formula: K′sp = Ksp / αM = s².

• More NH₃ → less free Ag⁺ → smaller αM → larger K′sp → more AgBr dissolves.