CHEM 120L: Acid-Base titration

acid strength to conjugate base strength

inversely related; strong acid → weak conjugate base

when do acid-base reactions go to completion

when at least one of the reactants is a strong acid or strong base

le chatelier principle

when a system at equilibrium is disturbed, it tries to counteract the disturbance

H3O+

hydronium, strong acid

reaction of NaOH with CH3COOH

net effect is conversion of CH3COOH → CH3COONa. happens indirectly via reaction of hydronium and hydroxide ions

endpoint

color of solution changes permanently

primary standard importance

• accuracy of analysis based on this therefore must be 100% pure, weighed and diluted precisely

• any error will affect accuracy of the entire analysis

what is the primary standard

• oxalic acid dihydrate, H2C2O4

• very pure, crystalline solid

• diprotic

• has known concentration, used to standardize NaOH

phenolphthalein color change occurs when the pH of the solution changes from….

8 - 10

phenolphthalein and its role in titration

• weak organic acid with different protonated - unprotonated form colours

• when weak acid + NaOH reaction nears completion, phenolphthalein gives its own H to replace those consumed by NaOH

equivalence point

acid and base in stoichiometric proportion, reaction complete, no excess reactant

monoprotic acid

donates 1 proton

diprotic acid

donates 2 protons

triprotic acid

donates 3 protons

standardization

determining concentration

why isnt NaOH a good primary standard

absorbed atmospheric water and CO2 changing its mass → must be standardized to be used in titration

how is NaOH standardized: primary standard prep

• titrated against oxalic acid dihydrate

• weigh amount of oxalic acid dihydrate needed → dissolve with DI in a beaker → put into volumetric flask → rinse beaker into flask → top up flask until bottom of meniscus is on the marking (last ½ mL added with a dropper)

• stopper volumetric flask and shake 5 minutes

how is NaOH standardized: titration step

• rinse burette 3x with little NaOH

• record initial NaOH reading to nearest 0.01mL

• measure 10.00mL acid into erlenmeyer, add indicator

• titrate with paper under; when nearing end point, rinse sides of flask with DI

• record final NaOH reading

when is titration at end point

faint pink persists for at least 30 seconds

titrating unknown acid

• weigh 3 samples by difference into 3 erlenmeyer flasks

• add DI and dissolve

• add indicator

• titrate against standardized NaOH

what to do if unknown acid doesn’t dissolve upon addition of DI

gently heat; if warming doesn’t dissolve everything, proceed with the titration since it should dissolve as titrated. make sure all acid is gone at the endpoint