Alkalinity is the buffering capacity of water — its ability to neutralise acids — arising from the presence of hydroxide (OH-), carbonate (CO32-), and bicarbonate (HCO3-) ions. This experiment uses the classical double-indicator method with standardised N/50 HCl as titrant. Phenolphthalein signals the end point when OH- is fully neutralised and CO32- is half-neutralised to HCO3- (volume Vp); methyl orange signals the end point when all remaining alkalinity is neutralised (total volume Vt). The relationship between Vp and Vt uniquely identifies which alkaline species are present across five possible scenarios. Total alkalinity, expressed as ppm CaCO3, equals V2 × 100 when N/50 HCl and 10 mL of sample are used. The engineering significance is substantial: very low alkalinity risks acid corrosion of metallic pipework, while high hydroxide alkalinity in boilers causes caustic embrittlement — a historically important cause of boiler explosions — where concentrated NaOH attacks grain boundaries in steel at stressed crevices.