The measurement of weakly basic substances requires the use of an indicator that will change color at pH levels of 3.7 To 4.5. The indicator commonly used in the past for such purposes was methyl orange. It does not yield well-definded color change that are easily detected by all people and is readily bleached by chlorine. Other indicators have been proposed as a substitute but most suffer from interference by carbon dioxide. This is particularly true of methyl red. “Standard Methods” now suggest as alternatives to methyl orange that bromphenol blue be used as apH 3. Indicator for acidity measurement and bromcresol green or mixed bromcresol green-methyl red be used as pH 4.5 indicator for total alkalinity measurements.The best substitute, however, is potentiometric measurement.