Final answer:
A positive result for a mixed acid methyl red test is marked by the culture broth turning red after adding methyl red, indicating the presence of stable organic acids from fermentation. The red color signifies a pH lower than 4.
Step-by-step explanation:
A positive mixed acid methyl red test is indicated by a red color in the culture broth after the addition of the pH indicator methyl red. This outcome suggests that bacteria in the culture are performing mixed acid fermentation, which results in the production of stable organic acid end products, lowering the pH below 4. In contrast, when the bacteria produce less acidic metabolites like alcohols, the indicator will turn orange to yellow, denoting a negative result.
Using methyl red as an indicator in titrations involves understanding its pH range, which is centered around its pKin value of 5.0. Specifically, for a strong acid like HCl, the equivalence point is detected with a rapid color change while titrating with a strong base like 0.100 M NaOH, whereas for a weak acid like acetic acid, because the pKa is close to the pKin of methyl red, the color change occurs near the midpoint of the titration.
It is imperative to select the correct indicator based on the pH change interval of the titration. Methyl red, for instance, changes from red to yellow over a relatively large volume interval when used in a weak acid titration, which does not correspond well with the equivalence point. Indicators like phenolphthalein provide a more precise end point detection in such cases, demonstrating a sharp color change around the equivalence point.