Questions regarding the analysis of alkalinity in circulating water
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The indicators used for alkalinity analysis are phenolphthalein and methyl orange. When using methyl orange to analyze total alkalinity, the color does not reach the endpoint and the indicator’s color fades; the orange-red color at the endpoint is not visible. What is the reason for this?
This post was last edited by qugd on 2020-5-14 10:41. This situation may be related to the operator’s experience and proficiency. 1. There is a lack of experience in observing the color change of the indicator at the endpoint, as the color change of methyl orange is not particularly easy to distinguish; in such cases, it is best to first prepare a control sample that is titrated to the endpoint, in order to enhance the observation skills and sensory impressions ； Additionally, if conditions permit, methyl red can be used in place of methyl orange, or an appropriate mixed indicator can be selected. 2. If active chlorine is present in the sample, its oxidizing power is low in alkaline conditions; as the pH value decreases ( acidity increases), its oxidizing power increases, causing the indicator to fade. In this case, a small amount of reducing agent can be added to eliminate the interference of active chlorine; at the same time, a blank sample for comparison can be used to account for any effects that the reducing agent might have. 3. A pH meter is used to monitor the endpoint, with the first or second derivative of the titration curve during the titration process being utilized to determine it; this eliminates interfering factors in the sample, ensuring accurate and reliable measurement results
Okay, I understand. It’s explained in great detail, thank you