DPD is the most common reference method for free chlorine determination. This article explains how chloramines can interfere with DPD results and how to perform a simple field check.
Overview
- DPD is widely used, but not immune to water chemistry effects
- Ammonia and chloramines can change DPD readings over time
- Ambient light and atmospheric oxygen can also influence the final result
- Always follow your photometer and reagent manufacturer instructions
Why Chloramines Matter for DPD
When ammonia is present in chlorinated water, chloramines can form. Depending on reagent chemistry and timing, this can shift the measured DPD result.
Many DPD reagent systems include inhibitors to reduce chloramine response, but these effects can still appear if timing and handling are not controlled.
Simple Field Test (Repeat Read on the Same Sample)
This is a simple, practical check to see if your DPD free chlorine reading is drifting due to chloramine effects.
- Take a DPD measurement for free chlorine exactly as described in your manufacturer manual.
- Record the first reading.
- Keep the same sample vial and wait an additional 30 seconds.
- Press read again and record the value.
- Repeat steps 3-4 as needed to observe the trend.
If chloramines are influencing the measurement, the reading will typically increase over time. In that case, DPD may not deliver the accuracy required for tight free chlorine validation without additional controls.
Next Step
If you observe a rising trend, contact your reference measurement supplier for guidance on the correct method, timing, and reagent selection for your water matrix.
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