Why Temperature Matters In Photometric Water Testing

emperature: An Overlooked Variable In Photometric Testing

When two tests on the same water sample produce different results, temperature may be one of the variables responsible.

Temperature can influence the chemical reactions behind colorimetric testing, affecting how quickly color develops and, ultimately, the result reported by the instrument. For temperature-sensitive methods, even a relatively small difference between testing conditions can introduce measurable bias.

Why Temperature Matters

Many photometric methods are developed under controlled conditions. Hach vial tests and photometers, for example, are calibrated around a 20°C (68°F) reference temperature.

When testing conditions move away from that reference point, reaction behavior can change. A colder sample may react more slowly, while a warmer sample may react more quickly. The exact impact varies by method, which makes following method-specific temperature requirements important.

Ammonia testing provides a useful example. In Hach testing of three TNTplus ammonia vial tests, results at 15°C showed negative deviations ranging from 13% to 20% compared with results at 20°C. At 30°C, those same methods showed positive deviations between approximately 6% and 8%.

For laboratories making process or compliance decisions from those results, that difference can matter.

Where Temperature Variability Happens

Temperature-related variation can enter the testing process in several common ways. Testing samples or reagents immediately after removing them from refrigeration can affect results, as can measuring digested samples, including COD or total phosphorus tests, before they have adequately cooled. Seasonal changes in ambient temperature can also introduce variability in labs without consistent climate control.

Basic practices can help reduce these effects, including allowing refrigerated materials to equilibrate to the appropriate temperature, cooling digested vials before measurement, maintaining stable laboratory conditions, and using a controlled water bath when precise temperature adjustment is needed.

Adding Temperature Awareness to the Workflow

Instrumentation can also help technicians account for temperature during the measurement process.

The Hach DR4900 Spectrophotometer, for example, uses infrared technology to measure and display the vial temperature for TNTplus tests. Temperature warnings are available for many TNTplus methods when conditions fall outside the method-specific range.

For ammonia TNTplus methods, the instrument can take this a step further by using the measured vial temperature to compensate the result to the 20°C reference condition.

Temperature may be an easy variable to overlook, but accounting for it can help laboratories reduce unexplained variability and build greater confidence in their photometric results.