20.07.2006

Water Change – Conductivity vs. pH-Value Measurement for the Assessment of the Nitrate Content

The possibilities of indirect measurement of nitrate are discussed from time to time to find an easy indication for the need of water change.

For freshwater aquaria conductivity is usually used for testing the success of making soft water. In fact, it is easy to mix and test water with a certain hardness.

The pH is especially discussed in connection with CO2 fertilization and the keeping of aquarium fish.

Both values are changing also during the working process of the aquarium by the pollution of water with fish food and the excretory products of the fish and their degradation products. Assuming there is a functioning biological filtration, especially the nitrate level is increasing if the water changes are not sufficient. It is well known that carbonates can be destroyed by hydrochloric acid and thus the pH - value descends:

2 HCl + Ca(HCO3)2 → CaCl2 + 2 H2O + 2 CO2 ↑

At eutrophication, the nitric acid takes this part.

This chemical formula clearly shows the correlation of the carbonate degeneration, the increase in nitrate levels and the decrease in the pH-value:

2 HNO3 + Ca(HCO3)2 → Ca(NO3)2 + 2 H2O + 2 CO2 ↑

On this basis it will be discussed whether the nitrate content of the water and therefore the timepoint for a water change can better be determined by the measurement of pH - value or by conductivity.

Conductivity:

During conductivity measurement, all charged particles (cations and anions) will be measures, that also includes the nitrate content.

The following table shows the contributions of each kind of ions on the conductivity of the water.

On this an example calculation:

In Düsseldorf, the tap water has a conductivity of 769 µS/cm according to the analysis of the municipal utility, so it is pretty hard with a total hardness of 14.7 and an alkalinity of 9.3. The pH - value is around 7.5 and the nitrate content at 14.2 mg/l. Assuming that we want to make a water change on a nitrate level of 35 mg/l, a conductivity increase, which is caused by about 20 mg/l nitrate, can be measured. But this increase of nitrate causes only an increase in conductivity of around µS/cm. Even if we wanted to change only water at 80 mg/l nitrate content, only an increase of about 30 µS/cm could be expected.

Conclusion: The expected increase of conductivity should be very small but it should be independent of the hardness of the water.

pH – value

At stable CO2 concentration the pH-value should be decreased by about 0.3 at a bisection of the alkalinity. The alkalinity is reduced by the nitrification.

How much is it in fact ?

One mmol/L KH is equivalent to a hardness of 2.8 dKH. To measure a pH - value change of 0.3, the concentration of nitrate must increase to around 120 mg/L at a KH of 9.3.

In soft water, this effect is much more eye catching. At a KH of 2 already an increase of 20 mg/l nitrate conducts to a pH value change of 0.3.

© Andreas Jäger