The key difference between NTU and FTU is that NTU measurement uses white light for determination, whereas FTU uses infrared light.
In laboratories, there are three typical units for expressing the turbidity of a sample. They are the NTU, FTU, and FAU, which stand for nephelometric turbidity unit, formazine turbidity unit, and formazine attenuation unit, respectively. Essentially, these are the same in value, but different methods are used to determine these values.
What is NTU?
The term NTU stands for nephelometric turbidity unit. It is useful in water treatment for describing the turbidity of a liquid. We can measure NTU with a calibrated nephelometer, which is an instrument for measuring the size and concentration of particles that are suspended in a liquid or gas by determining the light that it can scatter. This unit indicates that the higher the concentration of suspended solids in the water, the dirtier it appears and the higher the turbidity of that solution.
A nephelometer measures the intensity of light scattered at 90 degrees when passing a light beam through a water sample. Unlike FTU, we need a white light source for the detection of turbidity using the NTU method (FTU method uses an infrared light source).
What is FTU?
The term FTU stands for formazine turbidity unit. It is also known as formazine nephelometric unit or FNU. It is similar to NTU in value but different in the method of measurement. However, there is no straightforward relationship between NTU and FNU/FTU because the turbidity measurement will depend on the optical characteristics of the components in the sample. Unlike NTU, FTU is measured in the presence of an infrared light source. Moreover, we need a formazine suspension for this measurement.
We can prepare a formazine suspension by mixing solutions of hydrazine sulfate and hexamethylenetetramine in the presence of ultrapure water. It is advised to keep the resulting solution for 24 hours at room temperature for the development of the suspension. The turbidity value of this produced mixture is 4000 NTU/FNU. Thereafter, we have to dilute this suspension to a value that is suitable for the instrument we have.
Generally, the purity of the water used in the preparation of formazine suspension is very important because it cannot initially contain colloidal particles, which is why we need to use ultrapure water. Otherwise, the colloidal particles that are already present in the water can give a measurement for the turbidity value.
What are the Similarities Between NTU and FTU?
- NTU and FTU represent the same value.
- Both terms provide the turbidity of a solution.
What is the Difference Between NTU and FTU?
The term NTU stands for nephelometric turbidity unit, while the term FTU stands for foramzine turbidity unit. These units represent the same value, but the detection methods can be different from each other. The key difference between NTU and FTU is that NTU measurement uses white light for determination, whereas FTU uses infrared light. Moreover, we can use a nephelometer to detect the turbidity in NTU units and a formazine suspension to detect the turbidity using FTU units.
The below infographic presents the differences between NTU and FTU in tabular form for side by side comparison.
Summary – NTU vs FTU
Turbidity is the cloudiness of a solution. In laboratories, there are three typical units expressing the turbidity of a sample. They are the NTU, FTU, and FAU, which stand for nephelometric turbidity unit, formazine turbidity unit, and formazine attenuation unit, respectively. The key difference between NTU and FTU is that NTU measurement uses white light for determination, whereas FTU uses infrared light.
1. “Units of Turbidity.” USGS.
1. “Turbidity” By Washington State Dept of Transportation (CC BY-NC-ND 2.0) via Flickr
2. “Signs of growth in manual blood culture systems” By Ombelet, S., Barbé, B., Affolabi, D., Ronat, J. B., Lompo, P., Lunguya, O., Jacobs, J., & Hardy, L. (2019). Best Practices of Blood Cultures in Low- and Middle-Income Countries. Frontiers in medicine, 6, 131. https://doi.org/10.3389/fmed.2019.00131 (CC BY 4.0) via Commons Wikimedia