How to Convert PPM to NTU

How to Convert PPM to NTU
••• MadamLead/iStock/GettyImages

Solutions that contain dissolved solids may not appear clear, because dissolved solids disrupt the amount of light that passes through. Turbidity measurements measure the attenuation of light passing through cloudy solutions, recording results on a meter in nephelometric turbidity units (NTU). You must calibrate the instrument to convert between NTU and parts per million (ppm), because each application is slightly different and instrument response varies. The standardized solution to calibrate the instrument is a Formazin solution, and 1 NTU is the response to a 1 mg/l solution of Formazin.

    Turn on the turbidity instrument and let it warm up. This time allows the light source to reach a point of stable light output.

    Select a series of standard solutions that match the range of NTU you expect to find. Many chemical catalog vendors have these standards available, such as one sold by Cole-Parmer that spans the range of 0.2 to 1 NTU and another that spans the range of 2 to 10 NTU. Commercially available standard solutions are also available that use latex beads to simulate particles in solution at various concentrations.

    Take measurements of the standards and draw a calibration curve of instrument response versus concentration (NTU).

    Correlate the values of NTU to mg/l defined by the standard. Define a conversion factor between the NTU reading and mg/l. For example, the turbidity meter may read 15 NTU and the concentration of the standard solution to yield this response may be 5 mg/l. The conversion factor would be 1 mg/l = 3 NTU = 1 ppm based on 1 mg/l = 1 ppm. Each application may have a different conversion factor due to the varying response of the turbidity instrument based on the particulars of the application and samples.

    Things You'll Need

    • Set of standard solutions
    • Turbidity instrument


    • Careful calibration is necessary because while you may measure turbidity in ppm, the particle sizes present in the water will change the response of the turbidity instrument.

Related Articles

Extract Your Own DNA and More With These Awesome Science...
The Advantages of Potentiometric Titration
How to Calculate the Calculations for Spectrophotometers
How to Standardize a pH Meter
Use of a Colorimeter
How to Calibrate a Refractometer
How to Measure Conductivity in Water
How to Convert Conductivity to Concentration
How to Calculate Percent Relative Range
How to Calculate Absorbance
How to Find a Z Score
How to Convert Foot-Candles to Lux
What is Turbidity & What Does It Indicate in Microbiology?
How to Calculate Capacitance for AC Coupling
Laboratory Glassware and Functions
How to Make a Calibration Standard for an HPLC
How to Calculate Limit of Detection (LOD)
How to Calibrate an Infrared Spectrophotometer
How to Calibrate a Spectrometer
Purpose of a Refractometer
Difference Between Spectrometer and Spectrophotometer