Determination of turbidity of water

Determination of Turbidity of Water Experiment

Turbidity is the specialized term referring to the cloudiness of a solution and it is a subjective characteristic that is imparted by solid particles obstructing the transmittance of light passing through a water sample. Turbidity regularly demonstrates the presence of suspended solids like earth material, silt, clay, organic matter green growth, and different microorganisms.


To determine turbidity of the given sample of water by using Nephelo turbidity meter.


Nephelo turbidity meter, glass tubes, beakers.

determination of turbidity of water
Digital Turbidity Meter

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Turbidity is depended on the comparison of the intensity of light scattered by the sample under characterized conditions with the power of the light scattered by a standard reference suspension under similar conditions.

The turbidity of the sample is thus estimated from the amount of light scattered by the sample taking a reference with standard turbidity suspension. The higher the power of scattered light the more is the turbidity.

For the primary standard reference, suspensionformazin polymer is used. The unit of turbidity is expressed on NTU (Nephelometric Turbidity Unit).

Determination of Turbidity of Water
Turbidity (NTU)


1. Turbidity free water:-

Generally, Distilled water is considered as turbidity free water for any test.

2. Stock turbidity solutions:-

Solution 1:- For making solution dissolve 1.0 grams hydrazine suplate (NH2)2.H2So4 in distilled water and dilute it to 100 ml in a make up flask.

Solution 2:- For 2nd Solution Dissolve 10.0 grams hexamethylene tetramine (CH2)6N4 in distilled water and dilute it to 100ml.

Solution 3:- Now, Mix solution 1 and 2 in 100ml flask each 5ml and left it stand for atleast 24 hs., then dilute it to 100ml and mix thoroughly. The turbidity of this solution is 400 NTU.

Standard Turbidity Solution:- For making standard turbidity solution take 10.0ml of solution 3 in a 100ml to make up the flask and dilute it to 100ml. with turbid free water. The turbidity of this suspension is 40 NTU.

Read More: Drinking Water Quality Standards (WHO Guidelines)

Determination of Turbidity of Water

Procedure of Turbidity of Water Test:

a) Calibration of Nephelometer:-

  1. Select proper range of NTU on Nephelometer.
  2. Fill distilled water sample in test tube and set the Nephelometer reading to zero by using the knobs provided for zero setting.
  3. Now, fill the standard solution of 40 NTU in test tube and set reading to 40 NTU by adjusting the Nephelemeter reading to 40 NTU using calibration knob.

b) Determination of turbidity of sample water:

1. Turbidity less than 40 units:

It is essential to allow samples to come to room temperature before analysis. Mix the sample to thoroughly disperse the solids. Wait until air bubbles disappear then pour the sample into the turbidity meter tube. Now, read turbidity value in NTU directly from instrument display or from the appropriate calibration curve.

2. Turbidity exceeding 40 units:

In case of turbidity of sample exceeds 40 units, and then add one or more same volume of distilled water or turbidity free water in turbid sample until the turbidity falls below 40 units. The turbidity of test sample is calculated from the turbidity of the diluted sample and the dilution factor.

For example, if 5 same volumes of distilled water added to the turbid sample, and the diluted sample showed turbidity of 30 units, then the turbidity of the original sample was 180 units.                 

                  Nephelometric Turbidity Units (NTU) = A(B + C) / C


A = Turbidity of diluted sample, B = Volume of diluted sample (ml)

C = Sample volume taken for dilution.


For undiluted sample                                        For diluted sample

Digital read out =                                              Vol. of sample ( C ) =

                                                                        Vol. of dilution water ( B ) =

                                                                        Digital read out ( A ) =


For undiluted sample                                   For diluted sample

Turbidity of sample (NTU) =                        Turbidity in NTU = A(B + C) /C


determination of turbidity of water
Turbidity of water Experiment

Application of Turbidity of Water Test Data

  • In water supply turbidity data information can be helpful to know the effectiveness and adequacy of the treatment delivered with various coagulants and the necessary doses.
  • The faulty operation of filter can be checked by estimating the turbidity of filtered water.
  • The effective removal of suspended solids from the waste can be known from the measurements of treated wastewater turbidity.
  • To produce great quality effluent by using the least amount of chemical, the dose of the chemical can be adjusted or balanced by turbidity information.

NOTE: According to  IS 10500:1991 the maximum limit of turbidity is 10 NTU and up to 5 NTU water is acceptable and above 5 NTU the consumer acceptance decreases. Hence 0.1 NTU as a goal; less than 1 NTU as a standard and 5 NTU as a special case of consumable water. Turbidity is an indicator of poor treatment plant efficiency, filter run timing, contamination of distribution system, and can fix the measurements of coagulants.

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