Determination of turbidity in water sample using nephelo turbidity meter

MEASUREMENT OF TURBIDITY USING NEPHELOMETER

AIM:- To determine the turbidity of the given sample water by Nephelometric method.

APPARATUS:- Nephelo turbidity meter.

THEORY:- Turbidity is a measure of the extent to which light is either absorbed or scattered by suspended material present in the water. Turbidity is surface waters results from the erosion of colloidal material such as clay, slit, rock fragments and metal oxides from soil, vegetable fibers and micro-organisms may also contribute to turbidity. Drinking water supplies requires special treatment by chemical coagulation and filtration before it may be used for public water supply.

            This turbidity can be brought down to required level by adding coagulants. Coagulants when added to water it will form a geletaneous substance known as floc and this will arrest the fine suspended and colloidal particles. These arrested particles will settle down rapidly because of increase in their size.

RELEVANCE:- Turbidity waters are aesthetically displeasant and are not accepted for domestic use. The colloidal matter associated with turbidity provides adsorption sites for chemicals and biological organisms that may be harmful or cause undesirable tastes and odour. Disinfection of the turbid waters is difficult and unsatisfactory, since the colloids partially shield organisms from the disinfectant. This IS values for drinking water is 10 to 25 NTU.

REAGENTS:-

1. Turbidity free water:- Pass distilled water through a lower turbidity than distilled water, discard the first 200ml, collected. If filtration does not reduce turbidity use distilled water. 
2. Stock turbidity solutions:-

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

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

iii)                Solution 3:- In a 100ml flask, mix 5ml. each of solution 1 and 2. Allow it to stand 24 hours, then dilute it to 100ml and mix thoroughly. The turbidity of this solution is 400 NTU.

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

PROCEDURE:-

a) Calibration of Nephelometer:-

i)                    Select proper range of NTU on Nephelometer.

ii)                  By placing distilled water in Nephelometer test tube, set the Nephelometer reading to zero by using the knobs provided for zero setting.

iii)                Using the standard turbid solution (i.e. 40 NTU), calibrate the Nephelemeter (i.e. adjust the Nephelemeter reading to 40 NTU using calibration knob)

b) Determination of turbidity of sample water:

i)                    For samples having turbidities less than 40 NTU:  Thoroughly shake the sample so as to remove any air bubbles and pour it into meter cell. Read out the turbidity of the sample from the digital display.

ii)                  For samples having turbidities above 40 NTU:- Dilute sample with 1,2 or 3 volumes of turbidity free water and convert the value obtained as below.

                 

If five volumes of turbidity free water were added to one volume of sample and the diluted sample showed a turbidity of 30 NTU, then the actual value is equal to 180 units. i.e.

                  Nephelometric turbidity units (NTU) = A(B + C)

                                                                                        C

Where

A = Turbidity found in diluted sample, B = Volume of dilution water in ml

C = Sample volume for dilution in ml.

OBSERVATIONS:-

For undiluted sample                                      For diluted sample

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

                                                                        Vol. of dilution water ( B ) =

                                                                        Digital read out ( A ) =

CALCULATIONS:-

For undiluted sample                                      For diluted sample

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

                                                                                                               C

RESULTS:-

INTERPRETATION OF RESULTS:-