Theory & Procedure, Studies on Turbidity, pH and Microbial Presence in Water | Additional Study Material for NEET PDF Download

Our Objective

Our aim is to study different water bodies for pH, clarity and presence of any living organisms.

Theory

Water covers about three-quarters of the earth's surface and it is essential for all known forms of life. Water is a habitat for a variety of plants and animals which have special adaptations to survive in their habitats. Some of the characteristics that control the quality of water are the turbidity of water, pH of water and different types of plants and animals and their density.

Turbidity of Water

Turbidity is the measure of the relative clarity of a liquid. Turbidity of the water body determines the depth to which light can penetrate and thus affects the distribution and photosynthesis of phytoplankton and macrophytes. The more turbid the water body, the less is the thickness of its photic zone (the layer of a body of water that is penetrated by sufficient sunlight for photosynthesis).

Turbidity can be measured using several methods. The easiest and least expensive method is through a Secchi disk. Water turbidity can be measured with a Secchi disk, a circular white or black-and-white disk attached to a tape measure that is lowered through the water to a depth where it disappears from view. In clear lakes, the Secchi depth may be greater than 10 metres, while in eutrophic lakes, with dense growth of algae and other organisms, creating low visibility, the Secchi depth can be less than 1 metre.

pH of Water

The pH is a measure of the relative amount of free hydrogen and hydroxyl ions in an aqueous solution. Solutions with a pH less than 7 are said to be acidic and with a pH greater than 7 are basic. The pH of pure water is 7 at 25°C.

The pH value of a water sample can be determined by indicator dye method and electrometric method using a pH meter. For routine purposes, the indicator dye method, using universal pH indicator solution or paper strips containing the pH indicators, is used.

Living Organisms in Water

The productivity and the trophic state of a water body are determined by quantities of nitrogen, phosphorus, and other biologically useful nutrients. Nitrogen and phosphorus are nutrients that induce plant growth in standing water bodies, so increased concentrations tend to result in increased plant growth.

A productive water body has high nutrient concentrations, and has a very high density of phytoplankton per unit area. These water bodies have high amounts of nutrients and dissolved oxygen and bear large number of organisms at different trophic levels. A water body with a very low density of phytoplankton per unit area is a non-productive water body. The status of health of a water body can be determined by analyzing water samples for the number and type of organisms present in it at a given time.  Such assays also help us to find whether a water body is polluted, as some of the organisms are strong indicators of water pollution. When you look at a water sample with a microscope you will likely see a variety of tiny living things.  Some of them are shown below.

Learning Outcomes

• Students understand the meaning of turbidity of water, pH of water and about the different living organisms in water bodies.
• Students understand the term Secchi’s disk.
• Students do the experiment better in the real lab having gone through the animation and simulation.

To Study Turbidity of Water Samples using Secchi’s Disc Method

Materials Required:

Procedure:

• Reach the center of the pond in a small boat.
• Slowly immerse the Secchi’s disc into water vertically holding the rope tightly till the black and white segments of the disc disappear.
• On reaching a particular depth, the disc becomes completely invisible.
• Using a pin, mark the length of the rope when the disc disappears (say A cm).
• Slowly pull up the disc till the black and white segments of the disc just reappear.
• Using a pin, mark the length of the rope where they just reappear. (say B cm).
• Using a meter tape, find the length of A and B.
• Find the mean length (Say X cm) of the rope by the following method. X=(A+B)/2

Observation:

The value of X represents the depth of the photic zone up to which sunlight penetrates in the water body and photosynthesis takes place.

Precaution:

Students are advised to perform this experiment under the strict supervision of a teacher to prevent danger, such as drowning.

To Study the pH of Water

Materials Required:

Procedure

• Let’s first check the pH of water sample using pH paper.
• Take a pH paper strip and dip into the beaker that contains back water.
• Keep the wet pH paper strip on the tile.
• Do the same procedure for borewell water and pond water.
• Wait for some time to dry the pH paper.
• Note the colour and compare with the colour chart given on the broad range indicator paper and get a rough estimate of pH of the sample solution.
• We can observe that backwater and bore well water has pH 8 and pond water has pH 5.
• Let’s check the pH of the water sample using universal indicator solution.
• Using a dropper take some universal indicator solution.
• Put 5 drops of indicator solution into the test tube that contains water sample.
• Note the colour developed and compare it with the colour chart.
• We can observe that backwater and bore well water has pH 8 and pond water has pH 5.

To Analyse Living Organisms in Water Sample

Material Required:

Procedure:

• Take beaker containing pond water and pour into a test tube.
• Leave the water sample undisturbed till the sediment settles to the bottom of the test tube.
• Decant the clear water, leaving concentrated sediment at the bottom of the test tube.
• With the help of a dropper, transfer a drop of water from the test tube to the center of a glass slide.
• Take a cover slip and place it gently on the slide with the aid of a needle.
• Remove the extra water using a filter paper.
• Place this glass slide on the stage of the compound microscope and view it.

Observations:

We can observe a large number of different organisms in water sample.

Simulator Procedure (as performed through the Online Labs)

You can select the experiment from the ‘Select the experiment’ drop down list (pH of water samples, urbidity of water samples, living organisms in water samples).

pH of Water Sample

• Choose any one of the water samples in the beakers by clicking on it.
• There are two ways of finding the pH value of the solution:

Using the pH strip:

• Click and drag the dropper from the stand and move into the solution in the beaker to collect the water sample in the dropper.
• Still holding the dropper, move it from the beaker over to the pH strip and release it.
• To find the pH value of the solution, select the colour from the colour chart by clicking and dragging it to the pH strip and comparing it.
• The colour that matches with the spot on the pH strip indicates the pH value of the solution.

Using the Universal Indicator:

• Click and drag the dropper from the Universal Indicator bottle and move it into the solution in the beaker to drop the Universal Indicator into it.
• To find the pH value of the solution, select the colour from the colour chart by clicking and dragging it next to the solution in the beaker and comparing it.
• The colour that matches with the solution in the beaker indicates the pH value of the solution.

Note:

• Once test is done using the Universal Indicator, you cannot do it with the pH strip. To do it with the pH strip, click the ‘Reset’ button and vice versa.
• The ‘Reset’ button can be used to redo the experiment with other solutions.

Turbidity of Water Sample

• You can select the method of checking turbidity of water from the ‘Select the method’ drop down list (Secchi’s disk method and measuring cylinder method).

Secchi’s disk method:

• You can immerse Secchi’s disc into water by moving the ‘Disc movement’ slider.
• On reaching a particular depth, the disc becomes completely invisible.
• The length of the rope when the disc disappears (say A =95 cm).
• You can pull up the disc until the black and white segments of the disc just appear by moving the ‘Disc movement:’ slider.
• The length of the rope where they just reappear. (say B =90 cm).
• We can find the mean length (Say X cm) of the rope by the following method. X= (A+B)/2. The mean length shown in the left pane.
• You can redo the experiment by clicking on the ‘Reset’ button.

Note:

• The value of X represents the depth of the photic zone up to which sunlight penetrates in the water body and photosynthesis takes place.
• You can move the Secchi’s disc by using the up and down arrow keyboard keys.

Measuring Cylinder Method

• Click on the ‘Start’ button to start the experiment.
• A timer is shown and you need to wait till the timer stops.
• Observe the amount of sediment settled at the bottom of each cylinder.
• Click on the information icon to see the inference.
• You can redo the experiment by clicking on the ‘Reset’ button.

Note:

• Turbidity is a measure of the amount of suspended particles in the water column. Pond water has fewer suspended particles, so it has a low turbidity level. Bore well water and backwater have more suspended particles, so they have a high turbidity level.

Living Organisms in Water

• Choose any one of the water samples by clicking on the corresponding icon.
• Click and drag the dropper from the stand and move into the water sample in the vial to collect into the dropper.
• Still holding the dropper, move it from the vial over to the glass slide and release it.
• Click and drag the cover slip and move towards the glass slide to place cover slip on it.
• Drag and drop the glass slide onto the stage of the compound microscope.
• Click on the eye piece of the compound microscope to view the living organism.
• We can observe a large number of different organisms in the water sample.
• Click on each species to display its name.
• You can redo the experiment by clicking on the ‘Reset’ button.