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Concept:

(i) A buffer is an aqueous solution designed to maintain a constant pH, even when exposed to small amounts of acids or bases.

(ii)Whether acidic (pH < 7) or basic (pH > 7), a buffer solution consists of a weak acid or base mixed with the salt of its conjugate base or acid, respectively.

(iii)To calculate the specific pH of a given buffer, you need to use the Henderson-Hasselbalch equation for acidic buffers:

\(pH = pka + \log \left( {\frac{{\left[ {A - } \right]}}{{\left[ {HA} \right]}}} \right)\)

Where,Ka is the "dissociation constant" for the weak acid,[A-] is the concentration of conjugate base and [HA] is the concentration of the weak acid.

pH + pOH = 14

Calculation:

According to Henderson equation pH of acidic buffer is:

\(pH = pka + \log \left( {\frac{{\left[ {A - } \right]}}{{\left[ {HA} \right]}}} \right)\)

Here, pka = 4.74, [A-] = 0.02 and [HA] = 0.02

\(pH = 4.74 + \log \left( {\frac{{0.02}}{{0.02}}} \right) = 4.74\)

∵ We know that, pH + pOH = 14

So, pOH = 14 - pH = 14 - 4.74 = 9.26

Concept:

Measure of the degree at which water looses its transparency due to the presence of suspended particulates is termed asturbidity.

Baylis Turbidometer

• This turbidimeter is based on color-matching techniques.
• Light intensity is measuredin the direction of incident light.
• Formazinis a chemical compound that produces more reproducible standards than SiO2and has replaced it as a reference.The turbidity unit is also sometimes called FTU.
• This is also preferred for turbidity measurement of water when turbidity of the sample is less than 1.

Nephelometer

• Water containing1 milligram of finely divided silica per liter has a turbidity of 1 NTU.
• It is based on color matching technique.
• The NTU is measured with an electronic instrument called anephelometer.
• The water to be measured is placed in a standard container. A light beam passes through the water and strikes a sensor on the other side of the container. A second sensor is mounted at right angles to the beam,measuring light scattered by particles in the water. From the ratio between the light intensities at the two sensors the turbidity in NTU can be calculated.

Jackson turbidity method

• Jackson turbidity unit (JTU) is equal to turbidity produced by 1 mg SiO2in 1 liter of distilled water.
• This method is based on theAbsorption principle.

Concept:

Taste and odour in water is determined by diluting the raw water sample up to an extent taste and odour in water is hardly detectable. Intensity of taste and odour in water is expressed in terms of unit called Threshold Odour Number (TON), which signifies the dilution ratio at which taste and odour is hardly detectable. It is measured in the apparatus called Osmoscope.

Note:

1. Acceptable limit: 1 TON.

2.Cause of Rejection Limit: 3 TON.

3.Sources for taste and odour in water: Inorganic salts like Na, Ca, Mg etc, Organic dissolved impurities, dissolved gases like H2S, CO2.

Tintometer:

• Itis a device to determinecolour in water.
• The unit of colour measurement is TCU.
• 1 TCU is the colour produced by 1 mg of platinum cobalt in the form of chloroplatinate ions dissolved in 1 litre of distilled water.

Turbidimeter:

Turbidity is measured ona silica scaleand expressed in terms ofTurbid Units (TU)

Devices used to measure the turbidity in water are as follows:

1. Jackson’s Turbidimeter

It is used to measure high turbidity, i.e25 – 1000 mg/l

It cannot measure the turbidity less than 25 mg/l

2. Bayli's meter

It is used to measure low turbidity, i.e less than 10 mg/l

3. Ratio Turbidimeter

When a turbidity meter is measuring with “ratio on” it will use data from several detectors and take an average, in order to correct for changes caused by this particle size difference

4. Nephelometer

This works on the principle of light scattering.

It is a digital electronic device that measures lowturbidity in water with very high precision in no time

It measures the turbidity in terms of NTU (Nephelometric Turbidity Unit)