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Schematics for the measurement of pH and humidity

M.VAMSI KRISHNA08115047

CONTENTS1) INTRODUCTION

2) pH MEASUREMENT

3) MEASUREMENT ELECTRODE

4) REFERENCE ELECTRODE

5) HUMIDITY MEASUREMENT

INTRODUCTION Humidity and pH are process parameters in chemical and manufacturing industries.

  pH is a measure of the activity of the (solvated) hydrogen ion.

Humidity is the amount of water vapour in the air .

pH is defined as the decimal logarithm of the reciprocal of the hydrogen ion in a solution.

The device used to measure humidity is called a psychrometer or hygrometer.

pH MEASUREMENT•pH is a measure of hydrogen ion concentration in aqueous solution.

•It is an important parameter to determine the quality of water.

The pH value is expressed as: pH= 1/logC

Where C is the concentration of H+ ions in a solution.

•In pure water, the concentration of H+ ions is 10-7

gm/ltr at 25o C. So the pH value is 7.

•The advantage of using pH scale is that the activities of all strong acids and bases can be brought down to the scale of 0-14.

Contd…

•The pH value of acidic solutions is in the range 0-7 and alkaline solutions in the range 7-14.

•The pH value of a solution is measured by using pH electrode.

•It essentially consists of a pair of electrodes: measuring and reference electrode, both dipped in the solution of unknown pH.

•These two electrodes essentially form two half-cells.

•The total potential developed is the difference between the individual electric potential developed in each half cell.

The measuring cell potential is dependent on the hydrogen ion concentration of the solution and is governed by Nernst’s equation:

V= e.m.f of the half cell under saturated condition R= Gas constant (8.314 J/ 0C) T= Absolute temperature (K) z= valance of the ion F= Faraday Constant = 96493 C a= Activity co-efficient (0≤a≤1) ; for a very dilute solution, a→1C= molar concentration of ions.

Measuring Electrode :

•The measuring electrode is made of thin sodium ion selective glass.

• A potential is developed across the two surfaces of this glass bulb, when dipped in aqueous solution

•The buffer solution inside the glass bulb has a constant H+ ion concentration and provides electrical connection to the lead wire.

Reference Electrode :

The basic purpose of a reference electrode is to provide continuity to the electrical circuit.

The potential is measured across the two lead wires.

A reference electrode should satisfy the following basic requirements:

(i) The potential developed should be independent of H+ ion concentration.

(ii) The potential developed should be independent of temperature

(iii) The potential developed should not change with time.

•Sometimes both the electrodes are combined into one called combination electrode.

HUMIDITY MEASUREMENT

•A  hygrometer or a Pshycrometer is an instrument used for measuring the moisture content in the environment.

•Humidity measurement instruments usually rely on measurements of temperature, pressure and mass.

•Humidity measurement is among the more difficult problems in basic meteorology.

•Most hygrometers sense relative humidity rather than the absolute amount of water present.

Types of Hygrometers

Most of the hygrometers are classified into the below three types of hygrometers:

Metal-paper coil type

 Hair tension hygrometers

 Electronic hygrometers

Psychrometers

•A psychrometer consists of two thermometers, one which is dry and one which is kept moist with distilled water .

• Relative humidity is computed from the ambient temperature as shown by the dry-bulb thermometer and the difference in temperatures as shown by the wet-bulb and dry-bulb thermometers.

•Psychrometers are commonly used in meteorology, and in the HVAC industry.

The vapour pressure is calculated from the formula:

e = ew- (A/755) p (t-tw)where

e: vapor pressure hPaew: saturation vapor pressure hPap: atmospheric pressure hPat: dry-bulb temperature °Ctw: wet-bulb temperature °C

Electronic hygrometers

•Electronic hygrometers detect changes in the electrostatic capacity or electric resistance of a sensor when it absorbs moisture.

•An electrical capacitive hygrometer uses a dielectric material made of high-polymer membrane as a sensor.

•The sensor is fitted with a special filter that protects it from contaminants such as toxic gases.

•It has pores to enable moisture intake.

•When relative humidity increases from 0 to 90% and the sensor absorbs moisture, its timeconstant is about six seconds.

•On the other hand, when relative humidity decreases from 90 to 0%and the sensor releases moisture, the time constant is about 10 seconds.

REFERENCES

1) www.wikipedia.com

2) www.extech.com

3) Bates, Roger G. Determination of pH: theory and practice. Wiley, 1973.

4) Choosing a Humidity Sensor: A Review of Three Technologies. Sensors Magazine (2001).

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