Aim
To determine the pH of the sample.
Introduction
To calibrate the pH meter and determine the pH of
different water samples. 2. Principle pH indicates the acid
content of water. The concept of pH was first given by Soren
Peder Lauritz Sorensen in 1909 from the Carlsberg
Laboratory. The pH scale (measured from 0.0 – 14.0 pH units) is a logarithmic scale of the
hydrogen ion concentration. Solutions with a pH higher than 7.0 are considered as basic and pH lower
than 7.0 as acidic. A pH of 7.0 is neutral. Each pH unit is ten times greater in hydrogen ion
concentration than the next.
A slight variation in pH provides a significant change in water quality. The general pH equation is:
H20 -> H+ + OH -
pH = -log[H+] pOH=PKw - pH
where the log is the base-10 logarithm and [H+] stands for the hydrogen ion concentration in units of
moles per liter. The term "pH" arises from the German word "potenz," which means "power," combined
with H. So, pH is expressed as "power of hydrogen."
METHODS:
• Electrometric method
(pH meter method )
Most prominent.
• Calorimetric method
• pH paper
MATERIAL AND APPARATUS: (Electrometric Method)
• pH meter
• Buffer solutionBuffer of the different pH can be prepared by dissolving standard pH tablets in
100 ml ofdistilled water
• 3.Distilled water
PRINCIPLE
• The pH electrode used in the pH measurement is a combined glass electrode. It consists of
sensing half cell and reference half cell, together form an electrode system. The sensing half
cell is a thin pH sensitive semi permeable membrane, separating two solutions, viz., the outer
solution, the sample
to be analyzed and
the internal solution,
enclosed inside the
glass membrane and
has a known pH
value. An electrical
potential is
developed inside and
another electrical
potential is
developed outside,
the difference in the
potential is measured
and is given as the pH
of the sample.
Preparation of reagents-
Preparations of reagents Preparation of different buffer solutions Generally, pH 4.0 and pH 7.0 Buffer
solutions are used for the
• 0.1 M Phosphate buffer pH 7.0
Take 80 mL of distilled water in a 100 mL
volumetric flask and add 1.548 g of sodium
phosphate dibasic (mw: 268 g/mol) to the
distilled water. Add 0.583 g of sodium
phosphate monobasic (mw: 138 g/mol) to
the solution. Adjust solution to final desired
pH using the addition of drops of HCl or
NaOH. Now, add distilled water to make
volume 100 mL.
• 0.1 M Acetate buffer pH 4.0
Take 80 mL of distilled water in a 100 mL volumetric flask and add 0.186 g of sodium acetate
anhydrous (mw: 82 g/mol) to the distilled water. Further, add 0.464 g of acetic acid (mw: 60.05
g/mol) to the solution. Adjust the solution to final desired pH using addition of HCl or NaOH drops.
Now, add distilled water to make volume 100 mL.
• 0.1 M Carbonate-Bicarbonate buffer pH 10.0
Take 80 mL of distilled water in a 100 mL volumetric flask and add 0.388 g of sodium bicarbonate
(mw: 84 g/mol) to the distilled water. Further, add 0.571 g of sodium carbonate anhydrous (mw:
106 g/mol) to the solution. Now, add distilled water to make volume 100 mL.
Procedure
Calibration of pH meter
The instrument used to measure pH is known as a pH meter. Fig. 7.1 shows the image of pH meter along
with electrode. Calibration of pH meter is necessary because chemistry of the electrode changes over
time which will alter the pH readings. The suggested time range for the calibrations of the pH meter
varies from one week to one month according to the configuration of instruments. Calibration is also
required when the battery was replaced or a new electrode was used.
pH meter calibration and pH
measurement
▪ Take about 25 mL of each buffer
solution into separate 50 mL beakers.
▪ Press the "CAL" button/relevant
button.
▪ Rinse the electrode with distilled
water and shake gently to remove
excess drops of water
▪ Keep the electrode in the pH 4.0
buffer solution and keep it
▪ vertically.
▪ Press the "RUN/ENTER"/relevant
button on the pH meter and set the
pH value 4.0.
▪ Now, Take the electrode into the pH
7.0 buffer solution and press
"RUN/ENTER"/relevant button and set the pH value 7.0.
▪ Wait to record the pH value until it measures a stable value. Once the reading becomes stable,
the results of the calibration will be displayed on the instrument.
▪ After two-point calibration you can either continue with a three- point calibration using pH 10.0
buffer solution or start testing.
▪ Press "M" (measuring mode)/relevant button to measure the pH of samples.
▪ Now, Measure the pH of the unknown samples and note the readings in the observation table.
Note:
The specific buttons of pH meter may vary from one company to other. It is advised that
please see the user manual of your pH meter carefully before use.
Observations
Tabulation of samples of H20
Result
pH of sample A=7.84
pH of sample B=7.53
pH of sample C=7.91
pH of water=7.76 ( taking average)
PRECAUTIONS
The following precautions should be observed while performing the experiment:
• Temperature affects the measurement of pH at two points. The first is
caused by the change in electrode output at different temperatures
• The second is the change of pH inherent in the sample at different temperatures. This type of
error is sample dependent and cannot be controlled; hence both the pH and temperature at the
time of analysis should be noted
• Before using, allow the electrode to stand in dilute hydrochloric acid solution for at least 2
hours.
• iv.Electrodes used in the pH meter are highly fragile, hence handle it carefully.
2.Using indicator to determine pH
Procedure:
As performed in lab:
1. Take six strips pH paper and place them on a glazed tile. Mark
them 1 to 6.
2. Take the test solutions in separate test tubes. Dissolve the solid
substance by adding distilled water to it. Label the test tubes.
3. Now, place a drop of the test solution on one strip of the pH
paper with the help of a fine dropper or glass rod. Use a fresh
dropper for each test solution.
4. Observe the colour produced and match it with the different
colour shades of the standard colour pH chart.
5. Note down the colour of the pH from the colour chart that
matches most closely with the colour produced on the pH paper.
6. Similarly, find the pH value of the remaining samples by using a fresh strip of pH paper and a
separate glass rod or fine dropper for each one.
Using the Universal Indicator:
Same as priv.
• Using pH indicator dye
Use an aqueous solution of a pH indicator dye at
a specific temperature and wavelength. The dye
will produce a color that depends on the pH of the
solution.
Precautions
• Use only the standard colour chart supplied with the pH paper for assessing the pH value.
• Wash the test tubes and other glass apparatus thoroughly with distilled water,before use.
• All solutions should be freshly prepared.
• Keep the pH strips away from chemical fumes.
