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pH is a logarithmic scale that gives a quantitative measure of the acidity or alkalinity of an aqueous solution. pH is a measurement tool used to determine the concentration of hydrogen ions (H+) in a solution.
- pH stands for ‘Potential of Hydrogen’ or ‘Power of Hydrogen’.
- pH Scale ranges from 0-14 describing the alkalinity or acidity of a solution.
- Values from 1 to 6 denote acidic nature with 1 being the most acidic.
- Values from 8 to 14 denote basic nature with 14 being the most basic.
- 7 is neutral, i.e. the solution is neither acidic nor basic.
- Acids are chemical substances that donate free hydrogen ions (H+ ions) when dissolved in water.
- Bases are chemical substances that produce hydroxyl ions (OH- ions) when dissolved in water.
Determination of pH of Some Solutions Experiment is performed to find out whether certain solutions are acidic, basic, or neutral.
Read More: Importance of pH in Everyday Life
Key Terms: pH, Acids, Bases, Universal Indicator, Alkaline Solutions, Hydrogen Ions, Hydroxide Ions, pH Scale, pH Paper
What is pH?
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pH is a measurement tool that helps to determine whether a given solution is alkaline or acidic in nature. It is a negative logarithm of the molar hydronium-ion concentration. pH provides the amount or quantity of hydrogen ion activity (H+) in a given solution.
- pH is the abbreviation of ‘Power of Hydrogen’ or ‘Potential of Hydrogen’.
- It is dependent on the concentration of hydrogen or hydronium ions.
- The idea of pH was given by Soren Sorensen, a Danish Chemist.
On the pH scale, the values range from 0 to 14. It classifies a solution as acidic, basic, or neutral on the basis of the following criteria:
- pH < 7: Acidic Solution
- pH> 7: Basic Solution
- pH = 7: Neutral Solution
It means a solution with a pH value between 0 to 6 is acidic while a solution with a pH value ranging from 8 to 14 is basic in nature. Solutions with a pH value of 7 are considered to be neutral.
pH Scale
How to Determine pH Through Experiment?
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pH of a solution can be determined through several experiments that can help to identify whether a given solution is acidic or basic.
- Acids are substances that generate free hydrogen ions (H+ ions) when they are dissolved in water.
- Bases are those substances that when dissolved in water produce hydroxyl ions (OH- Ions).
- Acidic solutions have hydrogen-rich ions whereas the basic solutions are poor in hydrogen ions.
- The strength or weakness of an acidic or basic solution is dependent on the release of H+ ions or OH- ions when it is dissociated.
- Universal Indicator is a pH indicator that is composed of different chemical compounds.
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pH Measurement
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Soren Peder Lauritz Sorensen, a Danish Chemist, invented pH. pH is defined as the negative logarithm (base 10) of the hydrogen ion concentration.
- The acidity and the alkalinity are measured using a logarithmic scale that is called pH.
- pH scale gives us the measure of how acidic or basic any substance is.
- pH scale ranges from 0 to 14.
- A pH value of 7 on the scale is neutral.
- A pH value less than 7 is acidic and a pH value greater than 7 is basic.
- The value of the pH is low if the concentration of the hydrogen atoms in the solution is high.
Methods of pH Determination
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The pH of any solution can be determined by several different methods:
pH Paper
- pH paper is a special strip of paper used to determine the acidity or basicity of solutions.
- These paper strips are submerged in the chemical compound and are then dried.
- When this paper is dipped in any liquid substance, it changes color.
- The change of color is compared then to the color chart or the pH scale.
- It is then used to determine the actual pH level or the category in which that certain liquid belongs.
pH Paper for Acids and Bases
pH Meter
- pH meter is a device that is used to determine the pH level of any substance or solution.
- It is an electronic device and has a special bulb that is sensitive to the hydrogen ions that are present in the solution.
- The electronic bulb produces a signal which then gets amplified and is shown in a meter format in the electronic meter.
- It is responsible for displaying the pH level and the category to which this substance belongs.
- For a precise measurement, the pH meter needs to be calibrated before every measurement.
pH Meter
Read More: pH Change In The Titration Of A Strong Base Using Universal Indicator
Determination of pH of Some Solutions Experiment
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Here is an experiment that aims at the Determination of pH of Some Solutions using pH Paper or a Universal Indicator.
Aim
Determining the pH of solutions like fruit juices and of different acids, bases, and salts using pH paper and the universal indicator.
Material Required
- Sample Solutions
- Test Tubes
- White Tile
- pH Paper
- Universal Indicator
- Dropper
Procedure
The procedure for determining the pH of solutions using pH Paper or a Universal Indicator is as follows:
(A) Determination of pH Using Universal Indicator
- Take the samples of fruit juices that are given like lemon, orange, pineapple, and apple in a separate test tube.
- Take at least 2 mL of juice in each of the test tubes.
- Use a separate dropper for transferring each of the juice samples into a test tube.
- Then add two drops of the universal indicator.
- After that mix the contents thoroughly by shaking the test tubes gently.
- Then, refer to the pH chart for matching the color that appears in the test tube.
- Record your observations in a tabular form.
(B) Determination of pH Using pH Paper
- Take a white tile and place on it a clean pH paper strip.
- Then drop the sample of the solution on the pH paper using a clean dropper.
- After that observe the changes in the color of the pH paper.
- Compare the color obtained on the pH paper with the shades on the pH scale.
- Record observations in a tabular form.
Check More: Study for pH Clarity and Presence of Any Living Organism in Two Different Water Bodies
Observation
Note down the observations accordingly as per both procedures:
(A) Determination of pH Using Universal Indicator
| Sample | Color with pH Indicator | pH | Interference |
|---|---|---|---|
| Apple Juice | - | - | - |
| Pineapple Juice | - | - | - |
| Orange Juice | - | - | - |
| Lemon Juice | - | - | - |
(B) Determination of pH Using pH Paper
| Sample | Color Produced on pH Paper | Approximate pH |
|---|---|---|
| - | - | - |
| - | - | - |
| - | - | - |
| - | - | - |
Precautions
- Always keep the pH strips away from chemical fumes
- Match the color of the solution to the pH chart carefully.
- Keep the pH paper clean.
- Equal quantity of indicators to be added into juice samples.
pH of Fruit Juices
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The above experiment involved the determination of pH of some solutions which were fruit juices. Here is the pH value of certain fruit juices for easy reference:
- Lemon Juice: 2.25
- Fine Grapefruit Juice: 3.22
- Minute Maid Cranberry Grape: 2.71
- Ocean Spray Strawberry Kiwi Juice Cocktail: 2.90
- Ocean Spray Cranberry: 2.56
- Minute Maid Natural Energy Pomegranate Berry: 3.33
- Ocean Spray Cran-Grape: 2.79
- Minute Maid Orange Juice: 3.82
- Minute Maid Cranberry-Apple Raspberry: 2.79
Check More:
Things to Remember
- pH is a measure of the concentration of hydrogen ions in a solution.
- It is a measurement tool used to determine the alkalinity or acidity present in a solution.
- Values in the pH Scale range from 0-14.
- pH is less than 7 in acidic solutions.
- pH value is more than 7 in basic solutions.
- pH is equal to 7 in neutral solutions.
- Determination of pH can be done through pH paper as well as the universal indicator.
Previous Years’ Questions
- What is the pH for a neutral solution at body temperature… (JIPMER - 2000)
- The pH value of 0.01 N NaOH solution is… (JIPMER - 2014)
- The conjugate base of hydrazoic acid is... [JEE Main 2014]
- The pH of human small intestine is around 7.5 and the pH of large intestine… (JIPMER - 2016)
- Hard acids prefer to combine with… [KEAM]
- The pH of 10−8 M NaOH is... (JIPMER - 2004)
- Find pH of 0.1 M NaOH solution... (JIPMER - 2018)
- The compound that is not a Lewis acid, is... (JIPMER - 2000)
- The pKa of a weak acid HA and pKb of a weak base BOH are 4.60... [KEAM]
- The pH of normal rainwater is… [KEAM]
- The pH of the solution obtained by mixing 100ml of a solution of pH... [KCET 2012]
- The pH of a 0.02M NH4Cl solution will be [given Kb(NH4OH)... [JEE Main 2019]
Sample Questions
Ques. How to check if a solution is Acidic, Basic, or Neutral? (3 Marks)
Ans. Solutions that have a pH below seven are considered to be acidic and the ones that have a pH above seven are considered to be basic. Solutions that have a pH of 7 are considered to be neutral.
- pH < 7: Acidic Solution
- pH> 7: Basic Solution
- pH = 7: Neutral Solution
Ques. What are Acids and Bases? (3 Marks)
Ans. Acids and Bases are two types of chemicals that lead to the formation of salt and water on combining with each other.
- Acids are those chemicals that when dissolved in water, produce some free hydrogen ions (H+ Ions).
- Bases produce hydroxide ions (OH- ions) when dissolved in water.
- Hydrogen ions are abundant in acidic solutions whereas scarce in basic solutions.
Ques. Liquids up to what pH level are safe to be consumed by humans? (3 Marks)
Ans. The liquids, solutions, or juices that have a higher pH level are safe for human consumption. The juices which fall on a scale lower than the mark of 4 on the pH scale should be completely avoided as the acidic concentration in such juices is more and thus, can have a damaging effect on the teeth of humans.
Ques. Consider that there are four solutions A, B, C, and D. The pH of solution A is 6, B is 9, C is 12 and D is 7. Arrange these four solutions given above in the increasing order of H+ ion concentration. (3 Marks)
Ans. The acidity of a solution is directly affected by the presence of the H+ ions. A solution that has a high number of hydrogen ions will be acidic and have a low pH value anything that is below 7.0 is acidic, and anything above 7.0 is alkaline or basic.
Therefore, the increasing order of concentration of H+ ion is: A > D > B > C
Ques. Consider that solutions with equal volumes with pH values 8 and 10 are mixed together. What is the pH of the resulting solution? (3 Marks)
Ans. pH = 8 means [OH-] = 10-8 hence [H+] = 10-6
pH = 10 means [OH-] = 10-10 hence [H+] = 10-4.
M1V1 + M2V2= Mx 2V
10-6 x V + 10-4 x V = M x 2V
10-4 (10-2 + 1) V/2V = M
10-4 (0.01 + 1)/2 = M
10-4 (1.01)/2 = M
M= 0.505 x 10-4.
pH = -log[H+]
pH = 4 - log [0.505]
pH= 4 - (-0.296)
pH = 4.26
Ques. Why is there a visible change in visual indicators when introduced to an acid or base medium? (3 Marks)
Ans. There is a visible change in visual indicators when introduced to an acid or base medium because of a reaction that takes place. When an indicator reacts with the H+ ions of the acid, it forms a species that has some color or can also be colorless, and when the indicator reacts with the OH- ions of the bases it gives some other species a different color. An indicator is an organic molecule that works like a "color dye".
The most common indicator is "litmus" paper, which is red below pH 4.5 and blue above pH 8.2.
| Color | Blue Litmus | Red Litmus |
|---|---|---|
| Acid | Turns Red | Stays Same |
| Base | Stays Same | Turns Blue |
Ques. If a drop of phenolphthalein would be added to 1 ml of water and a drop of NaOH solution is also added to it. What will be the observations? (3 Marks)
Ans. The water solution is neutral and has a pH of 7. Whereas, phenolphthalein gives pink color on pH 8 and NaOH has pH 13. Hence it will show pink color in addition. Hence on adding water drops to phenolphthalein there is no change in color but after the addition of drops of NaOH, it changes its color from colorless to pink color.
Ques. Determine the pH of 0.001M of the HCl solution. (2 Marks)
Ans. HCl is a very strong acid and, thus, completely dissolves into water and dissociates in H+ and Cl-.
pH = - log [H+]
pH = -log [0.001]= 3
Therefore, the pH of the HCl solution is 3.
Ques. Study the pH for solutions of sodium chloride, ferric chloride, and sodium carbonate using an experiment. (5 Marks)
Ans. To study the pH for solutions of sodium chloride, ferric chloride, and sodium carbonate using an experiment we would need basic lab equipment like Boiling tubes, test tubes, glass droppers, pH paper, and solutions to be tested.
The procedure to follow is the same for all 3 solutions which is to take some quantity of each solution in a test tube and find out their acidic, basic or neutral nature using pH paper. You can make your observations in a tabular form for ease of understanding. Always remember that the solutions being used should be prepared fresh for best results and the pH paper should always be stored in a dry place for proper observations.
The theory behind this is that the salts of strong acids and bases form neutral solutions while the salts of weak acid/base and strong base/acid are respectively basic and acidic in nature. The salts formed of weak acid/base with strong base/ acid are hydrolyzed in the water while the salts formed by the neutralization of a strong acid and a strong base do not hydrolyze in solution.
Ques. The concentration of Hydronium ion is 8.0 x 10-8M. What will be the pH of its solution? (3 Marks)
Ans. The concentration of Hydronium ion is given as 8.0 x 10-8 M.
Using the pH Formula,
pH = −log[H3O+]
pH = - log [8.0 x 10-8]
pH= 7.09
Thus, he pH of the Hydronium ion solution is 7.09.
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