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pH of a substance is a measure of its acidity or alkalinity. The acidity or alkalinity of a sample determines its pH. The pH scale ranges from 0 to 14, with 0 indicating acidity and 14 indicating alkalinity. A pH of 7 is considered neutral. The chemicals employed in laboratories are either basic, acidic, or neutral in nature. This property relies on the ions they emit. A chemical is said to be acidic if it releases H+ ions in its aqueous solutions. If a chemical releases OH– ions in its aqueous solutions, it is considered to be basic.
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Key Terms: pH, UNiversal Indicator, Acids, Base, Neutral Solution, Alkalinity, Hydrogen Ions, pH Paper, pH Meter, Indicators
Aim
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Using pH paper or a universal indicator, determine the pH of the provided samples. The pH of the following samples must be determined:
- Dilute CH3COOH
- Dilute NaOH
- NaCl (salt)
- Dilute NaHCO3
- Water
- Lemon Juice
Read More: Determination Of pH Experiment
Materials Required
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- Six test tubes
- Test tube stand
- Dilute acid CH3COOH
- Dilute base NaOH
- NaCl (salt) (preparation: dissolve 1-gram salt in 10 mL distilled water)
- Water
- Lemon juice
- Dilute NaHCO3
- Glass rod
- Measuring cylinder (10 mL)
- Standard pH colour chart
- pH paper
- Glass rod
- Dropper
- Universal indicator
Theory
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Definition of pH
pH refers to the concentration of hydrogen ions in the solution to determine the alkalinity or acidity of the solution.
- If the pH of a solution is less than 7, it is considered acidic.
- A solution is considered basic if its pH value is greater than 7.
- It is a neutral solution when the pH value of a solution is equal to 7.
Read More: Acids & Bases
pH Scale
The pH scale basically ranges from 0 to 14, with 0 indicating acidity and 14 indicating alkalinity. A pH of 7 is considered neutral. Litmus paper is a piece of paper that has been dyed with a natural water-soluble dye to be used as a pH indicator.
pH Scale
Measurement of pH
A equipment like a pH metre or different indicators that change colour depending on the type of test solution can be used to determine the pH of a solution.
- PH Meter: A pH metre is a device that measures pH to two decimal places.
- Indicators: A material that can be used to determine the chemical's type based on its colour shift. When the pH of the solution changes slightly, the indicator changes colour.
pH Paper
A pH paper is a sheet of paper that is used to determine if a solution is basic, acidic, or neutral. This is determined by dipping a piece of paper into a solution and watching it change colour.
A colour-coded scale indicating the pH is commonly included in the packaging of pH paper. This has the following effects:
| Solution Type | Colour |
|---|---|
| Acidic | Red |
| Basic | Greenish-blue |
| Neutral | Light Green |
pH Paper
Universal Indicator
It is a combination of multiple indicators that displays a spectrum of colours depending on the pH of any given solution. It comes in two forms: a liquid ethanol solution and a pH paper soaked in an indicator solution.
Read More: ph Formula
Procedure
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- Wash six test tubes in distilled water and place them on a test tube stand with the letters A, B, C, D, E, and F written on them.
- Add 2ml CH3COOH to test tube A, 2ml NaOH to test tube B, 2ml NaCl to test tube C, 2ml NaHCO3 to test tube D, 2ml of Water to test tube E, and 2ml of Lemon juice to test tube F.
- Place 6 pH papers on a white tile and label them A, B, C, D, E, and F.
- Place the corresponding sample solutions on the indicated pH paper on the white tile using a dropper or glass rod.
- Take note of the colour shift.
Observation
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| Sample | Colour on pH paper |
|---|---|
| A | Orange |
| B | Dark blue |
| C | Red |
| D | Light blue |
| E | Green |
| F | Pink |
Read More: Importance of pH in Everyday Life
Results and Conclusion
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| Test Tube | Solution | pH Colour Paper | pH | Nature |
|---|---|---|---|---|
| Sample A | CH3COOH | Orange | 3 | Weak acid |
| Sample B | NaOH | Dark blue | 14 | Strong base |
| Sample C | NaCl | Red | 1 | Strong acid |
| Sample D | NaHCO3 | Light blue | 9 | Weak base |
| Sample E | Water | Green | 7 | Neutral |
| Sample F | Lemon juice | Pink | 2 | Weak acid |
Precautions
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- For the experiment, use a newly prepared test sample.
- To obtain the right pH values, the fruit juice sample must also be fresh.
- Before using a glass rod or dropper for another sample, be sure it has been well cleaned.
Read More: List of Strong Acids
Things to Remember
- The pH of a substance can be defined as a measure of its acidity or alkalinity.
- The acidity or alkalinity of a sample determines its pH. The pH scale ranges from 0 to 14, where 0 indicates acidity and 14 indicates alkalinity.
- If the pH of a solution is less than 7, it is acidic whereas if its pH value is greater than 7, it is basic in nature.
- When the pH value of a solution is equal to 7, it is neutral in nature.
- pH can be measured through a pH meter and Indicators.
- A pH paper is a sheet of paper that is used to determine if a solution is basic, acidic, or neutral. This is determined by dipping a piece of paper into a solution and watching it change colour.
Read More: Chemical Indicators
Sample Questions
Ques. Who was the first scientist to propose the concept of pH? (3 Marks)
Ans. Danish scientist Soren Peder Lauritz Sorensen was the first person to propose the concept of pH in the year 1909. pH stands for "power of hydrogen," with "p" standing for the German word for power, potenz, and H stands for hydrogen's element symbol.
Ques. Define the pH of a solution. (3 Marks)
Ans. It is a measurement of a solution's basic or acidic power. It's a scale for determining the concentration of hydrogen ions in a solution. The pH rises as the hydrogen ion concentration decreases.
Ques. At 25 degrees Celsius, what is the pH of pure water? (2 Marks)
Ans. The pH of pure water at room temperature of 25 degrees Celsius is 7.
Ques. Two solutions exist, each with a distinct pH value. The pH of solution A is 6, while that of solution B is 5. Which of the given two solutions comes out the most acidic? (2 Marks)
Ans. Solution B, with a pH of 5, is the more acidic of the two.
Ques. What is the colour of a solution that has a pH of 9? (2 Marks)
Ans. The light blue colour will be observed for the solution as the pH is 9 which shows its basic in nature.
Ques. What is the pH of Saliva after a Meal? (2 Marks)
Ans. The pH of Saliva after a meal is 5.8.
Ques. What is a pH paper and what is it used for? (3 Marks)
Ans. A pH paper is basically used to assess whether a solution is basic, acidic, or neutral. The colour of the solution changes when the paper is dipped in it, showing the nature of the solution. The pH of a solution is around 11 to 14 when a piece of paper turns a vivid greenish-blue colour.
Ques. What is the blood pH? (3 Marks)
Ans. Our blood pH ranges between 7.35 and 7.45. It means that the blood is somewhat alkaline or basic. Stomach acid, on the other hand, has a pH of 1.5 to 3.5, making it acidic.
Ques. What makes a pH Paper Inaccurate? (2 Marks)
Ans. pH cannot detect pH values as low as 0 or as high as 14. For example, if the pH of a solution is less than 0, it will not display the correct value.
Ques. When a person's body is acidic, what happens? (3 Marks)
Ans. Acidosis can occur when body fluids become overly acidic. This illness occurs when the lungs and kidneys are unable to maintain a healthy pH balance in the body. Acidosis can cause a variety of health problems, and it can even be life-threatening.
Ques. Describe how antacids work. (3 Marks)
Ans. Excess hydrochloric acid in the stomach causes hyperacidity. An antacid is a natural substance. It neutralises excess acid and relieves discomfort associated with hyperacidity.
Ques. What method will you use to detect the gas that is released when hydrochloric acid combines with an active metal? (1 Mark)
Ans. Bring a lit matchstick close to the gas. It makes a 'pop' sound when it burns, indicating that it is hydrogen.
Ques. State the reason for the following statements: (5 Marks)
(i) Why does tap water conducts electricity while distilled water does not?
(ii) Dilute hydrochloric acid turns blue litmus red whereas Dry hydrogen chloride gas does not.
(iii) During the summer season, a milkman usually adds a very small amount of baking soda to fresh milk.
(iv) For a dilution of acid, we add the acid into water and not water into acid.
(v) Ammonia is considered to be a base but it does not contain a hydroxyl group.
Ans. (i) While tap water has ions that carry electricity, distilled water does not.
(ii) While dry HCl does not form ions, it does produce H+ and Cl–.
(iii) Baking soda prevents milk from converting to lactic acid, which turns milk sour.
(iv) Adding water to acid produces a lot of heat. As a result, water is gently added to the acid as it cools.
(v) Ammonia is fundamental in nature because it dissolves in water and forms H.13.
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