A Brief Note on Determining the pH of Samples
Chemicals have their distinct nature - they can be neutral or acidic or basic. These characteristics primarily depend on how many H+ or OH- ions a particular chemical is releasing in its aqueous solution.
For example, chemicals that release H+ ions are acidic, whereas those which release OH- ions are basic.
These experiments are an integral part of Class 10 practical chemistry syllabus. Hence, to know about the pH of samples and how to determine that, keep on reading this article!
Introduction
pH, a measure of acidity or authenticity of aqueous or other liquid solutions. pH is one of the most common terms in the field of chemistry, biology, and agronomy, converting the value of hydrogen ions — typically between 1 and 10-14 (1 g) per liter — into numbers between 0 and 14. In clean water. , neutral hydrogen ion concentration is 10-7 gram-equivalents per liter, Solution with a pH of less than 7 is considered acidic; a solution with a pH greater than 7 is considered basic, or alkaline.
The measure was originally used by Danish biochemist S.P.L. Sørensen represents a mixture of hydrogen ions, expressed in proportion to each liter of liquid solution: pH = −log (H +)
Definition of pH
The International Union of Pure and Applied Chemistry (IUPAC) has a slightly different pH scale based on electrochemical estimates of standard bath solution. Basically, the description uses a number:
pH = -log aH +
where aH + represents hydrogen activity, which is the active combination of hydrogen ions in solution. This may be slightly different from the actual focus. IUPAC pH measurement includes thermodynamic factors, which can affect pH.
In most cases, a standard definition of pH is sufficient.
PH scale
The pH range is from 0 to 14. The pH of a solution is a measure of its acidity or alkalinity (base). You have probably used litmus paper, a paper that has been treated with a natural water-soluble dye to be used as an indicator of pH, to check how much acid or base (alkalinity) is present in the solution. You may have used some to ensure that the water in the outdoor pool is handled properly.
PH measurement
The pH of a solution can be determined using a tool such as a pH meter or various indicators that change their color accordingly depending on the type of test solution.
PH Meter: A measuring device called a pH meter can give pH up to two decimal places.
PH meters are measured using bath solutions with pH known accuracy.
Measure pH more accurately than indicators.
Now the pH days are used to measure the pH.
Indicators: It is a substance that can be used to identify the nature of a chemical due to its color change.
The indicator changes color when the pH of the solution changes slightly.
If you use clues, it is best not to color the solution to be tested. If the solution is coloured, it can affect the color of the indicators.
The universal index or pH paper provides a relative pH value.
Test
Hydrochloric acid (HCl):
Hydrochloric acid occurs as an aqueous solution or as a gas of hydrogen chloride (anhydrous hydrochloric acid).
Hydrogen chloride gas is a colorless, non-combustible, edible gas with an offensive odor.
When added to water it dissolves completely to form H+ ions and Cl– ions.
Its pH is < 7.
Used in composting, textile and rubber industries.
Color Produced in the Universal Index by Dilute Hcl.
Sodium hydroxide (NaOH):
It is often called caustic soda and is a solid base, naturally decomposing.
When soaked in water it separates completely to form Na+ and OH– ions.
Its pH is > 7.
Used to make soaps and detergents.
Theory:
Definition of pH
pH is defined as the negative logarithm (base 10) of hydrogen ion concentration. It is the most common way to determine the strength of base/acid. Often it is termed as “potential of hydrogen ion”. It is the molarity of H+ ion.
Following is the pH value of different chemicals on a scale from 0 to 14.
Acid pH<7
a pH of Base>7
pH of neutral =7.
pH Scale
A pH scale is used to determine the nature of a substance. Its value ranges from 0 to 14. This scale is logarithmic. It means that if an integer value increases or decreases, the concentration changes tenfold, accordingly.
For example, pH 2 is ten times more acidic than pH 3. Furthermore, pH 12 is 100 times more basic than pH 10.
pH Paper
A pH paper is a piece of paper used to find out if a solution is basic, acidic or neutral. This is determined by dipping part of the paper into a solution and observing the color change.
A pH paper changes color in different solutions due to the chemical flavin. This molecule, which is an anthocyanin, is soluble in water and changes color in the presence of various types of solutions.
The packaging a pH paper comes in often includes a color-coded scale indicating the pH. This works in the following ways –
Universal Indicator
It is a mixture of several indicators and demonstrates a range of colors based on the pH of any particular solution. It is generally available in two forms, liquid solution of ethanol and pH paper, soaked in indicator solution.
pH Value Chart
Aim:
To find the pH of the following samples using A) pH paper or B) Universal Indicator. The samples are -
Dilute HCl
Dilute Solution of NaOH
Salt NaCl
Dilute solution of Acetic(ethanoic) acid (CH3COOH)
Lemon juice
Water
Dilute solution of sodium bicarbonate (NaHCO3)
Materials Required
7 test tubes
Test tube stand
Dropper
White glazed tile
pH paper universal indicator
Distilled water
Glass rod
Measuring cylinder
pH color chart
Solutions of given samples
For salt, make a solution by dissolving 1 gm salt into 10 mL distilled water.
Procedure
Using a pH paper,
Take clean and dry 7 test tubes.
Label the test tubes as A, B, C, D, E, F, G, and place them in order in a test tube stand.
Take 10 mL of each given sample in the test tubes.
Place a small portion of pH paper (about 2 cm) on a white glazed tile.
With the help of a glass rod or dropper, transfer 1 to 2 drops of dil. HCl from test tube A on the pH paper.
Compare it with the colors in the chart given above.
Note the pH given against the color which tallies with developed color on pH paper.
Repeat the same procedure for other samples as well and note their pH in the observation table.
Observation
Result and Conclusion
Using Universal Indicator,
Take clean and dry 7 test tubes.
Label the test tubes as A, B, C, D, E, F, G, and place them in order in a test tube stand.
Take 10 mL of each given sample in the test tubes.
Then add two drops of BDH (British Drug House) Universal indicator in each test tube.
Match the changed color of the solution with the pH color chart and note the pH on the observation table.
You will see that the observation table is looking the same as above. It indicates that you can use any of these two methods to determine the pH of any sample.
Inference:
Acid samples have a pH of less than 7. For example, the pH value of lemon juice.
Basic samples have a pH of more than 7. For example, pH of sodium bicarbonate solution.
Neutral samples have pH equals to 7. For example, the pH of water samples.
The pH of soil determines if the soil is acidic or neutral or basic or alkaline. Generally, the soil has a pH range of 3 to 9. Below mentioned table shows the different pH values of different soil samples.
Soil pH
Determination of ph of Soil Samples Helps with Following Factors.
Soil pH determination helps in understanding the physical condition, nutrient availability, permeability, etc.
It can determine the potency of toxic elements of soil.
It provides an insight into the presence of microbial communities and its influence in the growth of the crops.
Based on this analysis, we can segregate the soil suited best for a specific crop.
It also ascertains the charge discharging into the soil that affects the nutrient of soils.
Along with that, Following Table also Shows the pH of Different Water Samples.
Precautions
A few precautions should be taken while determining the pH of samples. Otherwise, the result can be erroneous. Following are the tips -
Always use the freshly prepared sample solutions; even the fruit juice sample has to be fresh.
Use a rinsed and clean dropper.
Mark each test tube carefully.
Rinse the droppers and test tubes only with distilled water.
Clean tile needs to be used.
Impact of pH
The pH solution of a chemical depends on the temperature. The pH varies as the temperature changes.
The water pH is 7 at 25⁰C.
When water is heated, its pH becomes less than 7, but the water remains neutral. It happens because, at high temperature, water breaks down and produces more ions. Consequently, exactly the opposite happens when
Rack Your Brains
1. Who Discovered the pH Scale?
S.P.L Sorenson
Henry Moseley
Benjamin Franklin
Wilhelm Rontgen
2. What is the pH of Saliva after a Meal?
4.8
5.8
6.8
Less than 4
Answers: 1-A, 2-B.
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FAQs on pH of Samples
1. What does the pH of a sample signify?
The pH of a sample indicates its acidity or alkalinity (basicity). It is a measure of the hydrogen ion (H+) concentration in a solution. The scale typically ranges from 0 to 14, where a pH less than 7 indicates an acidic nature, a pH greater than 7 indicates a basic nature, and a pH of exactly 7 is considered neutral.
2. How does the logarithmic nature of the pH scale work?
The pH scale is logarithmic, meaning each whole number change represents a tenfold change in acidity or basicity. For example, a solution with a pH of 3 is ten times more acidic than a solution with a pH of 4. Similarly, a solution with a pH of 11 is one hundred times more basic than a solution with a pH of 9.
3. How can you determine the pH of samples like lemon juice or baking soda solution using a universal indicator?
To determine the pH of a sample using a universal indicator, you can follow these steps:
- Place the sample solution in a clean test tube.
- Add a few drops of the universal indicator solution to the sample.
- Observe the colour change in the solution.
- Match this new colour against a pH colour chart. The corresponding number on the chart indicates the approximate pH of the sample. For instance, lemon juice will turn reddish-orange (acidic), while a baking soda solution will turn blue (basic).
4. What are some real-world examples of acidic, basic, and neutral substances?
Many common substances can be classified by their pH:
- Acidic: Lemon juice (pH ~2), vinegar (pH ~3), and stomach acid (pH 1.5-3.5).
- Basic: Baking soda solution (pH ~9), soap water (pH ~10), and sodium hydroxide (NaOH) solution (pH ~13).
- Neutral: Pure distilled water (pH 7) and a sodium chloride (common salt) solution (pH ~7).
5. How is the pH of a solid sample, such as soil or salt, measured?
To measure the pH of a solid, it must first be mixed with a neutral liquid. For a solid like common salt (NaCl), you dissolve it in distilled water to create a solution. For a sample like soil, you create a slurry by mixing it thoroughly with distilled water. The pH of this resulting aqueous mixture is then tested using pH paper or a pH meter.
6. Why is determining the pH of soil important for agriculture?
Determining soil pH is crucial in agriculture for several reasons:
- Nutrient Availability: The pH of the soil affects how well plants can absorb essential nutrients.
- Crop Suitability: Different crops thrive in different pH ranges. Knowing the soil pH helps in selecting the most suitable crops for a particular field.
- Microbial Activity: Soil pH influences the population and activity of beneficial microorganisms that contribute to soil health.
- Toxicity: It can indicate the presence and availability of toxic elements in the soil.
7. What is the fundamental difference between a strong acid like HCl and a weak acid like acetic acid in terms of pH?
The difference lies in their degree of ionization in water. A strong acid, like Hydrochloric acid (HCl), dissociates completely, releasing a high concentration of H+ ions and resulting in a very low pH (e.g., 1-2). A weak acid, like acetic acid (CH₃COOH), only partially dissociates, releasing fewer H+ ions. Therefore, at the same concentration, a weak acid will have a comparatively higher pH (e.g., 3-4) than a strong acid.
8. Why is maintaining the pH of human blood so critical for health?
Human blood must be kept within a very narrow, slightly alkaline range of pH 7.35 to 7.45. This precise balance is vital for the proper functioning of enzymes and metabolic processes. If the body's fluids become too acidic (a condition called acidosis), it can disrupt these essential functions, leading to severe health problems. The lungs and kidneys work constantly to regulate and maintain this delicate pH balance.
9. If heating pure water makes its pH drop below 7, does that mean the water becomes acidic?
No, the water remains perfectly neutral. When water is heated, its natural dissociation into hydrogen ions (H+) and hydroxide ions (OH-) increases. Since both H+ and OH- ions are produced in equal amounts, their concentrations remain balanced, and the water is still neutral. The pH value decreases because the definition of a neutral pH (where [H+] = [OH-]) is temperature-dependent.
10. What are the main limitations of using pH paper for scientific experiments?
While useful for quick estimates, pH paper has several limitations:
- Lack of Precision: It provides an approximate pH value based on colour matching, which is not as accurate as a digital pH meter.
- Interference from Colour: If the sample solution is already strongly coloured, it can be difficult to accurately judge the colour change of the pH paper.
- Subjective Interpretation: The colour match can be subjective and vary between different observers.
- Limited Range: It may not be effective for measuring extreme pH values, such as those below 1 or above 13.
