pH of Samples

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 pH of samples and how to determine that, keep on reading this article!   

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 colour change. 

A pH paper changes colour in different solutions due to the chemical flavin. This molecule, which is an anthocyanin, is soluble in water and changes colour in the presence of various types of solutions.

The packaging a pH paper comes in often includes a colour-coded scale indicating the pH. This works in the following ways –

Solution Type

Colour

Acidic

Red

Basic

Greenish-blue

Neutral

Light green


Universal Indicator

It is a mixture of several indicators and demonstrates a range of colours 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

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Aim:

To find the pH of the following samples using A) pH paper or B) Universal Indicator. The samples are -

  1. Dilute HCl

  2. Dilute Solution of NaOH

  3. Salt NaCl

  4. Dilute solution of Acetic(ethanoic) acid (CH3COOH)

  5. Lemon juice

  6. Water

  7. 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 colour 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 piece 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 colours in the chart given above. 

  • Note the pH given against the colour which tallies with developed colour on pH paper. 

  • Repeat the same procedure for other samples as well and note their pH in the observation table. 


Observation

Sample

pH Paper Colour

A

Red

B

Purple

C

Red

D

Yellow

E

Orange

F

Green

G

Blue


Result and Conclusion

Samples

Solution

pH Paper Colour

pH

Nature

A

Dilute HCl

Red

1

Strong acid

B

Solution of dil NaOH

Purple

14

Strong base

C

Solution of NaCl

Red

1

Strong acid

D

Dilute CH3COOH

Orange

3

Weak acid

E

Lemon juice

Orange

2

Weak acid

F

Water

Green

7

Neutral

G

Dilute NaHCO3

Light Blue

9

Weak base


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 colour of the solution with the pH colour chart and note the pH on 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

  1. Acid samples have a pH of less than 7. For example, pH value of lemon juice. 

  2. Basic samples have a pH of more than 7. For example, pH of sodium bicarbonate solution. 

  3. Neutral samples have pH equals to 7. For example, 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 

<5

5.5

6

6.5-7.5

7.5-8.5

>8.5

Strongly acidic

Moderately acidic

Slightly acidic

Neutral

Appropriate for most crops.

Moderately alkaline 

Strongly alkaline


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.  

Water Type

pH

Reverse osmosis distilled water

5-7

Tap water

About 7.5

Common packaged bottle

6.5-7.5

Water bottles mentioning alkaline

8.9

Ocean water

About 8

Acid rain

5-5.5


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 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?

  1. S.P.L Sorenson

  2. Henry Moseley

  3. Benjamin Franklin

  4. Wilhelm Rontgen

2. What is the pH of Saliva after a Meal?

  1. 4.8

  2. 5.8

  3. 6.8

  4. Less than 4

Answers: 1-A, 2-B. 

For more information regarding the pH of samples, stay tuned to Vedantu’s website. We have several practical lessons on our website; you can also download our app to stay updated about all the vital topics of chemistry on the go. 

FAQ (Frequently Asked Questions)

1. What is the Function of a pH Paper?

Ans. By using a pH paper, we can determine if a particular solution is basic, acidic or neutral. On dipping the paper in the solution, the colour changes, indicating the nature of the solution. 

For example, when a paper turns deep greenish-blue, the pH of that solution is around 11 to 14. 

2. What is the pH of Blood?

Ans. The pH of our blood ranges from 7.35 to 7.45. It indicates that blood is little alkaline or basic in nature. On the contrary, the pH of stomach acid is between 1.5 and 3.5, making it acidic. 

3Why is a pH Paper not Accurate?

Ans. pH cannot measure the extreme pH values like 0 or 14. For example, if pH of a solution is below 0, it does not show the accurate value of it. 

4What Happens When a Body is Acidic?

Ans. When body fluids become too acidic, it can result in acidosis. This condition happens when lungs and kidneys cannot keep balance in a body’s pH. Acidosis may lead to several health issues, and sometimes it can be life-threatening as well.