Conventional Methods Of Measuring PH

This experiment was carried out to get a better understanding of what the pH of certain solutions are by carrying out measurements using pH meter and pH paper and why they are acidic or alkaline. These solutions varied in terms of their number of moles, but a 0. 1M concentration of each solution was ensured throughout.

Introduction

Acids and bases play a significant role in our lives, but we hardly pay attention to that. The use of acids. In this experiment, we look into the connection of pH to the molarity of hydronium ions. In order to find the pH, we have used 2 methods- the pH meter and the pH meter. The pH meter is a device that has to be calibrated regularly in order to give accurate results while the pH paper are thin strips of paper that changes colour depending on the property of the solution.

Power of Hydrogen (pH)

First introduced by the Danish chemist Soren Peder Lauritz Sorensen in 1909 and further revised in 1924, “pH” is a figure or number to express the acidity or alkalinity of a solution on a logarithmic scale from 1-14. (i. e. Each number represents a 10-fold change in the acidity/alkalinty of the water. A solution with a pH of five is ten times more acidic than solution having a pH of 6). pH = -log10[H+] or -log10[H3O+]The range from pH 1 till pH below 7 is deduced as acidic while the alkaline substances have the pH range above 7 until 14. Additionally, a pH of exactly 7 is neutral.

The value for pH depends on the amount of hydrogen ion or hydroxyl ion present in an aqueous solution. The higher the concentration of hydrogen ion present in an aqueous solution, the lower the pH value and the more acidic the solution. Similarly, the higher the concentration of hydroxyl ions, the higher the pH and the more alkaline the solution.

Acid vs Base

An acid and a base can be defined in 3 different ways.

Arrhenius theory: According to this theory, a substance which originates H+ (aq) ions in aqueous solution is Acid and the opposite compound which produces OH- (aq) ions aqueous solution is called base. Bronsted-Lowry theory: substances that can release proton is acid and substances which accept proton is base. Lewis theory: the base is the electron pair donor and acid is the electron pair acceptor.

Hydrolysis

Hydrolysis is a chemical reaction in which water is used to break down a compound; this is achieved by breaking a covalent bond in the compound by inserting a water molecule across the bond. The opposite of this is a dehydration-condensation reaction. There are three main types of hydrolysis in the chemical world, which are the acid hydrolysis, base hydrolysis, and salt hydrolysis.

Buffer

A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base, or vice versa. The pH of this solution changes very little when a small or moderate amount of strong acid or base is added to it and that’s why it is used to prevent changes in the pH of a solution. Thus, in this experiment, we see how the pH differs for different solutions.

Hypothesis

The pH system is a numerical scale that indicates the acidity of an aqueous solution. It can be used to ascertain the approximate hydrogen ion molarity. Depending on the selected solutions, those with pH less than 7 will show acidic properties and those with a pH more than 7 will show alkaline properties. Additionally, a pH of 5. 0 would indicate hydrogen ion concentration of 1x10-5. The two conventional ways of measuring pH will help us with identifying which solutions are acids and bases.

Procedures:

At the beginning of the experiment, the pH meter was calibrated using standard pH of a few known solutions with pH of 4, 7 and 10. The procedure is as follows:

1. Take out the electrode from the cab and rinse with deionized water and dry it with tissue paper.
2. Place the electrode in the pH 7 buffer solution, allow the display to stabilize and take out the electrode once stabilized
3. Rinse the electrode and repeat the step 3 for pH 4 and pH 10 buffer solution.
4. The pH of the samples, the buffer solution and hot water were all measured using the following procedure:
5. Rinse the electrode with deionized water and dry it.
6. Place the electrode in the sample and wait for the reading to stable. 4. Take the stable reading as the final reading of pH.
7. Measuring pH using pH paper.
8. The pH of the samples, the buffer solution and hot water were all measured by dipping the pH paper into the solution. The change of colour was noted and the colour series was identified. The corresponding value of pH was found.

Results

Each experiment for pH meter was repeated 5 times and the average was calculated. For pH paper, the experiment was carried out once.

Discussion

Due to the experiment, we now have a better understanding of how we can use two conventional methods of measuring pH- the pH meter and the pH paper- in order to find the pH. While pH papers are surely easier to handle, it comes with several drawbacks. Prolonged exposure of pH paper in the air might change the color series as air contains impurities and the dust particles may stick to the paper. It must be ensured that the pH paper is not held with naked or wet hands, and more preferably held with forceps. After dipping the pH paper, the color series is referred to a color chart of the pH paper, ensuring that the lighting of the room is adequate for the eye to distinguish each colour and avoid errors.

For pH meter, the electrode of the pH meter must be always immersed and kept in distilled water, because this electrode is highly sensitive. Before and after every use of pH meter, the electrode must be rinsed to prevent inaccuracy of result. While dipped for measurement, the electrode must not reach the bottom of the beaker or beaker. It should be held in and upright position and not be shaken or moved. It is also recommended to take the readings every two minutes because that is how long it should take to achieve a constant value.

We can tell that 0. 1M hydrochloric acid is the strongest acid because it has the lowest value, followed by 0. 1M boric acid and 0. 1M ammonium sulphate. The 0. 1M sodium chloride and household chemical is neutral, while 0. 1M sodium hydroxide is the strongest base, followed by 0. 1M sodium bicarbonate and 0. 1M aqueous ammonia. The substances with numbers higher.

We can also see that when hydrochloric acid was added into the buffer solution and to the deionized water, only slight changes were observed in the recorded readings of the pH of the buffer solution. This is because buffer solutions are used to keep pH at a nearly constant value when acid or base is added to it. Since the buffer is a solution consisting of a weak acid and its salt, upon adding a base, the hydroxide ions will react with the hydronium ions present. Then more acetic acid reacts with the water, thus restoring the hydronium ion concentration.

CH3CO2H + OH− ⇌ CH3CO2− + H2O

However, when we add in an acid, most of the hydronium ions from the acid will react with the acetate ions, forming acetic acid molecules. A reversible reaction also takes place here, restoring the acetate ion concentration.

CH3CO2− + H3O+ ⇌ HC2H3O2 + H2O

Additionally, as we can see in table 1. 5 the pH of the deionized water changed drastically when HCl or NaOH was added. Thus there is little increase in the concentration of the hydronium ions, and the pH remains practically unchanged or slightly changed.

We can decipher that deionized water, when boiled, has a pH lower than 7 due to an excess of hydrogen ions. This is because according to Le Châtelier's Principle, if you make a change to the conditions of a reaction in dynamic equilibrium, the position of equilibrium moves to counter the change you have made. Hence, if you increase the temperature of the water, the equilibrium will move to lower the temperature again. The forward reaction is favored, and more hydrogen ions and hydroxide ions are formed. A pH of 6. 09 is still considered neutral for water at that temperature. Additionally, the pH of the deionized water changes 5 minutes after the source of heat was removed. It continues to become more acidic due to dissolving of air.

Some precautions we should keep in mind is to take readings at eye level to avoid parallax errors. We should ensure solutions are measured in measuring cylinders using their lower meniscus. The electronic balance should be airtight to prevent dust to get in the way. Clumps of powder should be broken down because they may contain air in them. Upon stirring the powder into water, it must be ensured that all the particles dissolve. Every apparatus must be rinsed with distilled water before use to avoid contamination.

Conclusion

From this experiment, we found that HCl, H3BO3, (NH4)2SO4, and NaCl are acidic solutions, and NaOH, NH3(aq), NaHCO3, and the household chemicals are basic solutions. We also witnessed how the buffer solution resists to change in pH and the reason why this happens. We have also seen the effect of hydrolysis.

Furthermore, we have found that pH value can be measured with two conventional methods- pH meter and pH paper. But through our experiment we have understood that pH meter gives a more precise result, as it takes value in 2 decimal places while the pH paper only determines the pH value in whole numbers. The hypothesis is therefore accepted.