ClassAct SRS enabled. • In this presentation you will: • Explore the titration method to find the concentration of a solution
Titration is a method of analysis that allows us to determine the precise equivalence point of a reaction and therefore the precise quantity of reactant added to the titration flask. From this information we can calculate the concentration of the reactant. To learn about the titration method, it is important to understand first what the pH level of a base or an acid is. Next >
pH values of some substances H+ concentration moles per liter Sodium hydroxide Potassium hydroxide Magnesia milk Borax Sodium bicarbonate Distilled water Boric acid Vinegar Lemon juice Hydrochloric acid Nitric acid pH Scale The concentration of hydrogen ions in an acid aqueous solution can be measured with the pH scale, where the pH value is the negative logarithm of the oxonium ion concentration: pH = -log[H3O+] The concentrations of hydrogen ions in most acid aqueous solutions are between 10-14 mol dm-3 and 1 mol dm-3. Therefore, the range of values of these solutions are between 0 and 14 since: –log10(1) = 0 and –log10(10-14) = 14. Next >
pH values of some substances H+ concentration moles per liter Sodium hydroxide Potassium hydroxide Magnesia milk Borax Sodium bicarbonate Distilled water Boric acid Vinegar Lemon juice Hydrochloric acid Nitric acid pH Scale The pH of any solution when the concentration of hydrogen ions is written as 1 x 10-n will be the numeric value of n. For example: 1 x 10-9 mol dm-3 = pH 9. Pure water at 25 ºC has a concentration of hydrogen ions of 1 x 10-7 mol dm-3, hence a pH of 7. Every solution of pH 7 is neutral. The higher the pH of a solution, the more basic or alkaline it is. The lower the pH, the more acidic it is. Next >
Question 1 What is the pH value of a neutral solution? A) 0 B) 1 C) 7 D) 14
Indicators Some chemical substances react to different pH levels by changing their color. These substances, called indicators, are used in titration experiments to establish the acidic-alkaline values of substances and mixtures. Next >
Indicators Different colorant indicators have different color combinations which change at different pH levels. One color will represent an acidic solution and the other color a more basic solution. For example, the litmus indicator will show red in acid solutions and blue in basic solutions. Other indicators change color at different acid-alkaline levels. By selecting the right indicator we can establish the pH level of nearly any aqueous colorless solution. Next >
Question 2 What is an indicator used for in a titration experiment? A) To establish the acidity-alkalinity of a solution. B) To increase the acidity of a solution. C) To increase the alkalinity of a solution. D) To alter the pH level of a solution.
Titration Titration is a laboratory technique used to determine the concentrationCr of an acid or base solution. The aim of the experiment is to find out the volume Vs of a titrating substance (with a known concentration Ck) necessary to neutralize the acidity or alkalinity of a substance. With the volume of the titrating substance and its concentration, the number of moles necessary to neutralize the substance can be calculated. Next >
Titration In a titration, an acid solution of accurately known strength called standard acid is gradually added to a known volume of base until a pH meter reads exactly 7 or an indicator pigment dissolved in the mixture changes its color. The change of color of the indicator signifies the reaching of the equivalence point, where the substances neutralize each other. Next >
Titration The unknown concentration of the base can then be calculated based on the number of moles necessary to neutralize each other, together with the base volume, with the equation: where Cr is the concentration of the base solution, Nr is the neutralizing number of moles, and Vr the volume of the titrated solution. Next >
Question 3 What is the final aim of a titration experiment? A) To find out the volume of the titrating substance added to the titrated solution. B) To find out the concentration of the titrating substance added to the titrated solution. C) To find out the volume of the titrated solution. D) To find out the concentration of the titrated solution.
So the acid’s moles in this experiment = Calculating Concentration 25 cm3 of a base changed color (was neutralized) when 15.8 cm3 of a 0.025 mol dm-3 acid was added. To calculate the unknown concentration Cr of the titrated solution, the number of moles in the acid which neutralized the solution needs to be calculated first. This is given by: number of moles Nr = volume in dm3 Vr x acid concentration in mol dm-3 Cr = 3.95 x 10-4 mol Next >
Question 4 How is the number of moles of a titrating substance calculated? A) Divide the volume by its molarity concentration. B) Multiply the volume by its molarity concentration.
3.95 x 10-4 mol Calculating Concentration Knowing the number of moles that were necessary to neutralize the base solution, its concentration can be calculated. Applying the moles formula to the base solution, we obtain: Base moles Nr = volume in dm3 Vr x acid concentration in mol dm-3 Cr 3.95 x 10-4 mol = (25 dm3 / 1000 ) x Cr Therefore; The concentration of the base solution is therefore 0.0158 mol dm-3 Next >
Question 5 How is the concentration of a titrated solution calculated? A) Multiplying the concentration of the titrating substance by its volume. B) Multiplying the number of moles in the titrating substance by its volume. C) Dividing the concentration of the titrating substance by its volume. D) Dividing the number of moles in the titrated substance by its volume.
Titration curves A titration curve is a graph representing the pH change in a unknown solution during a titration experiment. The curve is drawn by plotting the data obtained during the titration, titrant volume on the x-axis and pH on the y-axis. The titration of a strong acid with a strong base produces the titration curve shown in the diagram. The titration of a strong base with a strong acid will produce a descending curve in the pH axis. Next >
Titration curves The important features of titration curves are: Final pH 1. The initial pH of the solution. 2. The equivalence point when equimolar amounts of both acid and base are present. equivalence pH range 3. The volume of titrating liquid required for equivalence. Equivalence point 4. The pH range at equivalence. 5. The final pH. Equivalence titrating volume Initial pH Next >
Question 6 What does a titration curve represents? A) The change of the equivalence point of an unknown solution during a titration experiment. B) The change of the number of moles in an unknown solution during a titration experiment. C) The pH change in an unknown solution during a titration experiment. D) The molarity change in an unknown solution during a titration experiment.
Summary Having completed this presentation you should be able to: • Show knowledge and understanding of what a titration experiment is. • Show knowledge and understanding of the pH scale. • Show knowledge of what indicators are. • Show understanding of how to calculate the concentration of a solution. End >