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Titration • In this presentation you will: • explore the titration method to find the concentration of a solution Next >

Introduction Titration is a method of analysis that allows us to determine the exact concentrations of solutions, such as acids and bases. The titration method is very widely used. Among other applications, it is used for analysis in hospitals and in the petrochemical industry. To explore and understand the titration method, it is important to understand the way in which acidic and basic (alkaline) solutions are distinguished. Next >

Number of moles of compound = mass in grams / molecular mass Number of moles of element = mass in grams / atomic mass Titration Calculations In order to perform titrations and successfully interpret the results taken from titration experiments, we need to be able to understand and use certain chemical equations and units. The mole is a quantity of substance that is equivalent to 6.022 × 1023 atoms or molecules of the substance. To calculate the number of moles of a substance, one of the following formulas is used. Next >

Number of moles of HCl = 9/36 Number ofmoles of HCl = 0.25 Titration Calculations To calculate the number of moles in 9 g of HCl (hydrochloric acid): Firstly, work out the molecular mass of HCl: Atomic mass of H = 1 Atomic massofCl = 35 Then, molecular mass of HCl = 36 The number of moles can then be found by dividing the number of grams by molecular mass. Next >

Molarity (M) = Number of moles (mol) Volume in liters or No. of moles = Volume in liters × Molarity Titration Calculations The molarity of a solution is measured in moles per liter. Molarity uses the symbol M and is a measure of the concentration of a solution. To calculate the molarity of a solution from the number of moles in a solution and its volume, the formula is: Next >

Molarity = 0.25 / 0.1 = 2.5 mol/L Titration Calculations All volumes must first be converted to liters (L). Therefore, a 100 mL solution of HCl that contains 0.25 moles of HCl has a molarity given by: Next >

pH Scale Acidic and basic (alkaline) solutions are distinguished using a scale known as the pH scale. The pH scale is a measure of the concentration of hydrogen ions (H+) that are present in a solution. Strong base A strong acid has a very high concentration of hydrogen ions and a low pH. A strong base has an extremely low concentration of hydrogen ions and a high pH value. Strong acid The extremes of the pH scale occur around pH 0 and pH 14. Next >

pH values of some substances H+ concentration moles per liter Sodium hydroxide Potassium hydroxide Milk of Magnesia Borax pH = -log10[H+] Sodium bicarbonate Distilled water Boric acid Vinegar Lemon juice Hydrochloric acid Nitric acid pH Scale The pH scale measures the concentration of hydrogen ions in an aqueous solution. The pH value itself is the negative logarithm of the H+ ion concentration: Next >

pH values of some substances H+ concentration moles per liter Sodium hydroxide Potassium hydroxide Milk of Magnesia Borax Sodium bicarbonate Distilled water Boric acid Vinegar Lemon juice Hydrochloric acid –log10(1) = 0 and –log10(10-14) = 14 Nitric acid pH Scale The concentration of hydrogen ions in most aqueous solutions is between 1×10-14 M and 1 M. Therefore, the range of pH values for these solutions is between 0 and 14 since: Next >

pH values of some substances H+ concentration moles per liter Sodium hydroxide Potassium hydroxide Milk of Magnesia 1 ×10-9 M [H+] = pH 9 Borax Sodium bicarbonate Distilled water Boric acid Vinegar Lemon juice Hydrochloric acid Nitric acid pH Scale The pH of any solution where the concentration of H+ ions is 1×10-n M is the positive numerical value of n. For example: Next >

pH values of some substances H+ concentration moles per liter Sodium hydroxide Potassium hydroxide Milk of Magnesia Borax Sodium bicarbonate Distilled water Boric acid Vinegar Lemon juice Hydrochloric acid Nitric acid pH Scale Pure water at 25 °C (298 K) has a concentration of H+ ions of 1 × 10-7 M, hence a pH of 7. A solution of pH 7 at 25 °C is neutral. The higher the pH of a solution, the more basic (alkaline) it is. The lower the pH, the more acidic it is. Next >

Question 1 What is the pH value of a neutral solution at 25 °C? A) 0 B) 1 C) 7 D) 14 Next >

Indicators Some chemical substances react to different pH levels by changing color. These substances, known as indicators, are used in titration experiments to establish the point at which an acidic solution and an alkaline solution neutralize each other. Next >

Indicators Different indicators change color according to the pH of the solution to which they are added. One color will represent a solution above a certain pH and another color will represent a solution below a certain pH. Next >

Indicators For example, the indicator litmus will show red in acidic solutions and blue in alkaline solutions. Other indicators change color at different acid-alkaline levels. By selecting the right indicator, we can establish the pH level of nearly any colorless aqueous solution. Next >

Question 2 Why is an indicator used in an acid-base titration? 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. Next >

Burette used to measure volume oftitrant needed toneutralize the analyte Titrantof knownconcentration Analyte of unknownconcentration Known volumeof analyte Titration Titration is a laboratory technique used to determine the concentration of an acidic or alkaline solution. The aim of a titration is to measure the volume of a titrantof known concentration needed to neutralize an analyteof unknown concentration. Next >

Burette used to measure volume oftitrant needed toneutralize the analyte Titrantof knownconcentration Analyte of unknownconcentration Known volumeof analyte Titration As the volume of the titrant and its concentration are known from the titration, the number of moles necessary to neutralize the analyte can be calculated. Next >

Titration An example of a titration is when an acidic solution of known concentration, called the standard acid (titrant), is gradually added to a known volume of alkali (analyte) until a pH meter reads exactly 7 (or an indicator pigment dissolved in the mixture changes its color). The volume of titrant needed to do this is measured. This indicates that the equivalence point has been reached, and the analyte solution is neutralized. Next >

Conducting Titrations The equipment required to carry out a titration typically includes: • Retort stand • Clamps • Burette • Erlenmeyer flask • Graduated measuring tube • Funnel • Plastic pipette Next >

Conducting Titrations Using the filter funnel, transfer the titrant solution to the burette, and record the exact volume in the burette. Next, measure a volume of analyte solution, and place this in the Erlenmeyer flask. Add a few drops of indicator to the analyte solution. Next >

Conducting Titrations Slowly titrate the solution from the burette into the Erlenmeyer flask. Allow the titrant solution to run through the tap as a slow but constant stream of drops. Swirl the Erlenmeyer flask continually to disperse the titrant through the solution. Next >

Conducting Titrations The end-point of the titration is shown by a permanent color change from the indicator. Stop the titrant flow when this occurs. Record the amount of titrant needed to cause a permanent color change. Next >

Conducting Titrations Now repeat the titration, adding the titrant from the burette to the Erlenmeyer flask onedrop at a time once you are within 1 mL of the first titration endpoint. Stop adding the titrant when the equivalence point is reached and record the volume of titrant used. Next >

Number of moles of titrant x 1000 Molar concentration of analyte = Volume of analyte Titration The molar concentration of the analyte can be calculated from the number of moles of titrant needed to neutralize the analyte and the initial volume of analyte. Provided the stoichiometric relation between the base and the acid is 1:1, the number of moles of analyte will be equal to the number of moles of titrant used in its neutralization, and because of that: Next >

Question 3 What is the final aim of a titration experiment? A) To find out the volume of the titrant solution added to the unknown analyte solution. B) To find out the concentration of the titrant solution added to the unknown analyte solution. C) To find out the volume of the unknown analyte. D) To find out the concentration of the unknown analyte. Next >

15.8 mL of acid (titrant)of 0.25M concentration 25 mL alkali (analyte) of unknownconcentration Calculating Concentration In the following worked example, an acid solution is the titrant, and an alkaline solution is the analyte. 25 mL of an alkaline solution was neutralized when 15.8 mL of a 0.25M acid was added. Next >

15.8 mL of acid (titrant)of 0.25M concentration 25 mL alkali (analyte) of unknownconcentration number of moles of acid = (volume in mL × molar concentration) / 1000 Calculating Concentration To calculate the unknown concentration of the titrated solution, the number of moles of acid that neutralize the solution must first be calculated. This is calculated from: So the number of moles of acid = (15.8 × 0.25) / 1000 = 3.95 × 10-3 moles Next >

Question 4 How is the number of moles of titrant used in a titration calculated? A) Divide the volume of titrant (in mL) by its concentration (M) and divide this amount by 1000 B) Multiply the volume of titrant (in mL) by its concentration (M) and divide this amount by 1000 Next >

3.95 × 10-3 moles (moles of acid × 1000) molar concentration of alkali = volume of alkali in mL (3.95 ×10-3 ×1000) = 0.158M 25 Calculating Concentration Once the number of moles of acid (titrant) that neutralized the alkaline solution (analyte) is known, the concentration of the analyte can be calculated. Assuming the stoichiometric ratio of acid:alkali as 1:1 then: The concentration of the alkaline solution (analyte) was therefore 0.158M. Next >

Question 5 In a titration experiment, what name can ALWAYS be given to the solution of unknown concentration in the Erlenmeyer flask? A) Acid B) Alkali C) Titrant D) Analyte Next >

Titration Curves A titration curve is a graph representing the pH change in an unknown solution during a titration experiment. The curve is drawn by plotting the volume of titrant added on the x-axis and pH on the y-axis. Next >

Titration Curves The titration of a strong acid (analyte) with a strong base (titrant) 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: 1. The initial pH of the solution. 2. The equivalence point when equimolar amounts of both acid and base are present. Final pH equivalence pH range Equivalence point 3. The volume of titrant required for equivalence. Equivalence titrating volume Initial pH 4. The pH range at equivalence. 5. The final pH. Next >

Question 6 What does a titration curve represent? 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. Next >

Summary In this presentation you have seen: • the purpose of titration • the pH scale • what indicators do • how to calculate the concentration of a solution End >

In this presentation you will: