Experiment 1 Milk Acidity PDF

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PRÁCTICA 1...

Description

Experiment 1

Determination of the lactic acid content of milk Objectives

- To work on the concepts of acids and bases in aqueous solution, pH, acid-base titration, equivalence point, indicators and standard solutions - To calculate concentrations in order to prepare a solution - To perform a volumetric analysis using the titration technique Introduction

The aim of this experiment is to determine the concentration of lactic acid in a commercial milk sample by titration, that is, the gradual addition of a standard titrant solution contained in a buret, to the analyte solution of unknown composition until all the reaction is complete. A reaction is complete when the reactants are combined in their stoichiometric proportions and none of them is in excess. In a titration this is the equivalence (or stoichiometric) point. At the equivalence point of an acidbase titration a steep change of the pH of the system occurs, that can be detected either directly, by measuring the pH using a pH-meter, or indirectly, by the change in color of an indicator added to the analyte solution. The point at which the indicator changes color is called the endpoint of the indicator. In this experiment, the indicator is phenolphthalein, which changes from colorless to pink in the pH range 8-10. According to the neutralization reaction (1), when sodium hydroxide is used in lactic acid titration, the stoichiometric proportion of the reactants is 1:1 (1 mol base CH3-CHOH-COOH

+

NaOH

 

lactic acid

+

sodium hydroxide

 

1 mol acid):

CH3-CHOH-COONa + sodium lactate

+

H 2O

(1)

water

The number of the OH–moles spent in titration and the number of moles of lactic acid present in the acid sample can be calculated from the molar concentration and the volume of the NaOH solution used to reach the equivalence point.

Chemistry, Spring 2019

Chemistry

Experiment 1

The number of moles of a solute in a solution is: n = V∙M where n is the number of moles of solute, V is the volume of the solution in liters, and M is the molar concentration of the solution, defined as the number of moles of solute per liter of solution.

Equipment and Reagents ● 25-mL buret

● 100-mL beaker

● 10-mL pipet

● NaOH solution of unknown concentration

● 5-mL pipet

● Standard 0.08M oxalic acid solution

● 250-mL Erlenmeyer flask

● Phenolphthalein solution

● Plastic (or glass) funnel

● Milk sample

Safety and risk description

Personal protection: gloves/ laboratory coat/ goggles/ laboratory mask. Most important: Oxalic acid: harmful in contact with skin or if swallowed. Sodium hydroxide: causes severe burns in skin and severe eye injuries.

Experimental procedure 1. Preparing the buret

Rinse the buret at least twice, using ~5 mL of NaOH each time. Use a clean, dry funnel to fill the buret with NaOH. Open

14 mL - incorrect 17.5 mL - correct

the stopcock and allow a small quantity of the liquid to flow in order to remove eventual air bubbles. Fill the buret with NaOH up to the 0.0 mark (see Figure 1). Important: the excess NaOH solution must be collected in a 100-mL beaker to be discarded into the proper waste container.

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Figure 1. Reading the volume from a buret

Chemistry

Experiment 1

2. Titration of the NaOH solution

Sodium hydroxide is not a primary standard; therefore, the exact NaOH concentration must be determined by titration with a primary standard of 0.08 M oxalic acid (HOOC-COOH).  Use a pipet to transfer 5 mL of 0.08 M oxalic acid to a 250-mL Erlenmeyer flask. Add ~30 mL of distilled water and 5 drops of phenolphthalein.  Put a sheet of white paper beneath the flask and add NaOH dropwise from the buret, swirling the flask vigorously, until the color of the solution in the flask changes to faint-pink. The equivalence point has been reached when the faint-pink color persists for at least15 s. Important: be careful not to add an excess of base. Over-titration is a common mistake. If the color of the solution in the flask is intense pink, the equivalence point has been missed and the titration is not valid.  Read and record the volume of the NaOH solution used in the titration. Repeat the titration at least once more with a new sample of the same NaOH solution. The reaction between oxalic acid and sodium hydroxide is the following: HOOC-COOH

+

2NaOH

 

NaOOC-COONa

+

2H2O

In reaction (2), the stoichiometric proportion of the reactants is 2:1 (2 mol base

(2)

1 mol acid).

3. Titration of the milk sample

 Use a 10-mL pipet to transfer 20 mL of milk to a 250-mL Erlenmeyer flask. Use the same pipet, without rinsing it first, to add 40 mL of distilled water to the milk sample.  Add 5 drops of phenolphthalein and put a white paper beneath the flask.  Add NaOH dropwise from the buret, swirling the flask vigorously, until the color of the solution in the flask changes to pink. The equivalence point has been reached when the pink color does not fade for at least 30 s.  Repeat the titration at least once more with another sample of the same milk.

(continued on the next page)

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Chemistry

Experiment 1

Results and calculations

1. NaOH concentration

Calculate the molar concentration of the NaOH solution, from the volume of NaOH used in the titration of oxalic acid, the volume and the molar concentration of the standard solution of oxalic acid and the stoichiometry factor in reaction (2). 2. Concentration of lactic acid in the milk

Calculate the molar concentration of lactic acid in the milk, from the volume of NaOH solution that was used in the titration of milk, the exact molar NaOH concentration determined in point 1 above, the volume of the milk sample and the stoichiometry factor in reaction (1). 3. Dornic Acidity

One of the most common ways to express the acidity of dairy products is in Dornic degrees (°D). One °D is equal to 0.01 g of lactic acid per 100 mL of milk. The acidity of good quality fresh milk is usually between 16 and 20 ºD.  Calculate the molar mass of lactic acid and then the Dornic acidity of the milk.

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