colorimetric determination of ph

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Colorimetric Determination of pHJudy Ann M. CocadizLouise Joy A. Martin

The Experiment VIObjectives of

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Determine colorimetrically the pH of an unknown solution.Calculate the ionization constant of weak acid.

Objectives

What is Colorimetry?An Introduction

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pH levels are used to express the concentration of H+ in an aqueous solution. These will let us classify whether it is acidic, basic or neutral. The pH levels can be determined either by using pH paper, pH meter or the Colorimetric method.

Introduction

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Colorimetry employs the use of complex organic dyes which change to distinctive colors through a specific pH range of the aqueous solutions. These dyes are called indicators.

Definition of Colorimetry

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pH IndicatorThe Acid-base indicators are weak acid or bases which can be used asa tool in colorimetry. The indicators are halochromic (a material which changes color when a change inpH occurs) chemical compound that, when added in solutions, can help detect changes in pH and present the result visually through color change

pH IndicatorsThis change in color occurs in narrow ranges of pH because of the equilibrium shift that occurred in the reaction. Two or more pH indicators are needed to determine the pH level of the solution.

IndicatorLower pH colorpH rangeHigher pH colorThymol blueRed1.2-2.8YellowBromophenol blueYellow3.0-4.6PurpleChlorophenol redYellow4.8-6.4VioletBromothymol blueYellow6.0-7.6BluePhenol redYellow6.8-8.4Red

The following is a table of examples of pH indicators with their respective pH range for color transition to occur and respective colors in pH levels higher and lower than this pH range8

McIlave Buffers

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Ionization Constant of a Weak AcidWith the colorimetric determination of pH, it is important to know the ionization constant of the weak acid involved. The ionization constant is the extent to which the substance will dissociate in water. Since weak acids only partially dissociate, they have low ionization constants.

Aldehyde ->RCHOKetone -> RCOR10

Ionization Constant of a Weak Acid

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Henderson-Hasselbalch equation

ofpH:

ColorimetricDetermination

ExperimentProcedure

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Part A: Preparation of Buffer SolutionsPrepare a set of McIlave Buffers in test tubes of uniform sizes labelled according to their respective pH levels.

2.02.22.42.62.8

3.0

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Preparation of McIlave BufferspHmL DSPmL CApHmL DSPmL CApHmL DSPmL CA2.20.209.804.24.145.866.26.613.392.40.629.384.44.415.596.46.923.082.61.068.914.64.675.336.67.342.662.81.588.424.84.935.076.87.722.283.02.057,955.05.154.857.08.241.763.22.477.535.25.204.807.28.691.313.42.857.155.45.584.427.49.080.923.63.226.785.65.804.207.69.370.633.83.556.455.86.053.957.89.570.434.03.256.156.06.313.698.09.720.28

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Part A: Preparation of Buffer SolutionsAdd five drops of the appropriate indicators for each pH level to each of thebuffer solutions.

6.26.46.6

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Part A: Preparation of Buffer Solutions

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Add equal amounts of 0.01M HOAc placed in five test tubes each labelled with the 5 indicators. Add 2 drops of proper indicator to the following solutions, then determine pH.Part B. Colorimetric Determination of pH

Thymol Blue

Bromophenol BlueChlorophenol RedBromothymol BluePhenol Red

Compare each of the resulting colors to the previously prepared buffer solutions in Part A while limiting the comparison to those included in the pH range of the indicatorusedPart B. Colorimetric Determination of pH

For example, if thymol blue is used, the buffers that will beused for comparison will be those labelled 2.2,2.4, 2.6 and 2.8.If the color of the solutionmatches that of a buffers, the pH of the solution is taken as the same as the pH of that bufferIf the color of the solutiondoes not match that of any ofthe buffers applicable, other test tubes of the remaining indicators will be used for comparison until the colors match24

Part B. Colorimetric Determination of pH

BCD

ResultsExperimental

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Test Tube A

Test Tube A

Test Tube A

Test Tube B

Test Tube B

Test Tube C

Test Tube C

Test Tube D

Test Tube D

Overall Results

discussionExperiment VI

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Appropriate indicators for certain pH ranges must be used because there will be no color transition in pH levels higher or lower than the respective range of the indicator. Discussion: Preparation of Buffer Solutions

Overall Results

Discussion:

Discussion:

Discussion:

Discussion:

Sources:http://chemwiki.ucdavis.edu/Physical_Chemistry/Acids_and_Bases/Buffers/Henderson-Hasselbalch_Approximationhttp://www.carlos-arano.com.ar/pHing.htmlhttp://www.anaesthesiamcq.com/AcidBaseBook/ab1_4.php

Thank you :D

colorimetric determination of ph

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