Determination of Water Hardness Using Standardized Edta Solution

Determination of Water Hardness using Standardized EDTA Solution

Cayme, Jan Michael1, Avelino, Japheth2, Asuncion, Alfredo2

1Professor, School of Chemical Engineering, Chemistry and Biological Engineering, Mapua Institute of Technology; 2Student, CHM112L/A41, Chemical Engineering, Chemistry and Biological Engineering, Mapua Institute of Technology

ABSTRACT

In the experiment, the students need to determine the hardness of water using standardized EDTA solution. Complexometric titrations are titrations that can be used to discover the hardness of water or to discover metal ions in a solution. Ethylenediaminetetraacetic acid, also known as EDTA, is commonly used in complexometric titrations. The students were asked to use the Eriochrome Black T as an indicator and titrate it with 0.005 M EDTA until the first appearance of a clear blue. In the course of experiment, the students were able to achieve the objectives of the study. After standardizing the solution, the average molarity was found to be 4.039 x 10-3 M. On sample A, the calcium content was computed as 63.4 %, the total hardness was 0.799 ± 0.0129 ppm. On sample B, the calcium content was found to be 55.9%; the total hardness was 0.714 ± 0.0222 ppm. The higher the calcium content increases, the degree of hardness also increases.

Keywords: EDTA, Eriochrome Black T, complexometric titration, water hardness  

INTRODUCTION

Complexometric titration is a form of volumetric analysis in which the formation of a colored complex is used to indicate the end point of a titration. Complexometric titrations are particularly useful for the determination of a mixture of different metal ions in solution. An indicator capable of producing an unambiguous color change is usually used to detect the end-point of the titration. The titrant used in the experiment is an EDTA solution that is standardized using primary standard[pic 2][pic 3].

EDTA or called ethylenediaminetetraacetic acid is used in this experiment as titrant. It is an organic agent which has two or more groups capable of complexing with a metal ion is called chelating agent. Titration with a chelating agent is called a chelometric titration which is a type of complexometric titration. EDTA is a hexadanate ligand that has six pair of unshared electrons capable on complexing with a metal ion, is located on each of the two amino atoms and each of four carboxyl groups.

Eriochrome Black T is an example of indicator that is paired to EDTA. It is an acid/base indicator and metal ion indicator that is used to determine calcium and magnesium content in the water. It contains a sulfonic acid group which is completely dissociated in water and two phenolic groups partially dissociate. When combined in EDTA its initial color is red and after all the calcium ions have reacted its color changes to clear blue.  Moreover, it is used to detect the presence of rare earth metals.

Water hardness is a measure of the quantity of divalent ions such as calcium, magnesium and iron in water. Hardness is traditionally measured by chemical titration. The hardness of a water sample in milligrams per liter as calcium carbonate. Calcium carbonate hardness is a general term that indicates the total quantity of divalent salts present and does not specifically identify whether calcium, magnesium or some other divalent salt is causing water hardness. Hardness can be a mixture of divalent salts.

Water hardness is used in biological processes of fish.  It is used for bone formation, blood clotting and other metabolic reactions.  Fish can absorb calcium for these needs directly from the water or food.  The presence of free ionic calcium at relatively high concentrations in culture water helps reduce the loss of other salts like sodium and potassium from fish body fluids like blood.  Sodium and potassium are the most important salts in fish blood and are critical for normal heart, nerve and muscle function.  In low calcium water, fish can lose substantial quantities of these salts into the water.  Fish must then use energy supplied by their feed to re-absorb lost salts.  That can reduce the energy available for growth and may extend the time necessary to grow fish to market size.  For some species environmental calcium is required for good survival.

The objectives for this experiment are to let the students learn how to prepare and standardize EDTA solution, and to let them be familiar with the concepts of the complexometric titration and to determine the water hardness using EDTA solution.

METHODOLOGY

Preparation of EDTA solution

In preparing EDTA solution, dissolve about 1.0g of [pic 4][pic 5], 0.5g NaOH and 0.05g [pic 6][pic 7] in 450mL of distilled water. Dilute and mix it to 500mL of volumetric flask.

Standardization of EDTA solution

In standardization of EDTA solution, dry 2.0g of primary standard grade [pic 8][pic 9]. Weigh accurately duplicate 0.20 to 0.25 grams of the dry [pic 10][pic 11] and transfer to a 250mL Erlenmeyer flask. To each of the sample, carefully add 5mL of concentrated HCL solution. After dissolution is complete, add about 50mL of EDTA solution, distilled water while rinsing down the sides of the flask in the process. Boil for 5 minutes to remove the carbon dioxide and then cool. Transfer the cooled solution to a 500mL volumetric flask and dilute to mark using distilled water. Pipette 25mL aliquot portion of the calcium carbonate solution into a 250mL Erlenmeyer flask and add 10mL of ammonia buffer to each flask. Then add 5 drops of Eriochrome Black T indicator and titrate with the 0.005 M EDTA until the first appearance of a clear blue color. After that, calculate the molarity and its average molarity of the EDTA solution.

Determination of Water Hardness using EDTA

In determination of water hardness using EDTA, dry 2.0g of unknown sample. Weigh accurately duplicate samples of 0.20 to 0.25 grams of the dry Limestone and transfer to a 250mL Erlenmeyer flask. To each of the sample, carefully add 5mL of concentrated HCl solution. After dissolution is complete, add about 50mL of EDTA solution, distilled water while rinsing down the sides of the flask in the process. Boil for 5 minutes to remove the carbon dioxide and then cool. Transfer the cooled solution to a 500mL volumetric flask and dilute to mark using distilled water. Pipette 25mL aliquot portion of the limestone solution into a 250mL Erlenmeyer flask and add 10mL of ammonia buffer to each flask. Then add 5 drops of Eriochrome Black T indicator and titrate with the 0.005 M EDTA until the first appearance of a clear blue color. After that, calculate the total water hardness as ppm CaCO3.