Water Hardness Lab

Lab #1: Complexometric Determination of Water Hardness by EDTA Titration

Morganne O'Brien and Nika Moussavi

Dr. Todd Windman

Mesa Community College

Experiment Date: 02 June 2016

Abstract

The purpose of this research is to determine the hardness of an unknown water sample. A disodium EDTA solution will be standardized by way of a stock calcium ion solution. An unknown water sample will then be titrated with this same standardized disodium EDTA solution. Eriochrome Black T will be used as an indicator. The standardization of EDTA will be performed first and the determination of calcium concentration will be performed second. After standardizing the EDTA, the average molarity was found to be XXX M. The calcium concentration was found to be XXX ppm. This technique stands as an effective method for analyzing the hardness of unknown water samples.

Background

Understanding the intricate nature of water hardness is essential for an array of individual, ecological, and industrialized environments. Water is the key to hydration; therefore, both the individual and his surrounding environments must remain in constant awareness of its chemical composition and abundance (or lack thereof). Humans must consume water to sustain life, just as plants, animals, and the earth all depend upon water for its inimitable qualities.

Many recent epidemiological studies have examined the effects of water hardness on such conditions as cancer, Alzheimer’s disease, cardiovascular disease, growth retardation, varying skin conditions, infertility, and more (Sengupta, 2013). Although distinctive cause and effect relationships are inconclusive, understanding the possible associations between the two elements is critical for advancing such topics as public education and proper health practices (Sengupta, 2013). Water hardness may also pose potential threats to several different species of fish that lack the evolutionary ability to withstand or filter out excessive amounts of extremely hard water (Playle, Gensemer, & Dixon, 1992). In commercial and industrialized settings, such as factories and restaurants, excessive amounts of hard water can build up in underground plumbing fixtures, leading to clogged pipes and even permanent plumbing damages (Perlman, 2016).  

Water hardness can be defined as a measure of the amount of calcium and magnesium salts contained in any given volume of water (Perlman, 2016). Data from the present study can be juxtaposed with a standardized water hardness scale (see figure 1 below). Notice the positive correlation between parts per million and increasingly hazardous classification levels (What is Hard Water, 2016).

[pic 1]

  Figure 1. Standardized water hardness scale (What is Hard Water, 2016).  

The current experiment will employ titration as a means of determining water hardness. Titration is a simple technique, whereby a solution of known concentration is used to determine the concentration of an unknown solution (Purdue University, 2016). The unknown solution for this experiment is a hard water sample. The figure below demonstrates what the unknown mixture should look like before, during, and after the titration trial (Harvey, 2016).

           [pic 2]

Figure 2. Illustrated end point for titration of water hardness (a) Pink prior to the end point, (b) violet approaching the end point, and (c) blue after the end point (Harvey, 2016).

The data collected from doing titration trials will act as the Segway towards determining the hardness of an unknown water sample. The equation below will be used to calculate the estimated precision (in parts per thousand) of the three titration trials.

             Estimated Precision (ppt) = [pic 3]

The final calculation of water hardness (in parts per million) may be calculated after the estimated precision is complete. The equation for determining water hardness is given below. Note that the V represents the actual delivered volume of Na2EDTA solution and the M is the actual molarity.

Water hardness (ppm) = xxx[pic 4][pic 5][pic 6][pic 7]

Procedure

To begin the experiment, prepare about 500 mL of approximately 0.004 M disodium EDTA solution. Start by filling a 500 mL plastic bottle with 500 mL of deionized water. Then, weigh out 0.7-0.8 grams of Na2EDTA. Carefully combine this solution with the DI water in the bottle. Seal the lid on top of the bottle and shake for a moment, long enough for the salt to disband amongst the DI water. When complete, set this mixture aside.