Determination of Water Hardness

Complexometric Determination of Water Hardness

By,

David Neils

January 30, 2013

Abstract

A mixture of Na₂EDTA solution was created in which we titrated the solution three separate times with a 1.000 g CaCO₃/L solution. EDTA chelates Ca²⁺ in a one-to-one ratio so we were able to find the molarity of Na₂EDTA using this one-to-one ratio. Since we did the titration three separate times we were able to get a pretty accurate average of the molarity of Na₂EDTA. Three more titrations were performed in which we titrated an unknown water sample with our Na₂EDTA solution. Using the values achieved from these three titrations and the mean molarity of the Na₂EDTA we now had enough information necessary for determining the waters hardness. We found that trial 1 contained 272.1 ppm, trial 2 contained 282.0 ppm, and trial 3 contained 285.6 ppm which gave us an average of 279.9 ppm in our unknown water sample. We expect an estimated precision of 9.457 ppt.

Introduction

"Water containing Ca²⁺ and/or Mg²⁺ ions is called hard water, and water that is mostly free of these ions is called soft water" (1). According to the American Modern Society of Engineers water hardness classification table, water is thought of being soft if there are between 0 and 60 ppm, moderate if between 61 and 120 ppm, hard if between 121 and 180 ppm, and very hard if the water contains more than 180 ppm. Calcium carbonate which creates limestone is a common problem that we face because of how bad it can be for pipes and other water delivery systems that we own in our homes. My textbook states that "solid calcium carbonate formed in this way is the main component of the scale that accumulates in boilers, water heaters, pipes, and teakettles. A thick layer of scale reduces heat transfer and decreases the efficiency and durability of boilers, pipes, and appliances. In household hot-water pipes it can restrict or totally block the flow of water" (1). We all should care about how hard our water is not only because we are drinking it, but because of the damage it can do to the appliances in our homes. At my home in Tempe, we have seen an increase in the hardness of our water over the last few years and I believe it is why our water heater just recently broke. Our kitchen sink and faucet frequently builds up a scaly surface and the dishes that come out of our dish washer have a thick white film on them. It is becoming a problem and now we are in a position of needing to purchase an expensive water softening kit in order to solve this problem that we are facing. Whether or not any serious health risks can arise from the consumption of hard water is somewhat unclear, but I did find that "In most large-scale studies, an inverse relationship between the hardness of drinking-water and cardiovascular disease has been reported" (2).

Procedure

We first prepared a 0.004 M disodium EDTA solution by adding 0.7967g of Na₂EDTA to about 500 mL of deionized water. Next we added 10.00 mL of standardized calcium ion, approximately 30 mL of deionized water, 3.4 mL of ammonia/ammonium chloride buffer (pH 10), and 4 drops of Eriochrome Black T indicator solution to an Erlenmeyer flask. While stirring the flask using a stir-bar, we then titrated the solution with our 0.004 M disodium EDTA solution. The solution changed from pink to violet and then to blue. When the solution turned blue we finished our titration. We repeated this titration two more times and then found the molarity of our disodium EDTA solution for each of our three titrations. We then calculated an average molarity using the sum of our three molarities and were then able to calculate the precision. Then we titrated an unknown water sample using our 0.004 M disodium EDTA solution that we prepared. In an Erlenmeyer flask we added approximately 25.00 mL of the prepared unknown water sample, 20 mL of deionized water, a stir-bar, approximately 3 mL of ammonia/ammonium chloride buffer (pH 10), and 4 drops of Eriochrome Black T indicator. We then titrated the solution with our 0.004 M disodium EDTA solution. We repeated this titration two more times and then were able to calculate the water hardness in mg of CaCO₃/L, calculate our average water hardness and experimental precision.

Results and Discussion

In preparing our approximate 0.004 M disodium EDTA solution, our next

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