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Complexometric Determination of Water Hardnesss Essay


This lab was about determining water hardness. Water hardness is the amount of metal ions in the water. The most common found ion in the water is calcium ions and typically with a charge of +2. Water hardness plays a big significance in our daily life because to many metal ions in our drinking water can have adverse effects on our body. You can measure the water hardness by EDTA titrations. EDTA is a disodium salt that stands for ethylenediaminetetraacetic acid and it is a chelating agent. Chelation is the process of a ligand forms a complex with a metal ion. Eriochrome Black T is an indicator you add to the water sample that will turn the water pink if metal ions exist. Through the process of titration, carefully adding EDTA to the water sample with the Eriochrome Black T will cause a chemical reaction to occur, slowly changing the water color from pink to violet and then violet to a light blue which will be the end of the chemical reaction. Once the titration is complete you can calculate your water hardness.

Above in the formula V represents the actual delivered volume of Na2EDTA solution and M is your actual molarity.

Procedures: Start by preparing 500mL of 0.004 disodium EDTA solution. This is what will be added to the buret. Next take a 250-mL Erlenmeyer flask and add 10.00mL of standardized calcium and 30mL of deionized water. Place a magnetic stir-bar in the flask and set on top of a piece of white paper on a magnetic stirrer. After place 3mL of ammonia/ammonium chloride buffer (pH10) inside the flask and let it stir for thirty seconds. Lastly add four drops of Eriochrome Black T indicator to the solution which will turn it a pink color. Begin titration and watch as the color changes from pink to violet and then violet to a light blue color. Record the data when the color turns a light blue and repeat the process two more times.

Fill the buret with the same 500mL of 0.004 disodium EDTA solution. Take 25.00mL of an unknown water sample and add it to a 250mL Erlenmeyer flask. Mix in 20mL of deionized water. Set a magnetic stir-bar inside the solution and place on the magnetic stirrer. Next add 3mL of ammonia/ammonium chloride buffer (pH10). After thirty seconds of stirring add four drops of the Eriochrome Back T indicator and watch as the solution turns pink. Begin your titration by slowly adding small droplets of the 0.004 disodium EDTA solution. The color will slowly change to a violet color and then to a blue color which will be the stopping point.

Record the data and repeat two more times. With the data collected calculates the hardness of each sample, then the average water hardness of all three samples, and lastly the precision of each trial. Compare your data to the expected range of a local cities water hardness. Results and Discussion: The first three titrations involved the known Calcium stock, titration 1 used 22.91ml of Na2EDTA solution before the titration complete. The second Titration used 21.91ml while the third and final used 21.55ml of solution.

During the procedure a 250ml Erlenmeyer flask was used as well as a 50ml burette, the buret was filled with our Na2EDTA solution while the flask was filled with 30ml of DI water, 3ml of ammonia, and 4 drops of an indicator, in this case Eriochrome Black T. Experimental error was calculated by taking the sum of all absolute deviations/3 than divided by the mean concentration of Na2EDTA, in this case the mean concentration was .00452M, and lastly, multiplied by 1000 to get the answer in PPT. Table 1 below shows the readings of both solutions before and after titration.

The mean concentration of these titrations is 0.00452, This figure was used to find the estimated precision which came out to 23.270% off of 100% meaning the experiment was 76.73% accurate. Finally the volume of the unknown was calculated to find the ppm for each titration and the mean of all three titrations was used to find the estimated precision of the unknown in ppm.

The mean in ppm for the unknown came to 212ppm, this number was plugged into the equation for estimated precision which is the sum of all absolute value deviations/ number of trials, all of which is divide by the mean and multiplied by 1000ppt. The final figure came out to 25.2% off of 100% meaning that the experiment was 74.8% accurate. Conclusion: In conclusion the water hardness of a solution can be found by titrating a known solution containing metal ions and using a chelating agent such as EDTA to determine the impurities of each of the water samples. Also, using the data collected from each titration the concentration and mean can be found from each titration and used to determine the water hardness in ppm. The estimated precision of this experiment can be found by running multiple titrations on the same sample more than one time.

The results obtained in this experiment include a mean concentration for the Calcium stock of .00452M, and an estimated precision of 76.73% and the average water hardness was 400.39ppm. The estimated precision for the unknown was 74.8% and an average water hardness of 204ppm. In this experiment 500ml of a known solution EDTA was prepared; a 10ml pipet was used to transfer a standard calcium ion stock solution into a 250ml Erlenmeyer flask. 30ml of DI water was added to the flask and the contents were than stirred using a magnetic stirrer and stirring rod.

3ml of Ammonia was added to the flask mixed for a few seconds underneath the fume hood and 4 drops of an indicator was added, in this case Eriochrome Black T. The contents of the flask were stirred for thirty seconds before titration began, as the titration progressed, the color changed from pink to violet and finally to a sky blue color indicating that the titration was complete. This process was completed with minor changes for the Unknown #141, 25ml of the unknown was placed in the flask and 20ml of Di water was used rather than 30ml, the rest of the titration however, used the same steps as above.

Work Cited

Klenck, Thomas. “How It Works: Water Softener.” Popular Mechanics 1 Aug. 1998: n. pag. Web.

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