Determination of Water Hardness

7 July 2016

When neutral molecules or anion gives an electron pair and attaches itself to a metal ion center the atoms become a single complex ion, thus forming covalent bonds. These complex ions are called ligands. Some of these polyatomic ligands have lone pairs available for bonding to the central metal ion. When a ligand forms with a metal ion it is called chelation and the ligand is called a chelating agent. In this experiment we used a chelating agent called ethylenediaminetetraacetic acid (EDTA).

When rain falls the water picks up impurities from dissolving the salts that are present in the soil. The water’s hardness comes from the water dissolving these salts. Water with a hardness value of less than 60 ppm is considered “soft”water and water with more than 200 ppm CaCO3 is considered “hard” water. Using the EDTA and an indicator called Eriochrome Black T we are able to calculate the hardness of water. In the presence of a metal cation, Eriochrome Black T forms a pink color.

Determination of Water Hardness Essay Example

But as the EDTA solution is added from the buret, the metal ions are attracted to the EDTA leaving the indicator solvated. As it looks in this equation. In this lab we are trying to find out what the water hardness of an unknown water sample is and compare it to the range of municipal water hardness in our area. Methods We used our 500ml plastic bottle and weighed out 0. 7999g of (on the scale with a serial number 12321601), and placed it in our plastic bottle then filled the bottle with deionized water and shook until dissolved.

This became our standardized EDTA solution. Then using a Grade A 10ml transfer pipet and added 10ml of a 1. 000g (Prepared by BTJ, Jan 13, 2014) into a 250ml Erlenmeyer flask mixed with 30ml of D. I. water. We than added 3ml of ammonia chloride buffer inside the fume hood added 4 drops of Erichrome Black T Indicator and slowly stirred while titrating the solution with the EDTA solution until it turned from pink to blue. Then repeated two more times calculating the Molarity and average molarity.

We then did the same exact thing but this time instead of using 10ml of the 1. 000g we used an unknown water sample, my unknown sample was #6. Then we used the information to calculate the hardnes of the water. We needed to make sure that we did not over titrate or under titrate, so paying close attention to the color change was a must. That is why we made sure to do each titration 3 times so we could get the averages of all the attempts. Recording all the data to the smallest possible number value was a must.

We were able to learn a few things from our experiment on Water Hardness. For the first half of the experiment we see that our relative precision (parts per thousand) was 9.6 ppt. That is really high compared to what we expecting in precision. For this experiment we were expecting a precision of five parts per thousand at the most. Some reasoning why it was so high could because I did not employ careful volumetric technique and I may have been trying to titrate too long and got a different shade of blue each time.

If I was to do this experiment again I would do my best to pay careful attention to the amount of liquid put in and also titrate slower and pay closer attention to the shades of the color. When it came time for the unknown water sample #6 I feel like my calculations and titrations where all well done. For each of the three trials I was at 400.0 mg / L . I was able to compare this to the city of Tempe’s water hardness range. According to their websitei they say that the range is from 150-400 mg/L. So my water sample fits on the upper end of the range. So it is possible that my unknown sample came from a City of Tempe water source.

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