Complexometric Determination of Water Hardness

Using complexometric method of calculating water hardness, an EDTA solution was made and calculated to be around .00209M. This EDTA solution was then used to titrate a water sample with unknown hardness. The sample used was sample #18. The hardness of the sample was calculated to be 180505 ppm. This is within normal levels for the Mesa area.

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This experiment focuses on titration with EDTA. Titration is the determination of a given component in solution by addition of a liquid reagent of known strength until the endpoint is reached when the component has been consumed by reaction with the reagent. An indicator called Eriochrome Black T will enable detection when the EDTA has completely chelated the metal impurities. Chelation happens when a ligand (with more than one binding site) forms a complex with a metal ion. Ligands are complexes that form when the electron donating groups form coordinate covalent bonds through empty orbitals on the metal ion. This experiment contains Ammonium chloride buffer which is an inhalation irritant.

About 500 mL of approximately 0.004 M disodium EDTA solution needed. The solution should have between 0.7–0.8 g of Na2EDTA and dissolve in about 500 mL deionized water in a plastic bottle. The bottled is Sealed and shaken vigorously for a few minutes to dissolve the salt. Standardize the Na2EDTA solution using a stock calcium ion solution as the primary standard. Using a 10-mL transfer pipet to add 10.00 mL of standardized calcium ion stock solution (1.000 g CaCO3/L solution) to a 250-mL Erlenmeyer flask which will have an additional 30 ml of deionized water to this titration flask. Adding a magnetic stir-bar the solution was then stirred. about 3 mL of ammonia/ammonium chloride buffer is added to the solution. Just prior to titrating the flask, adding four drops of Eriochrome Black T indicator solution will be needed.

At the endpoint, the color will change from pink to violet to blue. This will repeat two more times. Then Choose one prepared unknown water sample as provided. Record the unknown code in a notebook, then titrate this water sample with your standardized disodium EDTA solution. Transferring 25.00 mL of the prepared water sample to a 250-mL Erlenmeyer flask will be done. Then Add about 20 ml of DI water to the titration flask, which will have a magnetic stir-bar. The solution will have an added portion of 3 mL of ammonia/ammonium chloride buffer. The solution will then change color as previously examined in the first portion. This procedure will be repeated twice more.

Results and Discussion:

EDTA solution
Initial Reading (mL)
(mL) delivered

The average of the mL delivered of the three results is 47.72 mL The Molarity of the EDTA solution was calculated and is shown below.

Ppt: 15.99 for 50.01 mL delivered
Ppt: 4.68 for 47.05 mL delivered
Ppt: 11.31 for 46.10 mL delivered

Unknown solution
Initial Reading(mL)
(mL) delivered

The average of the mL delivered of the three results is 21.57 The hardness of the unknown water sample was calculated using the Molarity of the EDTA solution standardized in the first portion of the lab. The calculation is shown below. Ppt: 20.55 for 22.9 mL delivered

Ppt: 2.47 for 21.41 mL delivered
Ppt: 17.92 for 20.41 mL delivered

The tables reveal a difference in (mL) delivered in comparing the first experiment to the unknown. The unknown must have a weaker tolerance to the indicator in which the first experiment is stronger.

The water hardness in Mesa ranges from 12 gpg (grains per gallon) to 22 gpg (1). The Ppt of the unknown and EDTA do and don’t fall within Mesa’s range. There are some numbers that fall within the range and there are some that may fall out. Overall the data has some proof that the water hardness of the EDTA and unknown can potential match up to Mesa’s.


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