Water Hardness Lab Report PDF

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Water Hardness Lab Report...

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Complexometric Determination of Water Hardness Report by Austin Jones February 5th, 2020 Professor Paul Gilletti

Abstract: Complexometric titration was used to determine the water hardness of an unknown sample. Approximately 0.004M of disodium EDTA solution is titrated into a standardized stock solution to verify molarity and is then titrated into the unknown solution labeled #50 to determine the amount of calcium carbonate within it. The chemical equation is EDTA(aq) + CaIn–(aq) + 2H+(aq) → H2In–(aq) + CaEDTA(aq). The experimental value was found to be 209.8 mg/L of CaCO3 which is within the range of typical water hardness in Mesa, AZ (between 205-377 mg/L CaCO3).

Introduction Complexometric titrations are used to indicate the end point of a titration based on the formation of a colored complex. These titrations are primarily used to discover metal ions in a solution. In this experiment, complexometric titration is used to determine the hardness of water which is defined by the unit milligrams of calcium carbonate (CaCO3) per liter of water/liter of calcium

carbonate solution

( mgCaL CO ∨ parts per million ( ppm)) 3

. Ethylenediaminetetraacetic acid

(EDTA) is a common molecule used in complexometric titrations because it makes six bonds with metal ions to form a complex ion. These electron bonding groups in a complex are called ligands. When ligands form a complex, the process is called chelation; thus, EDTA is a chelating agent because it is a hexadentate (“six-toothed”) ligand that forms an octahedral complex with metal ions. In this experiment, Eriochrome Black T indicator will be used to show when the EDTA has completely chelated the calcium ions, this is called the endpoint. Since Eriochrome Black T forms a pink complex in the presence of metal cations, an endpoint can easily be determined when all calcium ions complex to the EDTA making the solution turns blue. The chemical equation for this reaction is EDTA(aq) + CaIn–(aq) + 2H+(aq) → H2In–(aq) + CaEDTA(aq). Ammonium chloride is used as a buffer in order to keep all the calcium ions in the solution. This process should allow for the determination of calcium carbonate per liter in the solution; thus, determining the water hardness (less than 60 parts per million is considered “soft”, more than 200 parts per million is considered “hard”).

Methods The experimental procedure was followed as given in the lab manual.3 A 0.004 M disodium EDTA (Na2EDTA) solution was created and used as the chelating agent. Transfer pipets were used to accurately measure the base solutions before titrating. Magnetic stir-bars were used to have consistent stirring throughout the procedure. A standardized calcium ion stock solution (1.000 g CaCO3/L solution) was used to standardize the EDTA solution. Eriochrome Black T indicator was used to determine an endpoint and an unknown water sample was used as the solution to determine water hardness from. Volume readings from the burette were the main data recorded.

Table 1. Standardization of EDTA Solution Trial

Volume of EDTA titrated (mL)

Molarity of EDTA solution

1

25.05

0.003988

2

24.95

0.004004

3

24.95

0.004004

Mean Molarity

Deviation from Mean

Estimated Precision (ppt)

0.000011 0.003999

0.000005

1.75

0.000005

Results Table 1. Volume readings that determined the approximate molarity of the EDTA solution Table 2. Water Hardness Hardness (ppm) 209.0 211.4 209.0

Trial 1 2 3

Average (ppm) 209.8

Table 2. Water hardness results in parts per million Sample Calculations Molarity of EDTA solution for each standardization trial.

10.00 mLCaC O3 so l' n

|

1.000 gCaC O3 so l' n

|

|

1 mol CaC O3 1 mol EDTA mol EDTA =0.003988 LEDTA so l n 24.95 x 10 L EDTA so l n 1000 mLCaC O3 so l n 100.1 g CaC O3 1 mol CaC O3 −3

'

'

'

Mean molarity of EDTA

Mean=

( 0.003988 + 0.004004 + 0.004004 ) =0.003999 M 3

Deviation from mean n=|0.003999− 0.003988|=0.000011

Estimated precision (ppt)

( 0.000011 + 0.000005 + 0.000005 ) ( ) x 1000=1.75 ppt 3 ppt= 0.0039999

Water hardness (ppm)

|

|

|

13.05 mL N a 2 EDTA so l' n 0.003999mol N a 2 EDTA 1 mol CaC O3 100.1 g CaC O3 mgCaC O3 =209.0 ' ' 1mol N a 2 EDTA 1 mol CaCO 3 0.02500 L CaC O3 so l n LCaC O3 so l' L N a2 EDTA so l n ¿ 209.0 ppm

Average hardness

Average Hardness=

Discussion

(211.4 +209.0 + 209.0 ) =209.8 ppm 3

The results from this experiment seem reasonably accurate due to all the data being consistent. The average molarity of the EDTA solution was found to be 0.003999 M. This is an accurate molarity because the procedure called for a solution of approximately 0.004 M of EDTA. According to the city of Tempe’s typical water quality values (2), the water hardness in the Tempe area is between 220-420 mg/L of calcium carbonate. According to the city of Mesa’s typical water quality values (1), Mesa’s water hardness is between 12-22 grains per gallon which calculates to approximately 205-377 mg/L of calcium carbonate. Based off this data, the experiment results for water hardness should be accurate (considering the lab was done in Mesa) and it can be concluded that Mesa’s water is considered “hard” since it exceeds 200 ppm CaCO3. References 1

City of Mesa, AZ. https://www.mesaaz.gov/residents/water-resources/services/water-

quality/hard-water (accessed Feb 1, 2020). 2

City of Tempe, AZ. https://www.tempe.gov/government/municipal-utilities/water/water-

quality/typical-water-quality-values#Hardness (accessed Feb 1, 2020). 3

Gilletti, Paul. CHM152LL Lab Manual Complexometric Determination of Water Hardness;

Mesa Community College: Mesa, AZ, 2012....