The Determination of the Concentration of Hard Water lons by the Titration of EDTA By Hannah Denby Lab partners: Heidi Kiziah, Leonie Hamel University of Central Florida, CHM 2046L December 3rd, 2015 Abstract. The “hardness” of water is a common dilemma attributed by high concentrations of metals such as calcium and magnesium. This is a result of the properties of water; because it is a good solvent, impurities are easily dissolved. As water moves through soil and rock, it dissolves minute amounts of minerals and holds them in solution. Although it is not hazardous to health, hard water is a nuisance to both industrial and domestic water consumers.
Mineral buildup on plumbing fixtures, poor soap and detergent performance, and a contribution to scaling in boilers and industrial equipment are several negative consequences of hard water (1). Determining whether or not water is hard involves the calculation of the concentration of metal ions in the given water sample compared to the suggested standards established by the Environmental Protection Agency. The following experiment tested samples of TAP water and Dl water to determine the concentration of metal ions contained in each. 1. Introduction The following lab involved the calculation of the concentrations of hard water ions in a given 50 mL sample of a university’s TAP water and in a 50 mL sample of deionized water. Two main reactions were used to perform the titration of EDTA that was needed to calculate these values.
The first reaction performed is displayed below. Equation 1 M2+ (aq) + In2- (aq) MIn(aq) Colorless Blue Pink In this reaction, an aqueous solution of Eriochrome Black T (blue indicator) was added to a known amount of M2+ (a divalent cation such as Mg2+or Ca2+), which resulted in a pink color change. The same result was seen upon the addition of M2+ to DI water. This was because the indicator reacted with the 3 mL of MgCl2 that had been added. The second equation performed is represented by the following equation. Equation 2 Min (aq) + EDTA4- (aq) M(EDTA)2- (aq) + In 2- (aq) Pink Clear Clear Blue The above reaction involved the titration of EDTA into the solution. This reaction was an example of coordination chemistry. In coordination chemistry, transition metals bind with ions or certain compounds in aqueous solutions.
These ions are ligands, while the compound in its entirety is known as a coordination complex. EDTA is one such ligand. In its nonprotonated form the EDTA has extra, unpaired electrons on the four oxygen atoms that have single bonds with the carbons and on the two nitrogen atoms. The unpaired electrons, the two nitrogens and four oxygens, make these coordinate covalent bonds (3). The buffer solution with a pH of 10 that added to each solution served a crucial role in the reaction. The buffer provided a basic pH that stimulated the deprotonation of EDTA so that it was able to bind the metal ions that were present in the solutions. Figure 1. EDTA Ligand An EDTA molecule binds a total of six times to a generic central metal, labeled M (3). Therefore, when EDTA was added to the pink solution, the concept of coordination chemistry was observed.
The hexadentate ligand EDTA bound the free hard water ions, those that had not already been bound by the Eriochrome Black T indicator, in a 1:1 mole ratio. Once all of the EDTA had coordinated with all of the hard water ions, the EDTA began to bind the metal ions that had previously reacted with the indicator. Finally, when all of the metal ions had coordinated with the EDTA, the end point was reached and the solution experienced a blue color change. The amounts of EDTA that were used to reach the end points of the titrations provided a means of calculation of the amount of metal ions that were present in each of the water samples. This data was then compared to the suggested concentrations for water hardness, and the determination of the hardness of the water was able to be determined.
2. Experimental Methods Two water samples were tested, one sample of 50 mL of TAP water and one sample of 50 mL of DI water. To begin, 50 mL burette was filled with standard 0.005 M EDTA solution. A 100 mL sample of TAP water was measured into the 100 mL volumetric cylinder. 3 mL of MgCl2 were then pipetted into the 50 mL of TAP water. This MgCl2 served as an internal standard to ensure that there were enough metal ions present in solution for the indicator to completely react. The 1 mL pipette was used to measure and distribute 1 mL of buffer solution into the sample. Then 2 to 3 drops of Eriochrome Black T (blue indicator) were added. This resulted in a pink color change. Finally the sample was titrated with the EDTA solution.
EDTA bound the free hard water ions, those that had not already been bound by the Eriochrome Black T indicator, in a 1:1 mole ratio. The end point of the titration was reached when the solution experienced a blue color change. This procedure was repeated so that the concordant value was achieved. This procedure was then repeated, the only difference being that deionized water was used in the place of TAP water. In the same way, 50 mL of Dl water, 3 mL of MgCl2,5 mL of buffer solution, and 2 to 3 drops of Eriochrome Black T indicator were titrated by 0.005 M EDTA and this procedure was also repeated so that the concordant value was achieved. The concentration of the 3 mL of MgCl2 was determined from this titration.
3. Results Table 1. Volume of EDTA used Depending on the Sample of Water EDTA coordinated with the MgCl2 metal ions added to both samples and the hard water metal ions present in TAP water. Sample TAP Water Di Water Volume of 0.005 M EDTA used in titration 150 mL 75 mL The data in the table above was used in the determination of the calculation of metal ions in the samples of both TAP and DI water. Primarily, hard water consists of magnesium and calcium. Therefore the ratio of magnesium to calcium ions was inferred to be 50/50. The average molar mass of the molar mass of Magnesium (24.305 g/mol) and the molar mass of Calcium (40.078 g/mol) was calculated to be 32.192 g/mol.
This calculated average molar mass was used as the molar mass of the metal that was present in the sample. The concentration of metal that was present in the TAP water was solved by the following calculations: 150 mL EDTA * (1 L/ 1000 mL) * (0.005 M) = 0.00075 moles of metal (0.00075 moles/ 0.050 L) = 0.015 moles/L 0.00075 moles of metal * (32.192 g/ 1 mol) * (1000 mg/ 1 g) = 24.144 mg The concentration of metal that was present in the DI water was solved by the following calculations: 75 mL EDTA * (1L/1000 mL) * 0.005 M = 0.000375 moles (0.000375 moles/ 0.050 L) = 0.0075 moles/L 0.000375 moles * (32.192 g/ 1 mol) * (1000 mg/ 1 g) = 12.072 mg Because the deionized water did not initially contain any metal ions, the concentration of metal ions that was calculated in the deionized water was equivalent to the concentration of the 3 mL of MgCl2 that were added to the deionized water sample.
Therefore, the total concentration of metal found to be in the TAP water sample was equivalent to the amount of added MgCl2 in addition to the initial amount of metal ions that was present in the TAP water. Therefore, the total amount of milligrams of metal ions calculated in the deionized water subtracted from the total amount of metal ions calculated yielded the amount of metal ions that were initially present in the TAP water sample. 0.00075- 0.000375= 0.000375 moles (0.000375 moles/0.050 L)= 0.0075 moles/L 0.000375 moles * (32.192 g/ 1 mol) * (1000 mg/ 1 g) = 12.072 mg (12.072 mg/ 0.050 | L) = 241.44 mg/L The following table depicts the data from the calculated above.
Table 2. Concentrations of Metal lons The total amount of milligrams of metal ions calculated in the deionized water subtracted from the total amount of metal ions calculated yielded the amount of metal ions that were initially present in the TAP water sample Deionized Water TAP water Initial concentration of metal ions present in TAP water prior to addition of MgCl2 Concentration (moles/L) 0.0075 0.015 0.0075 Concentration (mg/L) 241.44 482.88 241.44 The amount of metal ions that was initially present in the solution was then compared to the hard water concentration limit suggested by the Environmental Protection Agency and a conclusion on the acceptability for consumer use was determined.
The article “Water Hardness and Alkalinity” outlines the general guidelines for classification of water, stating that “O to 60 mg/L (milligrams per liter) as calcium carbonate is classified as soft; 61 to 120 mg/L as moderately hard; 121 to 180 mg/L as hard; and more than 180 mg/L as very hard” (2). Because these values were reported in mg/L, the 0.000375 moles of metal ions calculated were converted to mg using the average of the molar mass of both calcium and magnesium. 12.072 mg were then divided by the 0.050 L of TAP water. 241.44 mg/L of metal ion were found to be present in the TAP water sample. The comparison of this value to the standards provided by the EPA led to the classification of the TAP water sample as being very hard. Even though there was possible opportunity for error, 229.368 mg/L significantly exceeded 180 mg/L and, therefore, the results were concluded that it was recommended that water softeners should be purchased by the university
4. Conclusions The goal of the following experiment was to determine the hardness of campus water, primarily caused by concentrations of calcium and magnesium. In order to determine the hardness of water, the concentration of hard water ions was solved for and then compared to a criteria of classification. The experiment performed in the following lab involved the titration of EDTA that formed coordination complexes with the metal ions that were present in the solution. The concentration of metal ions was calculated by finding the amount of EDTA that had been used upon the end point of the titration.
This calculated concentration, compared with the standards for water hardness, allowed for a conclusion to be made about the campus water. Because the concentration of metal ions found to be in the TAP water sample (241.44 mg/L) exceeded the maximum suggested metal ion concentration by the government guidelines (180 mg/L), the purchase of water softeners was strongly recommended. In final analysis, the comprehension of this lab allowed for a better understanding of the chemical components of the “hardness” of water, the nature of coordination complexes and their properties, the process of deprotonation, and the performance of titrations.
