Complexometric Determination of Water Hardness Essays

Complexometric Determination of Water Hardness Essays Complexometric Determination of Water Hardness Paper Complexometric Determination of Water Hardness Paper ABSTRACT The average concentration of CaCO3 obtained was 212 ppm, with a standard deviation of 1 ppt. The results indicate that the unk B tap water can be considered as hard water. INTRODUCTION Hard water is due to metal ions (minerals) that are dissolved in the ground water. These minerals include Ca 2+, Mg2+, Fe3+, SO42-, HCO3-. When this water evaporates or boils, the difficult to dissolve metal salts remain as a scaly residue. Hard water inhibits the effectiveness of soap and detergents. Calcium ions typically make the most significant contribution to water hardness. This is why hardness is measured in terms of mg CaCO3/L of solution. Hardness is also reported in units of parts per million (ppm). Water with a hardness value of 60 ppm is considered â€Å"soft† and water with 200 ppm is considered â€Å"hard. † The purpose of this lab was to determine the water hardness of a sample of unk B tap water. The disodium salt of EDTA was used to determine the concentration of M 2+ metal ion impurities in hard water by a complexometirc titration. EDTA is a versatile chelating agent. A chelating agent is a substance whose molecules can form several bonds to a single metal ion. Chelating agents are multi-dentate ligands. EDTA acts as a chelating agent because each nitrogen and one oxygen from each of the four carboxylic acid groups has an electron pair to , donate (ligand- a substance that binds with a metal ion to form a complex ion) to a metal ion center, making EDTA a hexadentate ligand which forms an octahedral complex. An indicator, Eriochrome Black T enables the detection of when the EDTA has completely chelated the metal impurities. In the presence of a metal cation, Eriochrome Black T forms a pink complex. H2In- represents the anion of the free, solvated indicator, and M2+ represents Mg2+ or Ca2+. H2In-3 (aq) + M2+ (aq) MIn- (aq) + 2H+ (aq) Blue pink As you add EDTA solution from a buret, the metal ions preferentially complex to the EDTA, leaving the indicator solvated: EDTA (aq) + MIn- (aq) + 2H+ (aq) ? H2In- (aq) + MEDTA (aq) pink blue A color change from pink to violet to blue indicates the titration endpoint. Adding an ammonia/ammonium chloride buffer (pH 10) to the mixture prior to titration ensures that the calcium metal ions remain in solution and that the indicator works correctly. Proteins drop off and now chelates with calcium and forms a tetrahedral complex. EXPERIMENTAL First, a 500mL 0. 004M EDTA solution was prepared by weighing out 0. 7232g of Na2EDTA and dissolving it in 500 mL of deionized water. A standard calcium solution was prepared in order to standardize the Na2EDTA solution used in the titration of the unk B water sample with 1. 000g CaCO3/L solution. 3 mL of ammonia/ammonium chloride buffer (pH 10) was added under the fume hood. The ammonia buffer is an inhalation irritant. This solution was stirred for 30 seconds. drops of Eriochrome Black T indicator were added just before titrating. The titration was completed 3 times. The average molarity of disodium EDTA and precision were determined. Table 1. standardization titration EDTA RunVol (mL) 124. 20 224. 00 324. 10 Because EDTA chelates Ca2+ ions in a one-to-one molar ratio: [Na2EDTA] = 10. 00 mL CaCO3 soln x1. 000g CaCO3 x 1 mol CaCO3 x 1 mol Na2EDTA = 0. 004128 24. 20 mL Na2EDTA soln 1L CaCO3s oln 100. 1gCaCO3 1mol CaCO3 0. 004128 + 0. 004163 + 0. 004145 /3 = Average Molarity = 0. 004145M #1 # 2 #3 Estimated Precision(ppt)= |sum of absolute deviation|/3 x 1000 (ppt) [Na2EDTA] mean 1. 7 x 10-5 + 1. 8 x 10-5 + 0/3 = 1. 17 x 10-5/3 x 1000 = 1(ppt) #1 #2 #3 (0. 004145M) Next, an unk water sample – B was titrated with the standardized disodium EDTA solution. 25 mL of the unk B water sample was combined with 20 mL of deionized water. 3 mL of ammonia/ammonium chloride buffer (pH 10) was added and stirred for 30 seconds. Just prior to titrating, 4 drops of Eriochrome Black T indicator was added to the solution and stirred for 30 seconds. The titration was conducted 3 times. The hardness (mg CaCO3/L) of the prepared unk B water sample for each titration was calculated. The average hardness and precision from the three trials were then determined. Table 2. Titration of Unknown water sample B Unknown water sample B RunVol (mL) 112. 75 mL 212. 80 mL 312. 70 mL Water Hardness (ppm) = mg CaCO3 x 1000 = V mL Na2EDTA soln x mol Na2EDTA x 1 mol CaCO3 x 100. 1g CaCO3 1 L CaCO3 soln 25. 00 mL CaCo3 soln L Na2EDTA soln 1mol Na2EDTA 1 mol CaCO3 Run 1 = 210. 7 (ppm) Run 2 = 213. (ppm) Run 3 = 210. 8 (ppm) Average ppm = 212 (ppm) Precision = 2 sum of deviation in hardness (0. 9 + 1. 8 = 0. 8)/3 x 1000 (3) (211. 6 ppm) CONCLUSION The results show an average 212 ppm, with a standard deviation on 1 ppt. General guideline for water hardness classification are 0-60 mg/L of CaCO3 are classified as soft while water with more than 200 mg/L of CaCO3 are classified as hard. Based on the data gathered and calculated, it can be concluded that the unk B water sample is considered hard water. This is within allowable range for the City of Tempe, where the range for hardness is 150 -400 ppm with an average of 168 ppm. The unknown B water sample to slightly higher than the City of Tempe average. Some area of experimental error that may have deviated the obtained values from the â€Å"real† value would be the preparation of the standard solution due to contamination. Inaccurate measurements of reagents would also be a factor. Another possibility would be unsuccessful mixing of standard solution or not titrating the solution within the allotted timeframe.