In-person lab water hardness PDF

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Lab water hardness ...

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Nicole Fuerst – Analysis of Water Hardness – Student ID 20205503

Analysis of Water Hardness Introduction Measuring the calcium and magnesium content of a water sample.

Experimental Details EDTA (0.010 mol L-1) was added into the burette. Using a pipette, 40 mL of the water sample was measured and emptied into a conical flask. Sodium hydroxide was added to the water sample until a pH of 12 was reached. A small amount of hydroxynapthol blue indicator was added to the water sample. The solution was then titrated with the EDTA solution by adding EDTA from the burette to the conical flask until the indicator changed colour to blue, indicating that the endpoint has been reached. The volume of EDTA used was then recorded to +/- 0.05mL. The steps above were repeated until three concurring titres were obtained.

Titre Calcium

Titre Magnesium

Trial 1

8.7 mL

8.3 mL

Trial 2

9.0 mL

8.0 mL

Trial 3

10.6 mL

7.9 mL

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Nicole Fuerst – Analysis of Water Hardness – Student ID 20205503

Calculations Calcium (1) average titre Calcium 9.43 mL volume of EDTA solution = 9.43 mL = 9.43 x 10 -3 L

= concentration (mol L-1) x volume (L)

amount EDTA (mol)

= 0.010 mol L-1 x 9.43 x 10-3 L = 9.43 x 10-5 mol of EDTA were used (2) 1 mol Ca2+ : 1 mol EDTA V (sample water) = 40 mL = 40 x 10 -3 L = 0,04 L

concentration (mol L-1) x volume (L)

amount EDTA (mol)

=

concentration (mol L-1)

=

amount EDTA (mol) volume (L)

concentration (mol L-1)

=

[calcium]

= =

2.3575 x 10-3 mol L-1

The calcium concentration of the sample is 2.3575 x 10 -3 mol L-1 (3) Conversion of the concentration in mol L-1 to a concentration in mg L-1: 1 L of water contains 2.3575 x 10-3 mol L-1 of Ca2+ molar Mass (Ca) = 40.08 g mol-1 mass (g)

= amount (mol) x molar Mass (g mol-1)

mass

= 2.3575 x 10-3 mol L-1 x 40.08 g mol-1 = 0.09449 g L-1 x 1000 = 94.49 mg L-1

The calcium concentration of the sample is 94.49 mg L-1

2

9.43 x 10-5 mol 0,04 L

Nicole Fuerst – Analysis of Water Hardness – Student ID 20205503 Magnesium (1) average titre Magnesium

8.06 mL

concentration of EDTA solution = 0.010 mol L-1

volume of EDTA solution = 8.06 mL = 8.06 x 10 -3 L

amount EDTA (mol)

= concentration (mol L-1) x volume (L) = 0.010 mol L-1 x 8.06 x 10-3 L = 8.06 x 10-5 mol of EDTA were used.

(2) 1 mol Mg2+ : 1 mol EDTA V (sample water) = 40 mL = 40 x 10 -3 L = 0,04 L

amount EDTA (mol)

=

concentration (mol L-1)

=

[magnesium] = =

concentration (mol L-1) x volume (L) amount EDTA (mol) volume (L)

concentration (mol L-1)

=

2.015 x 10-3 mol L-1

The magnesium concentration of the sample is 2.015 x 10 -3 mol L-1 (3) Convert the concentration in mol L-1 to a concentration in mg L-1: 1 L of water contains 2.015 x 10 -3 mol L-1 of Mg2+ molar Mass (Mg) = 24.305 g mol-1 mass (g)

= amount (mol) x molar Mass (g mol-1)

mass

= 2.015 x 10-3 mol L-1 x 24.305 g mol-1 = 0.04897 g L-1 x 1000 = 48.97 mg L-1

The magnesium concentration of the sample is 48.97 mg L-1

3

8.06 x 10-5 mol 0,04 L

Nicole Fuerst – Analysis of Water Hardness – Student ID 20205503

Questions A pipette has more accuracy compared to burette, especially when it comes to readings. When a pipette is emptied, it delivers the exact volume of a solution it is designed to hold. The readings are very accurate since their long necks decrease the error in measuring volume of the meniscus. Burettes are tall and thin glass tubes with a tap at the bottom that can be opened and closed to allow the solution inside to flow out. The scale on its side allows the amount of solution that has been allowed to flow out to be read off. A burette is graduated to 0.1 mL and will be read correct to +/- 0.05 mL. A pipette, on the other hand, is measuring a fixed volume of a liquid, for example 20 mL, whereas with burettes you would continuously read the volumes as part of the experiment. Hence you normally would read a burette more often than a pipette. Human error in pipettes are much less compared to readings in burettes because a small change in height will only result in a small change of the actual volume (because of their narrow necks) making the readings much more accurate. Changes in height will affect a titre value of burette more as pipettes as their wider necks give more room to wrong readings. A burette is also being read to +/- 0.05 mL correctly. As specified on the Ballygowan Water website, the calcium content should be 114 mg L-1. According to our measurements and calculations the water sample contains a calcium content of 94.49 mg L-1 The discrepancy between these two values is approximately 18.5 %. According to our readings and calculations the magnesium content of the sample water is 48.97 mg L-1 which is roughly 3 times more than the manufacture’s specification of 16 mg L-1. Apart from sources of errors in pipetting and reading the burette other factors might be responsible the latter result and should be considered as part of the error analysis (e.g. pH adjustment is less than pH 10, wrongly determined endpoint).

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Nicole Fuerst – Analysis of Water Hardness – Student ID 20205503 Ballygowan Water Mineral Composition (source: https://www.ballygowan.ie/our-water/ballygowan-water-source/)

Element

mg/l

Calcium

114

Magnesium

16

Sodium

15

Potassium

3

Bicarbonate

400

Chloride

28

Sulphate

15

Nitrate (as NO3)

9

pH at source

7.2

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