CHEMISTRY 108
LABORATORY8
Analysis of solutions
Goals
Your team is called to compare two samples of bottled water bydetermining the pH, water hardness and alkalinity.
Materials Needed
pH meters
Calibration buffer pH 7 and 4
25 mL pipets (enough for five groups. Each group will need 2 pipets)
50 mL burets
Buret holders and stands
Magnetic stirring bars
0.0100 M Sodium EDTA solution, standardized
pH 10 buffer (NH3/NH4+ buffer plus MgEDTA)
Calmagite indicator
0.0100 M HCl solution, standardized
Methyl orange indicator
Two bottled water samples (not purified or distilled water): enough for five groups
Control water sample (distilled water)
Introduction
Bottled water is not pure water. Since it comes from wells or springs, it often has a relatively high mineral content. It has calcium and magnesium ions, the primary sources of water hardness. It also contains bicarbonate ion, the primary sources for the alkalinity or acid neutralizing capacity of the water. In addition, bottled water contains other soluble ionic substances.
We can determine the total amount of Ca2+ and Mg2+ ions in water by titrating a water sample with a standardized EDTA solution. EDTA forms strong 1:1 metal-EDTA complex with Ca2+ and Mg2+ ions. The indicator used in the titration is Calmagite indicator which is red/magenta in the presence of free Ca2+ and Mg2+ ions and blue in their absence.
Structure of EDTA (ethylene diamine tetraacetate)
Structure of Metal-EDTA complex
We commonly report water hardness as milligrams of CaCO3 per liter of water or ppm. Therefore, we calculate the number of moles of EDTA used in titration and convert it to the number of moles of Ca2+ and Mg2+ ions (it is a 1:1 reaction). Then it is converted to milligrams of CaCO3.
TableI. Classification of water hardness (hardness as calcium carbonate).Classification / mg/L or ppm
Soft / 0 - 17.1
Slightly hard / 17.1 - 60
Moderately hard / 60 - 120
Hard / 120 - 180
Very Hard / 180 & over
Example: A student used 11.35 mL of 0.0100 M EDTA solution to titrate 25.00 mL of the sample water. Report the hardness in milligrams of CaCO3 per liter of the sample (or ppm). What is the classification of the water sample (soft, slightly hard, moderately hard, etc.)?
moles of EDTA used = 0.0100 M 0.01135 L = 0.0001135 mol
moles of Ca2+ in 25 mL = moles of EDTA required = 0.0001135 mol
grams of CaCO3 in 25 mL= mol of Ca2+ molar mass of CaCO3 = 0.0001135 mol 100 g/mol = 0.01135 g
grams of CaCO3 in 1 L= 0.01135 g = 0.454 g
mg of CaCO3 in 25 mL= 0.01135 g = 11.35 mg
mg of CaCO3 in 1 L = 11.35 mg = 454 mg CaCO3 in 1 L= 454 ppm
According to the classification of water, this sample is very hard (over 180 ppm).
We can determine the amount of bicarbonate ions in water similarly by titrating a water sample with a standardized HCl solution. The indicator used in the titration is methyl orange indicator which changes color from yellow to red when an excess HCl is present and the solution pH drops below 4. It is a 1:1 reaction.
HCO3- (aq) + HCl (aq) → H2O (l) + CO2 (g) + Cl- (aq)
Example: A student used 5.20 mL of 0.0100 M HCl solution to titrate 25.00 mL of the sample water. Report the alkalinity in molar concentration of bicarbonate and milligrams of bicarbonate per liter of the sample (ppm).
moles of HCl required = 0.0100 M 0.00520 L = 0.0000520 mole
moles of bicarbonate in 25 mL = moles of HCl required = 0.0000520 mole
molarity of bicarbonate = moles/L = 0.0000520 mole / 0.02500 L = 0.00208 M
milligrams of bicarbonateper L= mole of bicarbonate molar mass of bicarbonate 1000 mg/g = 0.00208 M 61.02 g/mole 1000 mg/g = 127 mg/L = 127 ppm
Experimental Procedure
Water Hardness Test: 25 mL of water sample is combined with 2 mL of NH3/NH4+ buffer and five drops of calmagite indicator. EDTA out of the buret is added until the color goes from red to blue. The volume of EDTA required is noted and the concentration of CaCO3 is calculated. Use a 25 mL sample of distilled water as your control.
Alkalinity Test: 25 mL of water sample is combined with five drops of methyl orange indicator.HCl out of the buret is added until the color goes from yellow to red. The volume of HCl required is noted and the concentration of bicarbonate is calculated. Use a 25 mL sample of distilled water as your control.
Safety Precautions
The fumes from the NH3/NH4Cl buffer at pH 10 are toxic, corrosive, and irritating. Dispense under a fume hood. Wash hands thoroughly before leaving the lab.
Pre Laboratory Questions
- A student used 6.05 mL of 0.0100 M EDTA solution to titrate 25 mL of the sample water. Report the hardness in milligrams of CaCO3 per liter of the sample. What is the classification of the water sample (soft, slightly hard, moderately hard, etc.)?
- Is it better to drink hard water or soft water? Explain your answer.
- Is acidic water or basic water better for our health? Explain your answer.
REPORT SHEET
LABORATORY8: ANALYSIS OF SOLUTIONS
Name:______Section:______Date:______
Partner:______
IdentificationpHLabel
Bottled water sample 1______
Bottled water sample 2______
Distilled water ______
Water Hardness
Bottled water sample 1
Volume of water sample titrated, mL______
Trial#123
Volume of EDTA required, mL______
Number of moles of EDTA, mol______
Number of moles of Ca2+, mol______
Water hardness, mg CaCO3 in L sample______
Average Total Hardness, mg CaCO3/L______
Classification of your water sample______
Bottled water sample 2
Volume of water sample titrated, mL______
Trial#123
Volume of EDTA required, mL______
Number of moles of EDTA, mol______
Number of moles of Ca2+, mol______
Water hardness, mg CaCO3 in L sample______
Average Total Hardness, mg CaCO3/L______
Classification of your water sample______
Distilled water
Volume of water sample titrated, mL______
Trial#123
Volume of EDTA required, mL______
Number of moles of EDTA, mol______
Number of moles of Ca2+, mol______
Water hardness, mg CaCO3 in L sample______
Average Total Hardness, mg CaCO3/L______
Classification of your water sample______
Alkalinity
Bottled water sample 1
Volume of water sample titrated, mL______
Trial#123
Volume of HCl required, mL______
Number of moles of HCl, mol______
Number of moles of bicarbonate, mol______
alkalinity, mg bicarbonate in L sample______
Average alkalinity, mg bicarbonate/L______
Bottled water sample 2
Volume of water sample titrated, mL______
Trial#123
Volume of HCl required, mL______
Number of moles of HCl, mol______
Number of moles of bicarbonate, mol______
alkalinity, mg bicarbonate in L sample______
Average alkalinity, mg bicarbonate/L______
Distilled water
Volume of water sample titrated, mL______
Trial#123
Volume of HCl required, mL______
Number of moles of HCl, mol______
Number of moles of bicarbonate, mol______
alkalinity, mg bicarbonate in L sample______
Average alkalinity, mg bicarbonate/L______
POST-LABORATORY PROBLEMS: Must be included in your lab report.
- The pH of water for human consumption should be close to neutral. How closely do your samples compare with this goal?
- Report the water hardness in ppm and molar concentration of CaCO3. How would you classify your water samples according to table 1?
- Report the bicarbonate concentrations in ppm and molar concentration.
- Compare the water hardness and alkalinity of two water samples. Which one has higher water hardness? Which one has higher alkalinity? Can you decide which one is better?
1