Lab practical 2- water hardness Flashcards
What is water hardness a measure of?
the presence of magnesium and calcium in the water
What is alkalinity?
waters capacity to resist acidic changes in pH
What is total hardness?
Sum of Ca and Mg concentration expressed as calcium carbonate in mg L-1 or ppm.
What is a complexometric titration?
Use of complexing agent (chelator like EDTA) to capture metal ions
Amount chelator used calculate amount of metal in water
What is a chelate?
a compound containing a ligand bonded to a central metal atom at two or more points
What is a ligand? (part of chelate)
ion or molecule which donates a pair of electrons to the central metal atom
What is standardisation?
process of determining the exact concentration (molarity) of a solution
What are the 3 steps of the water hardness practical?
1- Standardisation of EDTA Solution
2- Determination of Total Hardness (Mg2+ and Ca2+)
3- Determination of Calcium Hardness (Ca2+ only)
What is the equation for moles?
moles= mass (g) / Mr
What are the degrees of hardness and the CaCO3L-1 equivalent?
Soft = <50
Moderately soft = 50-100
Slightly hard = 100-150
Moderately hard = 150-200
Hard = 200-300
Very hard = >300
What types of water were tested in the water hardness lab?
Distilled water
Tap water
Mineral water
“Sea” water (artificial)
What is the hypothesis for the water hardness practical?
sea water will be hardest and distilled softest
What is the aim of the water hardness practical?
use titration to determine the water hardness of 4 water samples
Why are hard waters undesirable?
Require considerable amounts of soap to produce lather
Produce scale in industrial boilers, heaters, and hot-water pipes
What are the ecological advantages of hard water?
neutralisation of acid deposition and the reduction of the solubility of toxic metals
What weathered materials contribute to water hardness?
limestone
dolomite
gypsum
How do you calculate Mg hardness? (water hardness practical)
Magnesium hardness = total hardness − calcium hardness
What is a titration?
technique where a solution of known concentration is used to determine the concentration of an unknown solution
What is the titrant and analyte? (titrations)
Titrant known solution
Analyte unknown solution
What is the endpoint for titrations?
physical change (colour)
What is the colour change expected from Eriochrome Black T indicator? (water hardness titration)
wine red to blue
What materials were used in the water hardness practical?
- pH meter and combination or glass-calomel electrode
- Tap water, bottled mineral water, distilled water, artificial “sea
water”. - Primary standard CaCO3 solution, 0.004 M
- Standard disodium ethylenediaminetetraacetate dihydrate
- 0.1M NaOH
- Eriochrome Black T indicator solution, 0.5% (w/v) in ethanol, freshly
prepared. - Patton & Reeder’s indicator (mix with 100 times its weight of
Na2SO4). - Buffer solution. Dissolve 7.0 g NH4Cl in 57.0 mL of concentrated
ammonia solution diluted to 1 L. - Hydrochloric acid (HCl), concentrated
What is step 1 of the standardisation of EDTA for the water hardness practical? (pipette and CaCO3)
Pipette, four 20 mL aliquots of the primary standard CaCO3 (0.004 M) solution into 250 mL conical flasks
What is step 2 of the standardisation of EDTA for the water hardness practical? (titrate and EDTA Eriochrome black)
Titrate, with 0.004 M EDTA by adding 15 mL of the EDTA solution the burette, then 10 mL of the buffer solution followed by five drops
of the Eriochrome Black T
What is step 3 of the standardisation of EDTA for the water hardness practical? (continuation of titration)
titration continue keep stirring until colour change
The first will act as a rough
then do 3 accurate titrations
What is a rough titration?
the first titration done to give a rough idea of the endpoint to allow next titrations to be more accurate
What is step 4 of the standardisation of EDTA for the water hardness practical? (conc EDTA and milligrams CaCO3)
avg of 3 accurate titrations
Use molar concentration formula
Then use moles equation substituting volume for mass
What is the molar concentration equation?
(M1V1/n1)= (M2V2/n2)
What does the formula for molar concentration mean? (M1V1 etc)
M1 and M2 are the moles of CaCO3 and EDTA (the unknown, that you want to calculate), V1 and V2 are the volumes of CaCO3 and EDTA used, respectively, and n1 and n2 are both 1 in this case, since 1 EDTA molecule reacts with 1 molecule of CaCO
What is step 1 of the determination of total hardness? (water hardness practical- water sample, rough titration)
Pipette 20 mL water sample into 250mL conical flask, add 10 mL buffer and 5 drop of indicator
Do rough titration
What is step 2 of the determination of total hardness? (repetition, titration)
Repeat titration with 20 mL water but add 3/4 expected titrant (based off rough end point)
Repeat several times for average
What is step 3 of the determination of total hardness? (calculate)
Calculate the total hardness as mg L-1 CaCO3 equivalents
What is the equation to calculate hardness as mg CaCO3L-1? (water hardness practical)
mg CaCO3 L-1 = 1000 * Vt * M/Vs
What does the equation for hardness as mg CaCO3L-1 mean? (water hardness practical)
Vt= mL titrant needed for endpoint
M= mg CaCO3 equivalent to 1 mL EDTA
Vs= volume sample analysed
What is step 1 of determining calcium hardness? (pH)
adjust pH of 20 mL water sample so between 12 to 12.25 using 1 M NaOH
What is step 2 of determining calcium hardness? (Patton & Reeders)
Add 0.2g Patton & Reeder’s to solution and titrate
Repeat in several individual samples of same water for average
What is step 3 of determining calcium hardness? (calculate)
Calculate Ca hardness expressed mg CaCO3L-1
What is the formula for finding Ca hardness?
Ca (mg L-1) = 401 * Vt * M/Vs
