Complexometric Determination of Water Hardness Knowledge Check Questions
What happens if we conduct the experiment at a pH of higher than 10?
Might denature something crucial to the experiment (guessing, possibly not true)
Describe what it means to quantitatively transfer? What are the steps involved?
Pour out all liquid from flask into other beaker, fill it with more water so no molecules are leftover in the flask.
Suggest a method to soften hard water
Temporary hardness can be boiled out
What is the difference between temporary and permanent hardness?
Temporary water hardness is caused by the presence of bicarbonate minerals (calcium bicarbonate and magnesium bicarbonate) Permanent water hardness is usually caused by chlorides or calcium and magnesium sulfates.
How is the indicator used for the EDTA titration different from the indicator in the NaOH titration?
The EDTA titration indicator observes the concentration of a certain ion (Mg2+) whereas the NaOH titration indicator observes changes based on pH
Why do we use a buffer of pH 10?
The magnesium ion forms a very unstable complex with EDTA and has to be kept at a pH of 10 to ensure complete titration
What is total water hardness and how can we use that to find the temporary water hardness?
Total hardness is all CaSO4, Ca(HCO3)2, and Mg2+ molescules, but we can't easily separate Ca2+ and Mg2+ ions, we can determine total hardness and permanent hardness to find the remaining hardness (which would be the temp water hardness)
Why can a magnesium-indicating reagent be used to detect the end point of the EDTA titration?
When the indicator is bound to Mg, it's red. When the EDTA takes the Mg from it, it's blue.
What is a chelate?
a compound containing a ligand (typically organic) bonded to a central metal atom at two or more points.
At what hardness is water generally considered to be soft?
below 60mg/l
How will EDTA be used to determine the moles of cations in the water?
it reacts in a 1:1 ratio with cations in water, so equal moles of EDTA and cation will react up until the equivalence point
How do buffers work to keep a constant pH?
the buffer is a relatively high pH in great concentration so that if anything that would interact with the pH enters the solution, it can react with it and neutralize it, maintaining a fairly stable pH.
What is the difference between how hardness of water was determined and how it is thought of now?
---actually don't know---
What's the ratio for the Ca/Mg-EDTA complex?
1:1 ratio
Which ions mainly contribute to water hardness?
CaSO4, Ca(HCO3)2, and Mg2+
What indicator is used and what does it indicate?
Calmagite, indicates presence of Mg2+
Why does a chelate combine with Mg and Ca ions?
Chelates only bond with metal ions, and Mg and Ca are both metal ions
How do we make the buffer and how do we know its pH?
Combining a determined amount of NH4Cl with water. We are able to know the pH by knowing the concentration of H+ ions in the solution, and by taking the -log of the concentration we have the pH value
Hardness is normally expressed as calcium carbonate concentration, but what are we actually determining in this lab?
Concentration of magnesium in unknown solid
If there are other metal ions in your water sample will they be complexed with EDTA?
EDTA can react only with multiply charged cations, so if it isn't multiply charged then no
What are the industrial and commercial consequences of hard water?
Hard water has minerals which are safe to ingest, but can create dry skin and dry hair if used while showering/bathing. Soda companies can't have hard water as it can alter the color and taste of the soda.
What does heating do to the soluble calcium bicarbonate in the water?
It converts to calcium carbonate, which is able to precipitate out of the water
Why was a buffer solution added to the titrations with the calmagite indicator?
It needs to be in a solution of pH 10 to titrate completely (Mg2+ would struggle otherwise)
What is the role of a magnesium-indicating reagent?
It provides a color change at the end point of the titration
