Water titrations

Question 1

Describe making up the EDTA in a solution

To dertermine the total hardness in a water sample using EDTA

Answer 1

A known volume of EDTA is made up into a solution using a solid, weighed on an electronic balance… made up to sln in a beaker.. volumetric flask…bottom of the mencisicus rests on the graduation mark

Question 2

describe filling the burette with the EDTA solution

To dertermine the total hardness in a water sample using EDTA

Answer 2

clap burette upright … fill the edta above the 0cm mark and then let the liquid slowly drain out of the tap until the bottom of the menciscus rests on the graduation mark.

Question 3

Describe transferring an excact solution of hard water into the conical flask.

**To dertermine the total hardness in a water sample using EDTA

Answer 3

it is common to transfer a mulitable of 25cm of hard water… transfer the known volume of water into a breaker

Question 4

What is in the beaker before the titration occours? What colour is in the titration before the reaction begins

To dertermine the total hardness in a water sample using EDTA

Answer 4

pH buffer solution 10
Erichrome Black T indicator
a RED-WINE colour forms in the conical flask in the presence of Ca2+/Mg2+ ions

Question 5

Name and justify the indicator used for To dertermine the total hardness in a water sample using EDTA

Answer 5

Erichrome Black T
(sometime called solochrome black)
Erichrome Black T is a red-wine colour in the presence of Ca2+/Mg2+ ions
E|richrome Black T turns blue in the absence of Ca2+/Mg2+

Question 6

State and explain the colour change at the end point

To dertermine the total hardness in a water sample using EDTA

Answer 6

The colour turns from red-wine to blue
the presence of of Ca2+/Mg2+ ions to the absence of Ca2+/Mg2+ ioins

Question 7

What is hard water?What is the cause of hard water?

To dertermine the total hardness in a water sample using EDTA

Answer 7

hard water is water that will not easily form a lather with soap
Hard water is caused by the presence of Ca2+ /Mg2+ ions.

Question 8

what is meant by tempoary hardness? In what compound are Ca2+ and Mg2+ ions in tempoary hard water present

To dertermine the total hardness in a water sample using EDTA

Answer 8

Ca(HCO3)2 - calcium hydrogencarbonate

Question 9

What is meant by permanent hardness? In what compound are Ca2+ and Mg2+ ions in permanent hard water present

To dertermine the total hardness in a water sample using EDTA

Answer 9

permanent hardness is hardness that cannot be removed by boiling water
caused by CaSO4

Question 10

Explain the term total hardness

To dertermine the total hardness in a water sample using EDTA

Answer 10

the sum of permanent hardness and tempoary hardness

Question 11

suggest a way of dertimining if a sample of water contained tempoary hardness and how the concentration of tempoary hardness could be achieved

To dertermine the total hardness in a water sample using EDTA

Answer 11

titrate the water sample against edta and find the value for hardness(total hardness)
Boil the water to remove tempoary hardness.
tirtate the boiled water sample against EDTA and find the value for hardness.(Permanent hardness only)

Question 12

What is the full name of the EDTA reagent

To dertermine the total hardness in a water sample using EDTA

Answer 12

ethylenediaminetetraacetic acid

Question 13

Why is a standard solution of EDTA used in this titration

To dertermine the total hardness in a water sample using EDTA

Answer 13

EDTA reacts with metal ions and forms and EDTA-metal complex. During this titration, it will react with the Ca2+/Mg2+ ions in the hard water to use them up. The end point is the moment where all the Ca2+/Mg2+ ions are used up.

Question 14

why is EDTA stored in a plastic container

To dertermine the total hardness in a water sample using EDTA

Answer 14

EDTA reacts with the metal ions in glass

Question 15

What are buffer solutions

To dertermine the total hardness in a water sample using EDTA

Answer 15

solutions that resist changes to pH and keeps the Ph of an eviorment at a desired value

Question 16

Why is pH buffer 10 required for this titration?

To dertermine the total hardness in a water sample using EDTA

Answer 16

A pH of 10 is needed for EDTA to react with the Ca2+/Mg2+ ions and form an edta metal complex
The Erichrome black T indicator changes colour only at a pH of 8-10. An innaccurate end point will be obtained at a pH other than 10

Question 17

Describe how to make up the sodium thiosulfate into solution

Measure the amount of dissolved oxygen in a sample of water by redox

Answer 17

weigh sodium thiosulfate on electronic balance, wash into beaker stir with stirring rod in beaker of deionied water… transfer into a volumetric flask

Question 18

describe filling the burette with sodium thiosulfate solution

Measure the amount of dissolved oxygen in a sample of water by redox

Answer 18

clamp retort stand with burette upright, fill above the 0cm mark and let drain until the bottom of the mencisus rests on the 0cm mark

Question 19

Describe making a solution of iodine using the dissolved oxygen in the water sample NB

Measure the amount of dissolved oxygen in a sample of water by redox

Answer 19

• An clean empty dry reagent bottle is rinsed with deionised water and the water it will contain
• The bottle is full submerged into the water sample and completely filled
• a dropper is used to add 1cm of Mangenses (II) sulfate (MnSo4) and Alkaline potassium iodide (KOH) snd (KI)
• the reagent bottle is carefully stoppered and shaken unil a Brown percipitate is observed
• Concentrated Sulfuric acid (H2SO4)

Question 20

Overall, 1mol of O2…

Measure the amount of dissolved oxygen in a sample of water by redox

Answer 20

…liberates 2 mols of I2

Question 21

Carrying out the titration

Measure the amount of dissolved oxygen in a sample of water by redox

Answer 21

The sodium thiosulfate in the burette
The iodine is in the conical flask
continously swirlling and washing down the sides of the conical flask with deionied water.
a yellow colour is observed
As the iodine is being used up a pale yellow colour is oberved
Starch solution indicator is added. The solution in the conical flask turns blue black
at the end point the blue black colour decolourises

Question 22

name and justify a suitable indicator for this titration

Measure the amount of dissolved oxygen in a sample of water by redox

Answer 22

Starch solution.
iodine turns blue black in the presence of starch

Question 23

state the colour change that occours during the titration

Measure the amount of dissolved oxygen in a sample of water by redox

Answer 23

red-brown to yellow to pale yellow. NOW ADD STARCH - a blue-black colour forms. Blue black to colourless

Question 24

explain the colour change that occours during the titration

Measure the amount of dissolved oxygen in a sample of water by redox

Answer 24

As the sodium thiosulfate is added and reacts with the iodine, the iodine is being used up, causing a less intense colour and changes from brown to pale yellow.
At the endpoint starch is added and a blue black colour forms due to the small amount of iodine left. The sodium thiosulfate is now slowly added in drops and as soon as iodine has been completely used up, the blue black colour decolourises

Question 25

Why is the starch added only when a pale yellow colour forms | Measure the amount of dissolved oxygen in a sample of water by redox

Answer 25

* waiting until a pale yellow colour forms in the conical flask tells us that the end point is very near. The sodium thiosulfate can be added slowly in drops resulting in an very inaaccurate end point. * iodine adsorbs onto starch preventing it reacting with the sodium thiosulfate, resulting in an innacturate endpoint would be obtained.

Question 26

What is meant by BOD | Measure the amount of dissolved oxygen in a sample of water by redox

Answer 26

Biolchemical oxygen demand. It is the amount of oxygen consumed by biological action when a sample of water is kept at 20C for 5 days in the dark

Question 27

Why should a sample of water be analysed for dissolved oxygen immediatly? | Measure the amount of dissolved oxygen in a sample of water by redox

Answer 27

due to biochemical oxygen demand. Living organisms in the water use of oxygen for respiration- lower result for dissolved oxygen would not be obtained is not analysed immediately

Question 28

What is the purpose of adding Manganese sulfate to the reaction bottle | Measure the amount of dissolved oxygen in a sample of water by redox

Answer 28

To provide a source of Mn2+ ions nessary for the formation of **Mn(OH)2**

Question 29

what is the purpose of adding alkaline conditions/ OH-/KOH to the reaction bottle | Measure the amount of dissolved oxygen in a sample of water by redox

Answer 29

provides alkaline conditions for the formation of Mn(OH)2

Question 30

why did a brown percipate form in the bottle | Measure the amount of dissolved oxygen in a sample of water by redox

Answer 30

due to **Mn(OH)3** being formed form the reaction of Mn(OH)2 with dissolved O2

Question 31

what does it mean if a white percipate forms/no brown percipate formed | Measure the amount of dissolved oxygen in a sample of water by redox

Answer 31

There is no dissolved oxygen in the water sample being dissolved The white percipate is due to the formation of Mn(OH)2

Question 32

why was potassium iodide added to the reaction bottle | Measure the amount of dissolved oxygen in a sample of water by redox

Answer 32

provides the I- ions nessary to produce I2

Question 33

**Why was exess potassium iodide added to the reaction bottle NB** | Measure the amount of dissolved oxygen in a sample of water by redox

Answer 33

**To ensure all of the oxygen in the water is used and detected and makes oxygen the limiting reagent** The I2 that us firmed will react with excess I- forming the **triiodide ion I3 which is a soluble version of iodide, it keeps iodine in solution**

Question 34

**Why is concentrated sulfuric acid added to the percipate NB** | Measure the amount of dissolved oxygen in a sample of water by redox

Answer 34

in an acidic enviorment **Mn(OH)3 will react with I- ions to liberate iodine**

Question 35

what is the colour observed on the additon of concentrated sulfuric acid | Measure the amount of dissolved oxygen in a sample of water by redox

Answer 35

a red-brown colour forms

Question 36

why are all the solutions added to the sample of water concentrated | Measure the amount of dissolved oxygen in a sample of water by redox

Answer 36

Minimises the volume of water displaced hence minimising the dissolved oxygen lost. ensure the other reagents are in excess despite being in small volume, meaning all dissolved oxygen reacts

Question 37

How was the manganese sulfate solution, the alkaline potassium iodide and the sulfuric acid added to the bottle containing the water sample and why were they added in this manner | Measure the amount of dissolved oxygen in a sample of water by redox

Answer 37

Using a dropper and ensuring it is under the level of water before adding the solution to avoid bubbling any air into the water

Question 38

What percaution should be taken when stoppering and shaking the bottle after each addition of each solution | Measure the amount of dissolved oxygen in a sample of water by redox

Answer 38

taking care not to bubble any air into the water as this would increase the amount of dissolved oxygen in the water

Question 39

What name is givento calculating the concentration of dissolved oxygen by liberating iodine from the oxygen in this way | Measure the amount of dissolved oxygen in a sample of water by redox

Answer 39

Winkler method

Question 40

explain why the winkler method is unsuitable for chloronated water | Measure the amount of dissolved oxygen in a sample of water by redox

Answer 40

chlorine in the water would react with the iodide ions for form iodine. The titration value would be too large, causing an innacurate end point