Titration - SODIUM THIOSULFATE Flashcards Preview

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Flashcards in Titration - SODIUM THIOSULFATE Deck (29):

Explain how iodine, a non-polar substance of very low water solubility is brought into aqueous solution

Reacts with iodide [potassium iodide]


The iodine solution was made up in a 500cm^3 volumetric flask. Describe the procedure for measuring 25.0cm^3 of this solution into a conical flask

Pour iodine solution into a clean,dry beaker
Use pipette which was rinsed with deionised water and solution to be contained
Fill using pipette filler until bottom of meniscus is on graduation mark and read at eye level
Tap pipette against wall of conical flask


Name a suitable indicator for this titration



At what stage is the indicator added

Colour in conical flask is pale yellow - indicates close to end point


State the colour change at the end point in the presence of the indicator

Blue/black -> colourless


Explain why the use of distilled water instead of deionised water throughout this experiment would be likely to ensure a more accurate result

Deionised - only has ions removed and could therefore contain non-ionic substances that could be oxidised/reduced


Describe how the crystalline thiosulfate was dissolved, and how the solution was transferred to the volumetric flask and made up to exactly 500cm^3

Add crystals to beaker and rise from clock glass into beaker. Stir and dissolve
Pour using funnel into volumetric flask and washings of beaker
Add deionised water until bottom of meniscus is at graduation mark at eye level
Stopper and invert several times


Pure iodine is almost completely insoluble in water. What must be added to bring iodine into aqueous solution?

Potassium iodide [source of iodide]


What sequence of colours was observed in the conical flask from the start of the titration until the end point was reached

Brown-pale yellow-blue/black-colourless


Why is hydrated sodium thiosulfate not suitable as a primary standard?

It loses water of crystallisation readily
It is not stable


Why are iodine solutions made up using potassium iodide solution?

Iodine is fairly soluble in water, however it reacts with iodide to form I^3- ions which are very soluble


Why does starch solution have to be freshly prepared?

It detiorerates quickly on standing


Why is starch indicator added close to the end point?

To give a sharp end point, while avoiding the formation of excess starch complex which would be difficult to decompose


Describe the appearance of the sodium thiosulfate crystals

Colourless crystalline solid


Give one advantage of using a clock glass rather than paper when weighing out the sodium thiosulfate crystals

Clock glass is more suitable than paper since any sodium thiosulfate remaining on the clock glass can be washed off, this is not possible with paper


Why is it necessary to remove the funnel from the burette prior to carrying out the titration

Failure to remove the funnel may introduce an error into the readings as drops of liquid may fall from the funnel into the course of the titration. This causes the titration result to be inaccurate


Why is it advisable to use deionised water rather than tap water when making up solutions in this experiment

Tap water contains many dissolved ions which could interfere with the reactions taking place


Describe how you would take the reading in the pipette when using KMnO4?

The reading should be taken from the top of the mensicus


Why is it not possible to make up a solution of I2 directly? How is this overcome?

Iodine vaporises slightly at room temperature. In addition iodine does not dissolve in water
A standard solution of iodine is obtained by reacting a standard solution of acidified potassium permanganate with excess potassium iodide


What colour change is observed in the conical flask when the dilute sulfuric acid and potassium iodide solution are added to the KMnO4

Purple - reddish/brown colour


What colour change is observed at the end point of the titration

Blue/Black -> Colourless


Why is it not necessary to know precisely the exact number of KI added

The KI is present in excess in order to ensure that the amount of iodine formed during the titration depends only on the amount of potassium permanganate present. In addition, I-ions are necessary in order to keep in solution the I2 that is formed


How is a standard solution of iodine generated?

Reacting a standard solution of potassium permanganate with excess potassium iodide in conical flask


What is the apperance of the iodine solution?



How can a sodium thiosulfate solution be standardised?

Standard solution of iodine


How can an iodine solution be standardised?

Standard solution of sodium thiosulfate


What is found in the conical flask during the titration?

Potassium permanganate + dilute sulfuric acid + potassium iodide


What is in the burette and pipette

Burette - Sodium thiosulfate
Pipette - Iodine solution


Is sodium thiosulfate an oxidising or reducing agent?

Reducing agent