Titration - SODIUM THIOSULFATE Flashcards
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
Starch
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
