[Dolphinsiu]

Reagent used

1. o-Phenanthroline solution
2. Hydroxylamine hydrochloride solution
3. Sodium citrate solution
4. Fe (II) standard solution
5. pH paper

After pipetting 1 cm3, 2cm3, 5cm3, 7cm3 and 10cm3 of standard Fe (II) solution.
Dilute to 70 ml and add 1cm3 of hydroxylamine hydrochloride solution and stand for 15 min.
then o-Phenanthroline solution is added and mixed.
Add sodium citrate solution until the pH value is about 3.5

Why the pH paper must not be dropped into the solution ?
(I only think that the dye absorbed by pH paper are washed away.
or the pH paper is not easily observable under the orange-red colour of solution)

[AWK]

Think about possible errors

[Dolphinsiu]

As I find that pH paper is a filter paper dyed by organic compound called dimethyl...(organic cpd) and sodium salt. Do these really affects the value in absorbance?

When I use glass rod to dip the solution and transfer to pH paper, the dye that absorbed by pH paper still can remains in the glass rod. It still can cause errors.

But I never think about sodium salt cause any error, as Na will not form complex with o-phenanthroline!

Do any books introduce this aspect of information? (as I find many book, no hints is provided)

[AWK]

Think about possible errors when removing piece of pH paper

[Dolphinsiu]

What I only think of is that the solution will be absorbed into pH paper, after removing it, causing loss of solution, lowering the concentration of solution, affecting the amount of sodium citrate added, thereby affecting the value in absorbance.

[AWK]

Fortunately, you use a huge volumes (probably 100 mL). Usually solution are prepared in 10 mL volumes.

[Dolphinsiu]

o i see. I really use 100 ml volumetric flask. Loss of solution is still really the major error?

[AWK]

I think, in your case, the highest error comes from pipetting of standard solution. Your error may be a few percent