Standard solutions

A standard is reference that can be relied upon to allow accurate comparison and measurement. In this context, standard solutions are made up using compounds whose purity can be assured, within the limits of a laboratory environment. Potassium hydrogenphthalate is a primary standard, having the following characteristics:

• It is a crystalline solid, available in pure form
• It is stable in the environment
• It has a high relative mass (204.23)

These characteristics allow the preparation of a standard solution to a good degree of accuracy.

Analytic reagents can be supplied that have a very small inaccuracy in terms of content.

Envoronmental stability means that we can be assured that the contents of the container have not degraded with time.

The high relative molecular mass allows for larger masses to be weighed out, reducing the percentage error introduced during the weighing process.

---

^ top

Procedure

Part 1 - preparing the standard solution

• Accurately weigh out approximately 5.1 g of potassium hydrogenphthalate (Mr = 204.23)
• Transfer to a volumetric flask and dissolve in about 100 ml of distilled (deionised) water. Then make up to the mark with distilled (deionised) water.
• Invert several times to ensure complete mixing.

Part 2 - preparing the sodium hydroxide solution

• Transfer a 25 ml aliquot of the 'bench' sodium hydroxide solution into a 250 ml volumetric flask using a pipette and filler.
• Add distilled (deionised) water to the mark.
• Invert several times to ensure complete mixing.

Part 3 - the titration

• Transfer a 25ml aliquot of the potassium hydrogenphthalate solution into a conical flask using a pipette and filler.
• Add 5 drops of phenolphthalein indicator solution
• Fill a 50 ml burette to the 0.00 mark with the diluted sodium hydroxide solution.
• Add the sodium hydroxide slowly to the acid/indicator solution in the conical flask swirling constantly until the first permanent hint of pink appears (the end-point).
• Repeat the steps above, adding the sodium hydroxide dropwise near the end-point.
• Repeat the titration as many times as needed to obtain concordant results (results within 0.1 ml of each other)

---

^ top

Recording the data

Ensure that all of the data is recorded with suitable units and inaccuracies.

The following shows example raw data and analysis.

Raw data

Mass of potassium hydrogenphthalate = 5.22g ± 0.01 (here we use the actual mass weighed out, as an example)

The initial solutions are all colourless.

Titre	Initial burette reading /ml ± 0.05	Final burette reading /ml ± 0.05	Volume added /ml ± 0.1	Observations
1	0.00	25.25	25.25	red end
2	0.05	24.85	24.80	pink end
3	0.00	24.75	24.75	pale pink

Data analysis

Titres 2 & 3 are concordant and were used to average the titration volume = (24.80 + 24.75)/2= 24.775

This is rounded to three sigificant figures as the inaccuracy is ± 0.1

Hence titre = 24.8 ml ± 0.1

The moles of potassium hydrogenphthalate used = mass/Mr = 5.22/204.23 = 0.00257

Molarity of the potassium hydrogenphthalate solution used = 0.00256/0.25 = 0.102 mol dm^-3

Moles of potassium hydrogenphthalate in 25 ml aliquot = 0.102 x 0.025 = 2.56 x 10^-3

Equation for the reaction between sodium hydroxide and potassium hydrogenphthalate (abbreviated to KHPh):

NaOH + KHPh KNaPh + H₂O

Hence moles of sodium hydroxide also = 2.56 x 10^-3

Volume of sodium hydroxide from the titration = 24.8 ml

Hence molarity of sodium hydroxide = 0.103

The was diluted in a ratio of 1:10 from the original stock bottle

Hence concentration of sodium hydroxide in the original stock bottle = 1.03 mol dm^-3

Treatment of errors and inaccuracies

These should be recorded as percentage inaccuracy and then propagated through a typical series of steps:

1 Making the potassium hydrogenphthalate solution:

Mass of potassium hydrogenphthalate = 5.22g ± 0.01 = 0.19% (inaccuracy)

Volumetric flask 250 ml ± 0.23 = 0.092%

2 Transfering 25 ml aliquot (pipette)

25 ml ± 0.06 = 0.24%

3 Titration inaccuracy

24.8 ml ± 0.1 = 0.40%

Total inaccuracy = 0.19 + 0.09 + 0.24 + 0.40 = 0.92%

Applying this inaccuracy to the sodium hydroxide give molarity = 1.03 ± 0.01

---

^ top

Titrations