Making a standard solution

In the world of laboratory work, accuracy is key. A primary standard like potassium hydrogen phthalate can help ensure that measurements are precise and reliable. By preparing a standard solution of this compound and using it in volumetric analysis, researchers can obtain accurate concentration measurements that are essential for a wide range of experiments. This article discusses the process of making a standard solution of potassium hydrogen phthalate, the importance of using primary standards, and the benefits of using volumetric analysis in the lab.

Aim

The purpose of this experiment is to prepare a standard solution of potassium hydrogen phthalate.

Introduction

Potassium hydrogen phthalate, is a primary standard because it meets certain requirements.

• It must be available in a highly pure state.
• It must be stable in air.
• It must be easily soluble in water.
• It should have a high molar mass.
• In solution, when used in volumetric analysis, it must undergo complete and rapid reaction.

You will need to weigh accurately a sample of potassium hydrogen phthalate and use it to make a solution of concentration close to 0.10 mol dm³.

Requirements

• safety spectacles
• weighing boat
• spatula
• potassium hydrogen phthalate (IRRITANT)
• balance capable of weighing to within 0.01 g
• beaker, 250 cm³
• wash bottle of distilled water
• stirring rod
• volumetric flask, 250 cm³
• filter funnel
• dropping pipette

Procedure

1. Transfer between 4.8 and 5.4 g of potassium hydrogen phthalate into a weighing bottle and weigh it to the nearest 0.01 g.
2. Put about 50 cm³ of water into a 250 cm³ beaker. Carefully transfer the bulk of the potassium hydrogen phthalate from the weighing boat into the beaker.
3. Reweigh the boat with any remaining potassium hydrogen phthalate to the nearest 0.01 g.
4. Stir to dissolve the solid, adding more water if necessary.
5. Transfer the solution to the volumetric flask through the filter funnel. Rinse the beaker well, making sure all liquid goes into the volumetric flask.
6. Add distilled water until the level is within about 1 cm of the mark on the neck of the flask. Insert the stopper and shake to mix the contents.
7. Using the dropping pipette, add enough water to bring the bottom of the meniscus to the mark. Insert the stopper and shake thoroughly ten times to ensure complete mixing. Simply inverting the flask once or twice does not mix the contents properly and is a very common fault.
8. Label the flask with the contents, your name and the date. Leave a space for the concentration to be filled in after you have calculated it. This can now be used to calculate the unknown concentration of a base.

Molar mass of potassium hydrogen phthalate,	g mol-1
Mass of bottle and contents before transfer, (m1)	g
Mass of bottle and contents after transfer, (m2)	g
Mass of potassium hydrogen phthalate, m = (m2 – m1)	g
Moles of potassium hydrogen phthalate, n = m/M	mol
Volume of solution, V	dm3
Concentration of potassium hydrogen phthalate, c = n/V	mol dm-3

Questions

What effect would each of the errors described below have on the concentration of potassium hydrogen phthalate?

(a) Some of the solid potassium hydrogen phthalate was spilled in making the transfer.

(b) Not enough water was added to bring the volume up to the mark.

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Answers

(a) If some of the solid potassium hydrogenphthalate was spilled during the transfer, the mass of the solid used to make the solution would be less than intended. This would lead to a lower number of moles of potassium hydrogenphthalate being present in the solution, which would result in a lower concentration of the solution than intended.

(b) If not enough water was added to bring the volume up to the mark on the volumetric flask, the resulting solution would have a higher concentration than intended. This is because the actual volume of the solution would be less than the volume of the volumetric flask, which would lead to a higher concentration of potassium hydrogenphthalate than intended.