Introductory investigations: Measure pH with pH-paper

Which components gives typically one of the following types of solutions?
You should focus on the actual solution.
Consider your answer before you proceed.

Acid solution?
Neutral solution?
Basic solution?

Solutions may be acidic for different reasons:

A strong acid will give an acidic solution, containing its corresponding base. Strong acids are strong electrolytes and are fully dissociated in aqueous solution.

Which anions are corresponding bases to strong acids?
(Consider before proceeding. You may use a table of acidity constants (see help)

Weak acids will also give acidic solutions.

A salt may be a weak acid if one of the ions is an acid. At the same time the other ion should only be a very weak base.

Hydrated metal ions are acidic.
Which metal ions ?

The table shows anions of strong acids, which are not or only marginally basic in aqueous solution. The identification reactions may be used to identify the anions.

Anions of strong acids
Name	Formula	Corr. acid	pK

Consider what you may conclude from the acid-base properties of the unknown.

Metal ion are hydrated in aqueous solution and are acids for that reason. The alkali metal ions are extremely weak acids and reacts neutral in solution.
The tabel shows some common examples of acidic hydrated metal ions:

Hydrated acidic metal ions
Name of acid	Formula	pK	Corr. base

Consider what you may conclude from the acid-base properties of the unknown.

Neutral solutions may correspond to different situations:

Soluble salt with
a cation being a very weak acid - typically the alkali metal ions - and
an anion being a very weak base with a corresponding strong acid

Buffer solutions with the pK of the acid component being around 7 will be neutral.

Which components?

Salts composed by a weak acid and a weak base of comparable strength

Neutral solutions will result if the same concentrations of an acid and a base with comparable strengths are mixed.

Amfolytes
Name	Formula	pK	Corr. base

Consider what you may conclude from the acid-base properties of the unknown.

Basic solutions may be of different kinds:

Ammonia and most anion are bases and give basic solutions when dissolved in water.
There are certain exceptions.

Which anions are too weak bases to give basic solutions i water?
(Consider the answer before proceeding. You may use a table. (see help))

The strength of different anionic bases are different. Some are rather weak and give a pH value between 7 og 10.

Which anions are only rather weak basic?
(Consider your answer before proceeding. You may use a table of pK values (see help))

Anions of strong acid are not basic anions.
They are:

Anions of strong acids
Name	Formula	Corr. acid	pK

Consider what you may conclude from the acid-base properties of the unknown.

Anions of the not that weak acids are only very weak bases. In water a pH value between 7 and 10 is obtained if no stronger acid or base is present .
They are:

Anions of the not-that-weak acids
Name	Formula	Corr. acid	pK

Consider what you may conclude from the acid-base properties of the unknown.

The poor accuracy of a pH measurement using pH paper means that solutions showing a pH value of 6 or 7 are neutral. pH paper actually measures a value of 6 in pure water. Accordingly if the solution in question gives a pH around 6, you have to consider the presence of components giving very weakly acidic as well as neutal solutions.