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Daniele Naviglio » 11.Acidimetry


Acidimetry

The term acidimetry refers to that part of volumetric analysis whereby an acid solution at known concentration, along with a specific indicator, is used to titrate a base solution and thus work out its concentration“.

Translated from: Treccani on-line dictionary


Acidimetry (cont.)

As we saw with alkalimetry, the aim of acidimetry is the determination of inorganic, organic and biological substances with intrinsic base properties.

Acidimetry enables us to:

  • Determine the alkalinity of water;
  • Carry out important analysis on different types of drugs.

Alkalinity of water

When we refer to the alakalinity of a solution what we mean is its ability to react with hydrogen ions and therefore neutralize acids.

There are two types of alkalinity:

  1. Stoichiometric alkalinity;
  2. Real alkalinity.

Stoichimetric alakalinity is defined as that deriving from the concentration of carbonates and bicarbonates. The presence of these ions is important to buffer pH changes resulting from photosynthesis. They also act as complexing agents for toxic metals.

Real alkalinity, on the other hand, refers to the concentration of OH- ions. It can be measured using a pH meter, making a note of the temperature when the reading was taken.

Alkalinity of water is due to the presence of carbonate (CO32-) and bicarbonate ions (HCO3-) and, if the pH is high enough, hydroxide ions as well. Many ions in weak acids, however, sulfide ions, disulfides, phosphates, borates and ammonia, can react with the H+ ions and thus increase the alkalinity.

Alkalinity of water (cont.)

The weak acid most commonly found in water is carbonic acid (H2CO3) which forms when carbon dioxide from the atmosphere or the ground, or from the aerobic decomposition of organic substances, dissolves in water.

 

  1. CO2 (gas) ↔ CO2 (aq)
  2. CO2 (aq) + H2O → H2CO3 (aq)
  3. H2CO3 (aq) → H+ + HCO3-
  4. HCO3- → H+ + CO32-

The equilibrium in number 3 and 4 tends to move to the right as the concentration of hydrogen ions reduces, thus, all other conditions being equal, the concentration of bicarbonate and carbonate ions increases as the pH increases. Natural water may have a base pH if there are salts dissolved in it that cause base hydrolysis, for example calcium or sodium bicarbonates, or even base substances like ammonia.

Alkalinity from carbonates and bicarbonates is not dangerous for people’s health so Italian law has not set any guidelines or maximum permitted levels for alkalinity.

Determining alkalinity of water

This method is used for the determination of alkalinity due to strong base. A sample of water under analysis is titrated with a strong acid solution. To reveal a strong base content in the presence of carbonates, we need to take the titration up to the two consecutive equivalence points, that of the bicarbonate and that of the carbonic acid. Those two points can be seen when the phenolphthalein changes colour (at pH 8.3), and then when the methyl orange changes colour (between pH 3.5 and 4.5).

Determining alkalinity of water (cont.)

Given that a is the volume of acid solution, with a normality N, that is needed to reach the first equivalence point, and that b is the total volume (including volume a) needed to reach the second equivalence point, the value of P (alkalinity at phenolphthalein) and T (total alkalinity) can be calculated using the following formulae:

 

  • First equivalence point: P(meq/L) = (a*N*1000)/V (1)
  • Second equivalence point: T(meq/L) = (b*N*1000)/V (2)

 

Where V is the volume (in mL) of the water sample. We can find out the alkalinity of the water, expressed in mg CaCO3/L by multiplying the result of the second calculation by 50 (equivalent weight of calcium carbonate).

Determining alkalinity of water (cont.)

Once titration of the sample has given us the P and T values (in meq/L), the alkalinity due to hydroxides can be worked out based on the assumptions described in the table.

Once P and T (in meq/L) have been found from titration of the sample, the alkalinity due to the hydroxides is determined on the basis of the assumptions in the table. Source: APAT

Once P and T (in meq/L) have been found from titration of the sample, the alkalinity due to the hydroxides is determined on the basis of the assumptions in the table. Source: APAT


Reaction responsible for alkalinity P

CO32- + H+ ↔ HCO3-
OH- + H+ ↔ H2O

Reaction responsible for alkalinity M

HCO3- + H+ ↔ CO2 + H2O

 

Equations to calculate alkalinity P (phenolphthalein) and alkalinity M (Methyl orange). Source: APAT

Equations to calculate alkalinity P (phenolphthalein) and alkalinity M (Methyl orange). Source: APAT


Practical application

mg CaCO3/L = 10,12 meq/L * 50 = 506

2,645 meq/L < 10,12 meq/L /2

Hydroxides = 0 meq/L
Carbonates = 2 * 2,645 meq/L = 5,29 meq/L
Bicarbonates = 10,12 meq/L – 5,29 meq/L = 4,83 meq/L

Example of how to calculate alkalinity of a water sample

Example of how to calculate alkalinity of a water sample


 To recap

Sources: Mygrass (Pipette and Burette);  Laboratorio di Fisica-Chimica dell’Istituto di Istruzione Secondaria Superiore “A. Greppi ” (Titration); Istituto MAgistrale “Leonardo da Vinci” di Alba (Reading); Istituto Tecnico “Enrico Fermi” di Modena (Indicators); Steroglass S.r.l. (Burette)

Sources: Mygrass (Pipette and Burette); Laboratorio di Fisica-Chimica dell'Istituto di Istruzione Secondaria Superiore "A. Greppi " (Titration); Istituto MAgistrale "Leonardo da Vinci" di Alba (Reading); Istituto Tecnico "Enrico Fermi" di Modena (Indicators); Steroglass S.r.l. (Burette)


To recap (cont.)

Read off volume of second turning point and record

Read off volume of second turning point and record


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