Complexometric Titration

Reagents for Complexometric Titrations

Sigma-Aldrich offers Fluka and Riedel-de Haën quality reagents for complexometric titration. The trademark IDRANAL comprises aminopolycarboxylic acids (EDTA and analogs) and their volumetric solutions for use in metal determination by titration. On this page you’ll find more information on the correct choice of complexing agent, indicator, pH value, titration method and masking agents.

Complexometric titration:

  • Metal ions from the sample react with complexing agents (chelating agents) to create the complex
  • Color change determines the end-point of the titration

Analytical applications:

  • Determination of water hardness
  • Determination of metal ions

Features & Benefits of Sigma-Aldrich’s complexometric reagents:

  • Stability at high temperatures
  • Readily soluble in water
  • Form stable chelates with most metal ions over a wide range of pH and temperature
  • Hydrolytic stability in both acidic and alkaline conditions

View complete product list for complexometric titration

View more titration topics:
Volumetric Titration
Non-aqueous Titration

Learn more about complexometric titration using IDRANAL reagents:
Complex formation
Stability constants
Titration methods
Masking agents
View complete product list for volumetric titration

Complex formation

Nitrilotriacetic acid NTA (IDRANAL I) is predominantly used as disodium salt for complexometric titration of heavy metals. For making a 0.1 mol/L solution of NTA, 19.11 g of IDRANAL I are added to a mixture of 500 mL distilled water and 200 mL 0.1 mol/L sodium hydroxide solution in a volumetric flask. The prepared mixture is adjusted to the color change of methyl red and filled up to 1000 mL with distilled water.

Many complexing agents are free acids and, therefore, hardly soluble in water. Consequently, their sodium salts are used for producing the volumetric solutions. Ethylenediaminetetraacetic acid EDTA (IDRANAL II), for example, is nearly insoluble in water; solutions of its alkaline salts are, therefore, used for titration. The disodium salt EDTA-Na2 (IDRANAL III) is readily soluble in water. The trisodium and tetrasodium salts are very soluble in water.

When determining water hardness with EDTA-Na2, the color change of Eriochrome® Black T is not very clear. This can be improved by spiking the volumetric solution of EDTA-Na2 with different amounts of EDTA-Na2-Zinc (IDRANAL II zinc). These mixtures can be obtained as ready-to-use solutions or concentrates:

IDRANAL solutions show following hardness numbers:
IDRANAL solution German Hardness index
1 mL IDRANAL A solution equals 5.6 German degree of hardness in 100 mL water
1 mL IDRANAL B solution equals 1 German degree of hardness in 100 mL water

Both of these solutions are also available as concentrates: IDRANAL A and IDRANAL B

The complex forming properties of diaminocyclohexanetetraacetic acid DCTA (IDRANAL IV) are very similar to EDTA, but a few differences should be mentioned. The complex formation constants of DCTA are slightly higher, which proves especially useful for determination of alkaline earth metal ions. The forming of aluminum-complexes proceeds easier and the usual heating needed during back titration can be eliminated. The nickel-DCTA-complex is highly stable and its cleavage with cyanide is difficult. As a result, separation of nickel from zinc, cadmium and copper is possible. The complex formation with DCTA runs slower than with EDTA, therefore, the titration speed should be slower or the titration should be carried out at an elevated temperature.

DCTA is insoluble in water and has to be dissolved in an alkaline solution for the titration. The solutions of various sodium salts show different pH-values:

Best suited for titration are tri- and tetrasodium salts since they provide for lesser acid formation, and therefore, smaller buffer concentrations are sufficient.
The application of diethylenetriaminepentaacetic acid DTPA (IDRANAL V) is limited to particular cases, e.g. determination of rare earth metals and titration of barium and strontium.
To prepare a 0.1 mol/L solution of DTPA, 39.33 g IDRANAL V is mixed with 500 mL of 1 mol/L sodium hydroxide solution in a volumetric flask, and filled up to 1000 mL with water (pentasodium salt). The titer is adjusted with 0.1 mol/L zinc sulfate solution at pH 10 against Eriochrome Black T.
Glycoletherdiaminetetraacetic acid EGTA (IDRANAL VI) is applicable for simultaneous determination of Ca2+ and Mg2+, as their stability constants are very different.
Hydroxyethylethylenediaminetriacetic acid trisodium salt HEDTA-Na3 (IDRANAL VII) is particularly suitable for determination of Fe3+ ions under acidic conditions.

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Stability constants

EDTA forms stable chelate complexes with most polyvalent metal ions according to the following reaction:
Na4Y + FeCl3 → NaFeY + 3 NaCl
Na4Y + MgCl2 → Na2MgY + 2 NaCl
(Y = EDTA anion)

Higher-valency metal ions form analog 1:1-complexes with EDTA. Also the other IDRANAL types solely form 1:1-complexes except for some NTA-complexes.

There are big differences in complex stabilities which are indicated by their stability constants shown as log K in table 4, according to Schwarzenbach, G.; Flaschka, H. Komplexometrische Titration (Ferdinand-Enke-Verlag, Stuttgart, 1965).

Selected log K values of different IDRANAL types

Mg2+ 5.4 8.7 11 5.2 7
Ca2+ 6.4 10.7 13.2 10.9 11 8.2
Sr2+ 5 8.6 10.5 8.5
Ba2+ 4.8 7.8 8.6 8.4
Mn2+ 7.4 13.8 17.4 15.6 12.3 10.8
Cu2+ 13 18.8 22 21.5 17.8 17.5
Zn2+ 10.7 16.5 19.3 18.6 12.9 14.6
Al3+ 16.1 18.3
Y3+ 11.4 18.1 19.8 22.2
La3+ 10.5 15.5 16.9 19.6 15.8

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Titration methods

Direct titration
A metal salt solution is titrated with a solution of a complexing compound. An added indicator shows the equivalence point. The pH-value of the titration is chosen according to the metal to be titrated and in consideration of other present foreign metals. Earth alkali metals must be titrated in an alkaline solution. Zinc could be titrated at a pH value of 5, where magnesium would be left out of the titration (selective titration). In contrast, a titration at pH 10 would allow for a combined titration of zinc and magnesium (sum titration).

Requirements for direct titration:

  • Complex stability under chosen conditions
  • Fast and stoichiometric course of reaction
  • Suitable indicator

Back titration
Direct titration is sometimes difficult. In many cases complex formation is too slow (aluminum), side reactions (precipitation of hydroxide) take place or a suitable indicator is not available. In these cases a back titration can be used. The solution of the metal salt (e.g. aluminum) is converted with an excess of complexing agent, and after complex formation (may take some time or need another pH value) the excess complexing compound is back titrated with another metal (e.g. zinc). The titration conditions have to be adjusted to the requirements of this second metal.

Substitution titration
Alternatively a substitution titration can be carried out, e.g. for lead. Lead is converted with EDTA-K2Mg where magnesium is substituted by lead to form EDTA-K2Pb and an equivalent amount of magnesium. The magnesium can now be titrated. For this titration, the emerging complex must show higher stability. Ideally suited is IDRANAL II Magnesium for substitution reactions at pH 10, and IDRANAL II Zinc at pH 5.

Indirect titration
If cations or anions do not form complexes with a complexing agent, a direct determination is not possible. In these cases, conversion with a titratable metal can enable indirect titration. A well-known example is the determination of sulfate through precipitation of barium sulfate. The precipitate is removed by filtration, dissolved, and the amount of barium can be titrated. Or the excess barium can be back titrated after precipitation of sulfate. Analog determination of phosphate or fluoride is possible.

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Masking agents

It is possible to protect single metal from reaction with EDTA by previous conversion with an auxiliary complexing agent. These so called masking procedures considerably expand the field of possible separation, and unmasking of the protected single metal is also possible. Changes in pH through addition of masking agents should be considered and their concentration should be kept low.

The following substances can be used as masking agents:

  • Potassium cyanide
    Forms highly stable complexes in an alkaline solution with zinc, cadmium, mercury, nickel, cobalt, copper (higher stability than EDTA-complexes)
  • Triethanolamine
    Forms stable complexes with iron and aluminum in an alkaline solution
  • Fluoride
    Fluoride ions can form stable insoluble compounds (with beryllium, magnesium, calcium, aluminum, titanium, tin) or soluble fluoro-complexes, preferably in a weakly acidic solution
  • 2,3-Dimercapto-1-propanol
    Some cations (mercury, zinc, cadmium, arsenic, antimony, tin, lead, bismuth) form uncolored complexes with 2,3-Dimercapto-1-propanol in ammonical solution that are more stable than their corresponding EDTA-complexes
  • Thioglycolic acid
    Forms stable, uncolored complexes with lead, bismuth, cadmium, zinc, mercury in an alkaline solution. Copper gives a weakly yellow complex. Iron forms a red complex; nickel and cobalt give intensely colored complexes with low stability.
  • Thiourea
    Thiourea is preferred for masking copper in a neutral solution.

Ordering information for masking agents

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Indicators for complexometric titrations

Visual indicators are preferred for complexometric titrations. They are mostly dyes that form intensely colored metal complexes giving different colors, predominantly triphenylmethane and azo dyes.

Indicator buffer tablets

Visual indicators are used in complexometric titrations. Eriochrome Black T is the most widely used indicator; but its aqueous solution is not stable. Therefore IDRANAL Indicator buffer tablets are used. These tablets contain Eriochrome Black T as metal indicator plus a buffer substance.

They are suitable for titrating calcium, magnesium, zinc, lead, and also for determination of water hardness.

Application: A 100 mL water sample is boiled after adding 2 drops of concentrated hydrochloric acid. It is then cooled to 40 °C, and mixed with one IDRANAL Indicator buffer tablet and 1 mL of 25% ammonia solution. Titration is carried out with IDRANAL solution for water hardness determination. Color change appears from red over gray to green.

Ordering information for metal titration indicators

View complete product list for complexometric titration

View more titration topics:
Volumetric Titration
Non-aqueous Titration

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