Chloride titrations with potentiometric end-point detection

Chloride titrations with potentiometric end-point detection

Summary
Together with acid/base titrations, the titrimetric chloride determination is one of the most frequently used titrimetric methods of analysis. It is employed more or less frequently in practically every laboratory. The purpose of this Bulletin is to demonstrate the possibilities for determining chloride in a wide range of concentrations using automatic titrators. Silver nitrate is normally used as titrant. (For ecological reasons one should refrain from using mercury nitrate). The titrant concentration depends on the chloride content of the sample to be analysed. It is especially important to choose the correct electrode for samples with low chloride contents.

Apparatus and accessories
• Titrino or Titrando with Dosino or Dosimate
• Magnetic Swing-out Stirrer
• Exchange unit
• 6.0726.100 Reference electrode (outer electrolyte KNO3, for use with separate Ag electrodes) with 6.2106.020 electrode cable
• Measuring electrodes; a wide range of suitable electrodes is available at Metrohm. Here a selection of these:
- 6.0430.100 Ag-Titrode **
-
- 6.0450.100 comb. Ag-ring electrode **
-
- 6.0331.010 Ag-rod electrode **
-
- 6.0350.100 Ag-ring electrode **
-
- 6.0502.120 AgCl-ISE
-
- 6.0502.180 Ag/S-ISE
-
** if desired bare or with AgCl resp. Ag2S coating.

Reagents
• Titrant: c(AgNO3) = 0.001 ... 0.1 mol/L
• Standard: c(KCl) = 0.1000 mol/L, e.g. Metrohm No. 6.2301.060, or dilutions from it Acid: c(HNO3) = 2 mol/L or c(H2SO4) = 1 mol/L

and for special applications:
• Acetone, p.a. as free from chloride as possible
• Acetic acid, w = 80 %, containing 1.9 g/L amidosulphamic acid
• Protective colloid: polyvinyl alcohol, e.g. Merck No. 114266 as 0.2 % aqueous solution (dissolve in hot dist. water)

General remarks
Silver nitrate with many anions causes more or less soluble precipitations. Thus, in mixtures of several anions, also several end-points can appear in the titration curve. Here, the anion causing the most insoluble precipitation is recorded first. Consequently, in a mixture of chloride, bromide and iodide, chloride would be titrated last. For the quantitative separation of mixtures, the solubility products of the Ag compounds must be as far apart as possible. In addition, no great differences in concentration should exist. In some cases, adding barium acetate and/or acetone can facilitate separation. Generally titration should take place in an acidic solution (acidify with HNO3 or H2SO4). Before the chloride determination, cyanide, sulphide and thiosulphate should be removed by means of oxidation, e.g. with H2O2. With samples containing peroxides (e.g. after digestions), these must first be destroyed before titration. For the determination of high chloride concentrations (in brines, salts), the sample is weighed, diluted with dist. water to a certain volume and a portion of this (aliquot) is titrated. To prevent an accumulation of the AgCl precipitation, protective colloid can be added to the sample solution. Polyvinyl alcohol (5 mL) 0.2 % per 100 mL sample solution prevents inclusions and keeps the electrode surface practically free from precipitation.
We prefer use of the Ag-Titrode. No electrolytes have to be replenished, nor will the diaphragms become blocked. For the titration of samples with small contents of chloride or of chlorides in aggressive solutions, we recommend use of an electrode coated with Ag2S.

General titration procedure
Place sample or an aliquot of this in a glass beaker and add 0.5 mL HNO3 or H2SO4. If content of chloride is high, dilute with dist. water 50 ... 100 mL. Immerse the electrode(s) and titrate on the mV measuring range with the selected AgNO3 concentration.

Calculation

1 mL c(AgNO3) = 0.1 mol/L = 3.5453 mg Cl- or 5.8443 mg NaCl or 7.4555 mg KCl

A few chosen examples

1. Chloride in drinking water
To 100 mL drinking water add 0.5 mL c(HNO3) = 2 mol/L and titrate with c(AgNO3)= 0.01 mol/L. Ag-Titrode with Ag2S coating.

mg/L chloride = EP1 x 0.3545 x 1000 / 100 = EP1 x 3.545

2. Chloride in dialysis and/or infusion solutions
To 5.0 mL sample add 2 mL c(HNO3) = 2 mol/L and 30 ... 50 mL dist. H2O. Titrate with c(AgNO3) = 0.1 mol/L using the Ag-Titrode.

mmol/L chloride = EP1 x 0.1 x 1000 / 5 = EP1 x 20

3. Chloride in Cr(VI)-bath
In a glass beaker pipette 5.0 mL bath sample and 20 mL each dist. H2O and ethanol. Add 0.5 mL conc. H2SO4, heat and boil for 5 min to convert Cr(VI) entirely to Cr(III). After cooling, titrate with c(AgNO3) = 0.01 mol/L. Use Ag-Titrode with Ag2S coating.

mg/L chloride = EP1 x 0.355 x 1000 / 5 = EP1 x 71

4. Chloride in acidic copper bath
In a glass beaker pipette 20.0 mL bath sample, 2 mL c(HNO3) = 2 mol/L and 50 mL dist. H2O. Using the Ag-Titrode, titrate with c(AgNO3) = 0.01 mol/L.

mg/L chloride = EP1 x 0.355 x 1000 / 20 = EP1 x 17.75

5. Chloride in nickel (sulphate/sulphamate) bath
Depending on the expected chloride content, pipette 1.0 ... 5.0 mL sample into a glass beaker. Add approx. 50 mL dist. H2O and 2 mL c(HNO3) = 2 mol/L and titrate using the Ag-Titrode with c(AgNO3) = 0.1 mol/L.

g/L chloride = EP1 x 3.5453 / C00

C00 = mL sample size

6. Chloride traces in cement and clinker
Weigh exactly 2.500 g sample into a glass beaker and mix to a slurry with 30 mL dist. H2O. Stirring, carefully add 6 mL conc. HNO3 and place beaker for 1 - 2 min in an ultrasonic bath. Filter through a paper filter (free from chloride) into a 100 mL graduated flask, rinse filter well with dist. H2O, fill up to mark and mix.Pipette 50.0 mL (corresponding to 1.25 g original sample) into a glass beaker, add 20 mL glacial acetic acid and approx. 0.5 g sodium acetate and titrate with c(AgNO3) = 0.01 mol/L using the MET-mode of the titrator.

% chloride = EP1 x 0.355 x 0.1 / 1.25 = EP1 x 0.0284
C00 = 1.25 g

7. Salt content in meats (Dried meat, sausage, ham, smoked fish etc.)
Cut the sample in tiny pieces with a knife. Weigh exactly 10 g of sample into a mixer together with 190 g dist. water and let run for 1 ... 2 min until the mixture is homogeneous.
Weigh 50 g of this homogeneous mixture into a glass beaker, then add 50 mL dist. H2O and 2 mL c(HNO3) = 2 mol/L. Titrate with c(AgNO3) = 0.1 mol/L using the Ag-Titrode.

% NaCl = EP1 x 5.844 x 0.1 / C00

8. Absorbable halogenated hydrocarbons (AOX)
The analysis of absorbable halogenated hydrocarbons traces represents a special case. After combustion, the gases are caught in 80 % acetic acid with 1.9 g/L sulphamic acid and titrated with c(AgNO3) = 2 mmol/L in 80 % acetic acid.

Electrodes:
6.0331.010 with Ag2S coating
6.0726.100 Ag/AgCl reference electrode. Outer electrolyte is
c(NaOOCCH3) = 2 mol/L in 80 % acetic acid

Literature
Metrohm Application Bulletin No. 130 / 2e