Based on Metrohm® Application Bulletin 206/3 e
Introduction
This Bulletin provides an overview of the potentiometric titer determination in common volumetric solutions. Many publications only describe methods with color indicators. However, the titration conditions chosen for the titer determination should resemble those used for the actual analysis as closely as possible.
The tables below contain suitable titrimetric standard substances and electrodes for selected titrants as well as additional information. Following this, a procedure for titer determinations is described.
Instruments and accessories
- Various instruments or combinations of instruments can be used:
- pH meter and Dosimat for manual titrations
- Titrando®, Titrino® or Titroprocessor with Dosimats or Dosinos® for automatic recording and evaluation of the titration curves and calculation of the results
- Titrators as listed above, combined with a sample changer
The instruments for automatic titrations offer the possibility of transmitting the data to a PC using Tiamo™ titration software.
Titrimetric standard substances
Titrimetric standard substances have the following characteristics/features. Their content remains virtually unchanged; they have a defined, high degree of purity; they can be dried and they can be directly traced back to standard reference materials, e.g., from NIST (National Institute of Standards and Technology, USA).
The weight of titrimetric standard substance depends on the concentration of the titrant and the buret volume used.
For accuracy reasons, the sample weight must not be too small. A sample weight >100 mg normally yields good analytical results. However, the minimum weight varies considerably, depending on the substance, the balance used and the required accuracy.
In order to increase the measuring accuracy, it may be a good idea to prepare a stock solution instead of weighing the titrimetric standard substance directly into the titration vessel.
Literature
- G. Jander, K. F. Jahr Massanalyse, Theorie und Praxis der Titrationen mit chemischen und physikalischen Indikationen Walter de Gruyter & Co., 2003 ISBN 3-11-017098-1
- Metrohm Monograph No. 8.015.5003 Electrodes in potentiometry Metrohm Ltd., 2001
- Metrohm Monograph No. 8.016.5003 Practical aspects of modern titration Metrohm Ltd., 2001
- Various Metrohm Application Bulletins
- Laborpraxis Birkhäuser, 1996 ISBN 3-7643-2528-3
- Merck «Spektrum», Sonderheft Titration und Elektrochemie
- Hydranal-Praktikum, Wasserreagenzien nach Eugen Scholz für die Karl-Fischer-Titration Riedel-de Haën AG, 1996Electrodes and titrimetric standard substances for determining the titer of different titrants (subtitle)
Electrodes and titrimetric standard substances for determining the titer of different titrants
Table 1: Acid-base titrations (acidimetry, alkalimetry) and complexometric/chelatometric titrations
| Titrant | Titrimetric standard | Electrode | Remarks |
| Aqueous acids HCl, H2SO4 | Tris(hydroxymethyl)-aminomethane (CH2OH)3CNH2 (TRIS) (105°C) | 6.0259.100 Unitrode® | Solvent: water |
| Aqueous bases NaOH | Potassium hydrogen phthalate C8H5KO4 (105°C) | 6.0259.100 Unitrode | Solvent: water |
| Perchloric acid HClO4 in glacial acetic acid | Potassium hydrogen phthalate C8H5KO4 or TRIS (105°C) | 6.0229.100 Solvotrode® | Solvent: glacial acetic acid Reference electrolyte: LiCl saturated in ethanol (6.2312.000) |
| Trifluoromethanesulfonic acid CF3SO3H in glacial acetic acid | Potassium hydrogen phthalate C8H5KO4 or TRIS (105°C) | 6.0229.100 Solvotrode | Solvent: glacial acetic acid Reference electrolyte: LiCl saturated in ethanol (6.2312.000) |
| Trifluoromethanesulfonic acid CF3SO3H in isopropanol | Potassium hydrogen phthalate C8H5KO4 or TRIS (105°C) | 6.0229.100 Solvotrode | Solvent: glacial acetic acid Reference electrolyte: LiCl saturated in ethanol (6.2312.000) |
| Tetrabutylammonium hydroxide in isopropanol | Benzoic acid C6H5COOH | 6.0229.100 Solvotrode | Solvent: isopropanol Reference electrolyte: c(TEABr) = 0.4 mol/L in ethylene glycol (6.2320.000) |
| Alcoholic KOH | Benzoic acid C6H5COOH | 6.0229.100 Solvotrode | Solvent: ethanol Reference electrolyte: c(TEABr) = 0.4 mol/L in ethylene glycol (6.2320.000) |
| Cyclohexylamine C6H11NH2 in methanol | Benzoic acid C6H5COOH | 6.0229.100 Solvotrode | Solvent: methanol Reference electrolyte: c(TEABr) = 0.4 mol/L in ethylene glycol (6.2320.000) |
| EDTA (Komplexon III, Titriplex III or Idranal III) | Calcium carbonate CaCO3 (105°C) | 6.0504.100 calcium ISE or 6.0502.140 copper ISE; 6.0726.107 reference electrode [filled with c(KCl) = 3 mol/L] | CaCO3 in water, dissolve in HCl and add buffer pH = 10.0 (NH3/NH4OH). For titrations with the Cu ISE, add 1 ml c(Cu-EDTA) = 0.05 mol/L to the sample solution (see also Metrohm AB No. 101). |
The indicated electrodes are listed here as suggestions. It is often also possible to use other electrodes or electrode.
Electrodes and titrimetric standard substances for determining the titer of different titrants
Table. 2: Precipitation titrations (argentometry) and redox titrations (cerimetry, iodometry, permanganometry, ferrometry, Karl Fischer titration)
| Titrant | Titrimetric standard | Electrode | Remarks |
| Silver nitrate AgNO3 | Sodium chloride NaCl (110°C) | 6.0450.100 combined Ag ring electrode (reference electrolyte: KNO3 saturated) or 6.0430.100 Ag Titrode® | Dissolve NaCl in 40 ml water, then add 2 ml c(HNO3) = 2 mol/L and possibly 2 ml 0.2% polyvinyl alcohol solution. [ Dissolve polyvinyl alcohol (Merck No. 114266) in warm water.] |
| Lanthanum nitrate La(NO3)3 | Sodium fluoride NaF (110°C) | 6.0502.150 fluoride ISE; 6.0726.107 reference electrode [filled with c(KCl) = 3 mol/L] | Dissolve NaF in 50 ml water, add 10 ml acetate buffer pH = 6.0 and titrate slowly (see also Metrohm AB No. 82). |
| Cerium(IV) in H2SO4 or HClO4 | Arsenic trioxide As2O3 (105°C) | 6.0451.100 combined Pt ring electrode or 6.0431.100 Pt Titrode | Dissolve As2O3 in 10 ml c(NaOH) = 1 mol/L, then add 6 ml c(H2SO4) = 1 mol/L and 2 g NaHCO3 (see also Metrohm AB No. 52). |
| Iodine solution KI3 | Arsenic trioxide As2O3 (105°C) | 6.0451.100 combined Pt ring electrode or 6.0431.100 Pt Titrode | Dissolve As2O3 in 10 ml c(NaOH) = 1 mol/L, then add 6 ml c(H2SO4) = 1 mol/L and 2 g NaHCO3. |
| Potassium permanganate KMnO4 | Sodium oxalate Na2C2O4 (105°C) | 6.0451.100 combined Pt ring electrode or 6.0452.100 combined Au ring electrode | Dissolve Na2C2O4 in 40 ml water, then add 5 ml concentrated H2SO4 and 1 g MnSO4. |
| Sodium thiosulfate Na2S2O3 | Potassium hydrogen diiodate KH(IO3)2 (100°C) | 6.0451.100 combined Pt ring electrode or 6.0431.100 Pt Titrode | Stock solution: Dissolve KH(IO3)2 in water and make up to 100 ml; use 10 ml of this solution, dilute with 40 ml water, then add 1 g KI and 4 ml c(HCl) = 1 mol/L. |
| Iron(II) solution (NH4)2Fe(SO4)2 | Potassium dichromate K2Cr2O7 (105°C) | 6.0452.100 combined Au ring electrode | Dissolve K2Cr2O7 in 40 ml water, then add 3 ml concentrated H2SO4. |
| Sodium nitrite NaNO2 | Sulfanilic acid | 6.0452.100 combined Au ring electrode | Dissolve sulfanilic acid in 50 ml water, add 30 ml w(HBr) = 20% and titrate immediately using the MET mode (0.10 ml, 25 s). |
| Karl Fischer reagent | Sodium tartrate dihydrate C4H4Na2O6 x 2 H2O or water standards in ampoules (Riedel-de Haën) | 6.0338.100 polarized double Pt electrode | Fill methanol or KF solvent into the titration vessel and condition. As soon as a steady drift is attained, add the standard and titrate with the KF reagent. The titer is specified in mg H2O / ml KF reagent (documentation available from Riedel-de Haën or Merck). |
The indicated electrodes are listed here as suggestions. It is often also possible to use other electrodes or electrode.
An example of a procedure for titer determination
Determination of the titer of c(HCl) = 0.1 mol/L.
The titration is carried out in the DET or MET mode. Tris(hydroxymethyl)-aminomethane is used as a titrimetric standard substance.
Reagents
- Tris(hydroxymethyl)-aminomethane (TRIS) (titrimetric standard substance)
- Distilled water (carbonate-free)
Procedure
The TRIS is dried in a flat bowl for 2 h at 105°C, then allowed to cool down in a desiccator and stored there.
Using an analytical balance, approx. 0.15 g of the dried TRIS is weighed exactly into a 100 ml beaker. After the addition of 40 ml carbonate-free water, the solution is stirred until the titrimetric standard substance has dissolved, then titration is performed using the following parameters:
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Calculation
Titer = SS * C01 * C02/C03/EP1
SS = weight of TRIS in g
C01 = 1,000
C02 = % assay of TRIS/100 (e.g., 0.999)
C03 = 121.14 (molar mass of TRIS in g/mol)
EP1 = titrant consumption in ml
Remarks
The titer determination is carried out five times.
Using the statistics function, the mean value as well as the absolute and relative standard deviation can be calculated. The mean value can be automatically stored. This makes it possible to directly use the current titer in other titration methods.
Common problems and their remedies
| Problem | Probable cause | Remedies |
| Early EP at start of titration | Minimum increment or Vstep too small | Increase minimum increment; use start volume |
| Multiple EP’s within a break | Minimum increment or Vstep too small | Increase minimum increment or Vstep |
| EP not recognized | EPC too large Window-shifting potentials Incorrect stop criteria | Decrease EPC setting Widen window limits Check & adjust |
| No ER on blank | Titrant volume too small | Increase solvent volume Decrease minimum increment Decrease MPDensity Use MET mode with small Vstep |
| Over-range | Electrode not connected Faulty cable Wrong input assigned Wrong electrode pH glass bulb empty | Check & connect Replace cable Change assignment Check & replace Shake to fill bulb |
| Spiky curves | Titration rate too fast | Decrease drift Increase waiting time |
| Non-aqueous medium | Rehydrate pH probe Program pause time Use 3-electrode set | |
| No pH or mV change (straight line graph) | Wrong electrode type Wrong titrant Delivery tip not in beaker Electrode not connected | Change electrode Change titrant Place tip in sample beaker Connect electrode |
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