Potentiometric TitrationKarl FischerAutomationpH/Ion Meter

Potentiometric TitrationKarl FischerAutomationpH/Ion Meter -

Titer determination in Potentiometry

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:

Parameters DET: Titration parameters meas.pt.density 4 min.incr. 10.0 µL signal drift 50 mV/min start V: rel. faktor 50 Stop conditions stop V: rel. stop V 120

Parameters MET: Titration parameters V step 0.10 ml dos.rate max ml/min signal drift 50 mV/min start V: rel. faktor 50 Stop conditions stop V: rel. stop V 120

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

Titrimetric determination of sulfate

Photometry

Article	Name
AB-140	Titrimetric determination of sulfate
This Bulletin describes three potentiometric and one photometric titration method for the determination of sulfate. Which indication method is the most suitable depends above all on the sample matrix and is illustrated with examples. Method 1: Precipitation as barium sulfate and back-titration of the Ba2+ excess with EGTA. The ion-selective calcium electrode is used as indicator electrode. Method 2: As in method 1, but with the electrode combination tungsten/platinum. Method 3: Precipitation titration in semi-aqueous solution with lead perchlorate using the ion-selective lead electrode as indicator electrode. Method 4: Photometric titration with barium perchlorate, thorin indicator and the 662 Photometer or 525 nm Spectrode. Particularly suitable for micro determinations!
AB-094	Potentiometric and photometric analysis of honey
Simple methods are described for the analysis of honey that permit any damage or adulteration to be detected. The pH and total acids are determined together with the lactone and formol numbers. The determination of the hydroxymethylfurfurol content (HMF) is carried out photometrically. A separate method exists for the determination of water by the Karl Fischer method.
AB-093	Potentiometric analysis of cadmium plating baths
This Bulletin describes titrimetric methods for the determination of cadmium, free sodium hydroxide, sodium carbonate and total cyanide. The free cyanide can be calculated from the total cyanide and the Cd content.
AB-063	Determination of silicon, calcium, magnesium, iron and aluminum in cements by photometric titration of the solubilized product
The insoluble silicon dioxide remaining after dissolution of cement is determined gravimetrically. The calcium, magnesium, iron and aluminium in the filtrate are determined by photometric EDTA (0.1 mol/L) titration using a 662 Photometer. The following instructions conform to the analytical methods of 11 November 1981 recommended by the Association of Austrian Cement Manufacturers.
AB-049	Colorimetric determination of copper
Application Bulletin no. 43 describes the polarographic determination of copper. For copper concentrations of 10 mg/L and below, however, colorimetric methods are also used, particularly in water analysis. The method using sodium diethyldithiocarbamate can be employed down to a minimum limit of 0.01 mg/L Cu, but suffers from the drawback that the determination can also be affected by other metal ions. The method using neocuproine has a minimum concentration limit of 0.1 mg/L Cu, but has the advantage of being unaffected by ions of other metals.
AB-033	Determination of the total, calcium and magnesium hardness of water samples by photometric titration
This Bulletin describes the determination of the total, calcium and magnesium hardness of water using the light-guide photometer and/or Spectrode.
AB-030	Photometric determination of chromium(VI)
With chromates and dichromates, diphenylcarbazide gives a red-violet coloration which can be measured with the 662 Photometer. The reaction is extremely sensitive, enabling concentrations of ρ(Cr) <> In electroplating effluents, chromium occurs as the Cr(VI) ion. It must first be reduced to Cr(III) and then precipitated as the hydroxide. With incomplete reduction, hexavalent chromium remains in solution in the effluent water. For this reason, periodic checks for chromium in the effluent are necessary.
AB-029	Determination of chlorine in water with the 662 Photometer
The well-known method for determining free chlorine in water with o-tolidine is described and the absorbance coefficients determined for the 662 Photometer are given.