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Accurate titer determinations of titrant solutions are essential for reliable and precise titration results. Titrations sometimes involve diluted solutions (e.g., 0.01 and 0.002 mol/L) which present unique challenges. The low concentration of the titrant can lead to sluggish electrode responses, making it difficult to obtain stable readings during the titration. This situation requires slow titration parameters to avoid overtitration and to smooth the titration curve, with the aim of obtaining the «S» shaped plot expected for a good potentiometric titration curve.

Additionally, the small amounts of primary standard required for these dilutions can introduce weighing errors and contribute to increased variability in the final titer values. To prevent this, the most accurate alternative is to prepare solutions of the primary standard with the same concentration as the titrant and use a macro pipette to carry out the aliquoting.

This Application Note describes the procedure for performing a titer determination of sodium thiosulfate (0.01 and 0.002 mol/L) using the OMNIS Titrator and a Pt Titrode.

The following solutions are required for this study. These should be prepared with analytical grade reagents.

  • 0.01 and 0.002 mol/L Na2S2O3 – titrant
  • 0.01 and 0.002 mol/L KIO3 – standard
  • 1% KI
  • 0.1 mol/L H2SO4

Na2S2O3 – 0.01 mol/L

Pipet 1 mL of 0.01 mol/L KIO3 standard solution, add 10 mL of 1% KI, then add 10 mL of 1 mol/L H2SO4 and deionized water until the bulb and metal ring of the Pt Titrode are covered. Titrate to the first endpoint at approximately 6 mL.

 

Na2S2O3 – 0.002 mol/L

Pipet 1 mL of 0.002 mol/L KIO3 standard solution, then add 2 mL of 1% KI, 2 mL of 0.1 mol/L H2SO4, and add deionized water until the bulb and metal ring of the Pt Titrode are covered. Titrate to the first endpoint at approximately 6 mL.

Results of the titer determination of Na2S2O3 – 0.002 mol/L (Table 1) and 0.01 mol/L (Table 2) – were calculated according to the following equation where cstandard is the concentration of KIO3 standard solution, Vstandard is the volume of KIO3 standard solution, VEP is the volume of thiosulfate titrant, and 6 is the stoichiometric factor. An example titration curve for the titer determination is shown in Figure 1.

Figure 1. Exemplary titration curve of the titer determination of sodium thiosulfate with the OMNIS Titrator and a Pt Titrode.
Table 1. Results of the 2 mmol/L sodium thiosulfate titer determination.
No. (n = 5) Mean value in mmol/L s(abs) in mmol/L s(rel) in %
1 2.008 0.013 0.6
Table 2. Results of the 10 mmol/L sodium thiosulfate titer determination.
No. (n = 5) Mean value in mmol/L s(abs) in mmol/L s(rel) in %
1 10.057 0.041 0.4

The drift is set to 30 mV/min; the minimum volume increment is 20 µL, and maximum increment is 150 µL.

 

The OMNIS Titrator equipped with a Pt Titrode reliably determines titer concentration in diluted titrants through sensitive and flexible analyses combined with high-end software.

Only fine adjustment of the titration parameters and the Pt Titrode electrode are necessary when using the OMNIS Titrator. The electrode is sensitive enough to respond adequately to potential differences during titration, resulting in an ideal titration curve.

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