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§ 16

Concentration from a titration

How strong is the sample when I've titrated it?

Answer: In a titration, you add a solution of known strength (e.g. 0.1 M NaOH) from a burette, until the indicator or pH meter shows a change. From the amount used and the sample's volume, the calculator finds the sample's concentration in mol/L and, with the substance's molar mass, in g/L and as a percentage. The molar ratio is read from the reaction equation: lactic acid, acetic acid and hydrochloric acid react 1:1 with NaOH, sulphuric acid 1:2.

Formula

n(titrant) = c(titrant) × V(titrant)

n(prøvestof) = n(titrant) × molforholdet

c(prøve) = n(prøvestof) ÷ V(prøve)

g/L = mol/L × molmasse, og % (w/v) = g/L ÷ 10

The standard values are lactic acid (90.08 g/mol) titrated with 0.1 M NaOH.

Symbols and units in the formula
SymbolMeaningUnit
c_titrantTitrant concentrationmol/L
v_titrantConsumption of titrant (read from burette)mL
v_proeveSample volumemL
molforholdMol test substance per mol titrant
molmasseTest substance's molar massg/mol
=Concentration in samplemol/L
=Concentration in grams per litreg/L
=Concentration in percent (weight/volume)%

Validity and source

Source
Syre-base-titrering: c(prøve) = c(titrant) · V(titrant) · molforhold ÷ V(prøve)

Does not apply when

  • Dairies often specify acidity in Thörner or Soxhlet-Henkel degrees. They are defined from consumption of NaOH with a specific strength per 100 mL sample, so follow the company's analysis specification when the figure goes into a table.

Worked example

The calculator's own default values inserted. The figures are calculated with exactly the formula the tool uses.

Substituted

  • Titrant concentration0.10 mol/L
  • Consumption of titrant (read from burette)10 mL
  • Sample volume10 mL
  • Mol test substance per mol titrant1
  • Test substance's molar mass90.08 g/mol

Giver

  • Concentration in sample0.1000 mol/L
  • Concentration in grams per litre9.01 g/L
  • Concentration in percent (weight/volume)0.901 %

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Sådan bruger du den

Procesoperatøren tager prøver og titrerer dem for at kontrollere syregrad eller koncentration, fx mælkesyre i et syrnet produkt, lud i CIP-væsken eller syre i et bad. Beregneren omregner buretteforbruget til koncentration.

Step by step

  1. Afmål prøven præcist med en pipette.
  2. Titrér med en opløsning med kendt styrke, til indikatoren slår om eller pH-meteret viser omslag.
  3. Aflæs forbruget på buretten.
  4. Vælg molforholdet efter reaktionsskemaet og indtast stoffets molmasse.
  5. Aflæs koncentrationen og sammenlign med specifikationen.

Typical errors

  • At glemme molforholdet. Svovlsyre bruger dobbelt så meget NaOH pr. mol som saltsyre.
  • At aflæse buretten skævt eller glemme startaflæsningen.
  • At bruge en titrant, hvis styrke ikke er kontrolleret. NaOH optager CO₂ fra luften og bliver svagere.
  • At blande mL og L. Forholdet mellem de to volumener er det, der tæller.

Examples from everyday life

Mælkesyre i yoghurt

Du titrerer 10 mL yoghurt med 0,1 M NaOH og forbruger 10,0 mL, til phenolphthalein slår om. Mælkesyre (90,08 g/mol) reagerer 1:1 med NaOH.

Titrant concentration
0.1 mol/L
Consumption of titrant (read from burette)
10 mL
Sample volume
10 mL
Mol test substance per mol titrant
1:1 (e.g. lactic acid, acetic acid or hydrochloric acid against NaOH)
Test substance's molar mass
90.08 g/mol

It means: Prøven har 0.1 mol/L mælkesyre, svarende til 9.01 g/L eller 0.901 %. Det er normalt for en færdigsyrnet yoghurt.

Svovlsyre i et bejdsebad

Du tager 10 mL fra et fortyndet syrebad og titrerer med 0,1 M NaOH. Forbruget er 17,2 mL. Svovlsyre (98,08 g/mol) har to syregrupper, så forholdet er 1:2.

Titrant concentration
0.1 mol/L
Consumption of titrant (read from burette)
17.2 mL
Sample volume
10 mL
Mol test substance per mol titrant
1:2 (e.g. sulphuric acid against NaOH)
Test substance's molar mass
98.08 g/mol

It means: Badet har 0.086 mol/L svovlsyre, altså 8.43 g/L eller 0.843 %.

Rule of thumb

Med 0,1 M titrant og samme volumen titrant som prøve er prøven 0,1 M (ved 1:1).

How to check the answer

c(prøve) × V(prøve) skal give c(titrant) × V(titrant) × molforholdet.