The method of Bertrand is a biochemical method which allows the proportioning of the Ose S reducers thanks to a back titration.

Principle

Glucose reduces partially the Fehling's solution in excès  the formed cuprous oxide (precipitated red) is proportioned by manganimetry. A table gives the correspondence between the copper mass and the mass of glucose. The reaction must proceed hot and during three minutes starting from boiling to respect the correspondence of the tables.

VE: Volume of the test

A quantity of glucose reacts with copper in excess to form a red precipitate brick. The copper excess is eliminated. The precipitate reacts with an iron excess to dissolve it. The iron excess is given by a solution of Permanganate of potassium.

Protocol

Material

• Oil-can
• Beaker S
• 2 erlenmeyers of 100  ml
• agitators
• Propipette
• Pipette of 5  ml
• pipette of 10  ml
• pipette of 20  ml
• grips
• filters of porosity 3
• Vacuum pump
• Filter-flask

solutions

• test to proportion
• Fehling's solution
• distilled water pulp
• permanganate of potassium with 0,02  mol/l
• Thiosulfate of iron to 0,02  mol/l
• Hydrochloric acid concentrated (in the event of hydrolysis of the test)

Stages

• in a erlen  :
• to take 5 ml of solution to proportion (according to what is proportioned, to pay attention to dilutions)
• 40  ml of Fehling's solution
• 10  ml of water distilled pulp
• to carry to soft boiling during 3 minutes exactly
• to cool under water as soon as possible
• to incline the erlen to gather the precipitate

• To regulate with weak mode to see the detail on Vacuum filtration

• transvasing the liquid on the filter while involving least possible of precipitate
• to wash the precipitate several times at water pulp until disappearance of the blue color.

• never not to leave precipitated in contact with the air (erlen and filters)

• to prepare the potassium permanganate solution in the oil-can

• to empty and rinse the filter-flask
• in the erlen, to pour 20m  L of solution ferric and to agitate (dissolution of the precipitate)

• to pour the contents on the filter to dissolve the remainder of the precipitate

• to wash the erlen with 5  ml of water pulp, to filter

• to immediately proportion the ferric solution with permanganate, the reaction finishes when the solution passes from the green to the pink

Equations of reactions

• oxidation of glucose

• $2\; Cu^\; \{2+\}\; +\; 2\; OH^\; \{-\}\; +\; 2e^\; \{-\}\; \backslash \; longrightarrow\; Cu\_2O\; +\; H\_2O\; \backslash ,$
• glucose $\backslash \; longrightarrow$ produced oxidation + N e^-

• oxidation of Cu₂O by the ferric acid

• $Cu\_2O\; +\; 2:00\; ^+\; \backslash \; longrightarrow\; 2\; Cu^\; \{2+\}\; +\; H\_2O\; +\; 2e^-\; \backslash ,$
• $(Fe^\; \{2+\}\; \backslash \; longrightarrow\; Fe^\; \{3+\}\; +\; e^-)\; \backslash ,$ $\backslash \; times\; 2\; \backslash ,$
• $Cu\_2O\; +\; 2Fe^\; \{3+\}\; +\; 2:00\; ^+\; \backslash \; longrightarrow\; 2Cu^\; \{2+\}\; +\; H\_2O\; +\; 2Fe^\; \{2+\}\; \backslash ,$
• Proportioning of Fe²⁺ by a permanganate solution titrated
• $\backslash \; longrightarrow\; Fe^\; \{3+\}\; +\; e^-\; \backslash ,$ $\backslash \; times\; 5\backslash ,$
• $MnO\_4^\; \{-\}\; +\; 8:00\; ^\; \{+\}\; +\; 5e^-\; \backslash \; longrightarrow\; Mn^\; \{2+\}\; +\; 4:00\; \_2O\; \backslash ,$
• $5\; Fe^\; \{2+\}\; +\; MnO\_4^-\; +\; 8:00\; ^+\; \backslash \; longrightarrow\; 5\; Fe^\; \{3+\}\; +\; Mn^\; \{2+\}\; +\; 4:00\; \_20\; \backslash ,$

Operating conditions

• the precipitate is never in contact with the air to avoid its oxidation.

• To use distilled water pulp to avoid oxidation by O₂ dissolves in water.
• To abundantly wash the precipitate to eliminate tartrate, if not the turn with permanganate will not be visible.

Relation for calculation

1  mole of MnO4^- react with 5  moles of Fe²⁺

$5\; N\; MnO4^\; \{-\}\; =\; N\; Fe^\; \{2+\}$

2  moles of Fe²⁺ come from 1  mole of Cu₂O

$n\; Fe^\; \{2+\}\; =\; 2\; N\; Cu\_\; \{2O\}$

1  mole of Cu₂O come from 2  moles of Cu²⁺

$2\; N\; Cu\_O\; =\; N\; Cu^$

thus $5\; N\; MnO4^\; \{-\}\; =\; N\; Cu^\; \{2+\}\; \backslash ,$

$m\; Cu\; =\; 5\; \backslash \; times\; V\_\; \{MnO4^-\}\; \backslash \; times\; C\_\; \{MnO4^-\}\; \backslash \; times\; M\; (Cu)\; \backslash ,$

Once the copper mass known, we should use the tables of Bertrand, which give the relation between the concentration of glucose and that of formed copper precipitate.

Precise details

It is possible to proportion the Saccharose by this method. It is then necessary to carry out hydrolysis in acid medium during 20 minutes has 70°C. It will be necessary to use the table of reversed sugars. proportioning is then made in neutral medium . To neutralize the Hydrolysat with Soda and Phenolphthalein. The proportioning of other reducing sugars is possible on the condition of knowing the tables to be able to interpret the results.

If the supernatant after boiling yellow or is coloured, the solution should be diluted.

If you proportion glucose in a fruit juice, one needs déféquer before proportioning.

For proportionings of glucose with weak concentration rather use the method of the Dosage of glucose by the DNS.

Conversion chart glucose

Table of correspondence of reversed sugars

Use of the Tables of Bertrand

There is no proportionality between the formed copper masses and the Glucose (or the Sucres). One cannot thus use the product in cross. Moreover, one seldom finds a copper mass which is written in the table.

In the following example, one finds a copper mass of 22  Mg

$\backslash \; frac\; \{AB\}\; \{AC\}\; =\; \backslash \; frac\; \{OF\}\; \{DF\}$

$\backslash \; frac\; \{x-10\}\; \{11-10\}\; =\; \backslash \; frac\; \{22-20,6\}\; \{22,6-20,6\}$

$\backslash \; frac\; \{x-10\}\; \{1\}\; =\; 0,7$

$x-10\; =\; 0,7$

$x=10+0,7$

$x=10,7\; \backslash ,$ Mg of glucose

Attention: not to forget to take account of dilutions to find the concentration in glucose of the solution initiale !

to see too

• Glucid
• Glucose
• Vacuum filtration