Determination of copper number:

When cellulose is bleached i.e oxidised and hydrolysed by treatment with mineral acid solution , reducing groups are formed. These reducing groups are capable of reducing an alkaline solution of copper sulphate, when insoluble cuprous oxide is formed is proportional to the amount of the reducing groups present in the cellulose sample. This is expressed in terms of Copper Number, which is defined as a gram of cupric copper reducing to cuprous oxide by 100g of the cellulose sample under standard conditions of boiling in an alkaline medium.

To determine copper number following procedure is followed:

        Solution A: 100 g/litre hydrated copper sulphate

        Solution B: 50 g sodium bicarbonate + 150 g anhydrous sodium 

                          carbonate in 1 litre of distilled water

        Solution C: 100 g/litre ferric alum and

        Solution D: N/100 Ceric sulphate

Ø  Take cellulose sample whose copper number to be determined and conditioned it at 30^oC at 65% RH for 48 hours

Ø  Determine moisture content of the conditioned sample and note down the value

Ø  Take 0.25g of the dry sample in dry test tube in triplicate. For highly degraded sample, 0.125g of sample may be taken

Ø  Mix solution A and B in the ratio of 5:95 and boiled. Take 15 ml of this solution and add to all the tree test tube. Now each test tube will have cellulose sample and 15ml of mixture of solutions.

Ø  Mouth of these tubes closed using glass stopper and placed in the boiling water bath.  Along with these three test tubes, also placed three blank test tubes (only contain solution, No sample) in the boiling water bath. Boiling continue for three hours.

Ø  Cooled above test tubes and content of each test tube filter using Gooch crucible (Sinterd glass, AG3) and washed with distilled water (now the colour of solution become red).

Ø  The residue in the crucible also treated with first 10 ml and then 5 ml ferric alum solution (Solution C).  The residue also washed with successively with 10 ml and 5 ml 2N sulphuric acid. All these process are carried out on each test tube.

Ø  The solution so collected is titrated against N/100 ceric sulphate (Solution-D) using a few drop of ortho-ferrus phenathroline indicator (the initial red colour of the solution turns Green at the end point).

Ø  The blank alkaline copper solution gives a reading of 0.02 ml of N/100 ceric sulphate solution. This blank reading is subtracted from the actual reading in each case, where the cellulosic sample is treated.

Ø  The copper number is calculated from the amount of ceric sulphate solution consumed (after deducting blank reading) by ferrous sulphate formed by the reduction of ferric sulphate sulphate by cuprous oxide deposited on the cellulosic sample during the three hours of boiling.

Copper Number= (63.5 x V x N x 100)/ W x 1000

=(6.35 x V x N)/W

Where V is the titration reading (ceric sulphate solution) after deducting the blank reading, N is the normality of ceric sulphate solution and W is the weight of the bone dry cellulosic sample