alcohol in spirit

8. Spectrometric determination of the alcohol in a spirit

BACKGROUND

The objective of the experiment is to determine the content of alcohol in a spirit (vol. %). The lowest acceptable content of alcohol for a spirit from fruits is 37.5 vol. %.

EQUIPMENT

Automatic pipettes
Blisters
Spektra^TM Spectrometer
Conical-shoulder bottle

REAGENTS

0.25 M K₂Cr₂O₇
2.25 M H₂SO₄
96 % ethanol

PREPARATION OF REAGENTS

0.25 M K₂Cr₂O₇: 3.675 g of K₂Cr₂O₇ is weighed and quantitatively transferred into a 50-ml volumetric flask, dissolved with deionised water and filled to the mark with deionised water.
2.25 M H₂SO₄: 6 ml H₂SO₄ (ρ = 1,816 g/ml) is diluted with deionised water in a 50-ml volumetric flask.

HAZARDS

	Potassium dichromate is very toxic and dangerous to the environment. It is harmful in contact with skin and may cause sensitization by skin contact. It is toxic if swallowed. Wear protective gloves and goggles and avoid releasing it into the environment. R: 49-46-21-25-26-37/38-41-43-50/53 S: 53-45-60-61

	Sulphuric acid is corrosive and causes severe burns. Wear suitable protective clothing, gloves and eye protection. R: 35 S: 26-30-36/37/39-45
	Ethanol is highly flammable. Inhalation may cause coughing, headache, fatigue and drowsiness. It causes dry skin and redness of the eyes. Wear protective gloves and goggles. R: 11 S: (2-)7-16

PROCEDURE

Solutions of potassium dichromate (0.25 mol/L), sulphuric acid (2.25 mol/L), ethanol (96 %), as well as deionized water and the sample are needed for the experiment.

The procedure for the blank and for the sample is summarised in the table below.

The reagent and acid are pipetted into the hollow of a blister. This is followed by the addition of deionised water and ethanol or sample. Immediately after preparation the transmittance is measured against the blank using the green LED.

Table 1: Procedure of preparation

	VK₂Cr₂O₇ µL	VH₂SO₄ µL	VH₂O µL	V_ethanol µL	V_spirit µL	vol %	T %	T	A
Calibr.sol.1	180	110	100	0	0	0
Calibr.sol.2	180	110	80	20	0	19,2
Calibr.sol.3	180	110	60	40	0	38,4
Calibr.sol.4	180	110	40	60	0	57,6
Calibr.sol.5	180	110	20	80	0	76,8
Calibr.sol.6	180	110	0	100	0	96
vzorec	180	110	0	0	100

CALCULATION

The calibration line is a function of the absorbance versus the vol. % of alcohol. The vol. % of alcohol in the sample can then be deduced.

RESULTS OF THE EXPERIMENT

Accessible to registered users

Developed and prepared by: mag. Marjana Dvoršek, Tadeja Polajnar, Kristina Frlic, Biotechnical school Kranj