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Alcohol Determination by Ebulliometry - Common Errors and Adjustments -

Sugar Concentration, Interference

Temperature Measurements

Condenser Water Temperature

Boiling Point of Water

Contamination

Volumes

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The presence of sugar.

• In the same way that the presence of alcohol affects the boiling point temperature of a solution, the presence of significant quantities of sugar (>0.5^oBaume), also affects the boiling point of a solution, and has to be allowed for, using the equation below.
  
  True Alcohol (%v/v) = Apparent Alcohol (%v/v) * (1 - ^oBaume * 0.015)
  
  Apparent alcohol is the value as determine by the ebulliometer procedure.
  
  ^oBaume (^oBe^/) is the value determined by hydrometry (expressed @ 20^oC).
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  

• Make sure that the thermometer bulb does not protrude into the liquid itself, but is situated above the liquid and is measuring the vapour temperature only.
  
  
• The temperature reading should be taken and recorded only when the temperature remains stable for at least 30 seconds.
  
  
• It is important that the same thermometer is used for determining both the boiling points of the wine and the distilled water.
  
  
• Both boiling points should be taken within the same session, ensuring that the both determinations are recorded at the same prevailing air/atmospheric pressure.
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  

• When analysing a number of wine samples, the condenser water can become quite hot, resulting in poor condenser operation. This can result in the loss of alcohol and low, false alcohol concentration measurements.
  
  
• The cooling water temperature should never exceed 40^oC.
  
  
• Therefore, when boiling the wine samples the condenser must always contain fresh, cool, cooling water for efficient refluxing, that is, to prevent the loss of alcohol or water through evaporation, and changing the alcohol concentration.
  
  
• This is not a concern when boiling the water as the evaporation will not result in concentration changes.
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  

• The boiling point of water is 100^oC at standard atmospheric pressure (101.325 KPa or 760mm of mercury (Hg)).
  
  
• However, atmospheric pressure varies from the standard pressure, and a change of 4mm Hg pressure will result in a final difference of 0.5 % v/v of alcohol.
  
  
• Changes in atmospheric/barometric pressure of this order can occur within a one hour period.
  
  Hence it is important to take both temperature measurements (T₁ & T₂) within a one (1) hour period.
  
  
• That is, if performing a number of alcohol analysis the boiling point of water only needs to be ascertained or confirmed once every hour.
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  

• Suspended solids will also alter the boiling point of a solution (see MW determination by ebulliometry).
  
  
• Hence only use wine samples that have been settled clear of suspended solids.
  
  
• The ebulliometer can be cleaned with a solution of caustic soda (NaOH) when necessary.
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  

• The volume of wine (50ml) and the distilled water (20ml), although approximate, are important in obtaining an accurate boiling point temperature.