Determination Moisture by Dean Stark Method | Dean-Stark apparatus

Definition:- This method determines moisture by distillation with an immiscible solvent. This method is applicable to all fatty alkyl sulfate products except those containing water-miscible volatile substances. Determination Moisture by Dean Stark Method.

Dean-Stark apparatus

The apparatus shall consist of a glass flask heated by suitable means and provided with a reflux condenser discharging into a trap and connected to the flask. The connections between to trap and the condenser and flask shall be interchangeable ground joints. The trap serves to collect and measure the condensed water and to return the solvent to the flask. A suitable assembly of the apparatus is illustrated in the Image.

A) Flask:- A 500 ml flask of either the short-neck, round-bottom type, or the Erlenmeyer type shall be used.

B) Heating Mental (heat source):- The source of heat may be either an oil bath or an electric heater provided with a sliding rheostat or other means of heat control.

Determination Moisture by Dean Stark Method

C) Condenser:- A water-cooled glass reflux condenser, having a jacket approximately 400 ml in length with an inner tube 9.5 - 12.7 mm in outside diameter, and not less than 6.35 mm inside diameter, shall be used. The end of the condenser to be inserted in the trap may be ground off at an angle of 30 deg. from the vertical axis of the condenser.

When inserted into the trap, the tip of the condenser shall be about 7.0 mm. above the distillation conditions have been established. The image shows a conventional sealed-in type of condenser, but any other condenser fulfilling the detailed requirements of this paragraph may be used 

D) Trap:- For the greatest accuracy, several trap sizes are allowable depending upon the percentage of moisture expected. Traps made of well-annealed glass, constructed essentially as shown image and graduated to contain one of the following specified volumes at 20 °C shall be used.

Note:- The condenser and trap should be thoroughly cleaned before use.

Reagents for Dean Stark Method

Xylene (or Toluene) - Saturate xylene with water by shaking with a small quantity of water and distilled. Use the distillate for the determination.

The procedure of Dean-Stark | Dean and Stark toluene distillation method

Transfer to the 100 ml flask, equipped with the size of trap specified in the image, an amount of sample according to the percentage of moisture expected as follows.

Procedure of Dean Stark Method

Add immediately about 100 ml of xylene or toluene. Place a small thin sheet of long-fiber, chemical-resistant glass wool (Pyrex and Kimax glass have been found satisfactory for this purpose) on the surface of the toluene or xylene. The glass wool should be thoroughly derided in the oven and held in the desiccator before use.

Connect the flask and receiver to the condenser and pour sufficient xylene or toluene down the condenser tube causing a slight overflow through the side tube. Wrap the flask and tube leading to the receiver with asbestos cloth, so that re-fluxing will be under better control.

Heat the oil bath with a gas burner or other source of heat, or apply heat directly to the flask with an electric heater and distill slowly. The rate at the start should be approximately 100 drops per min. When the greater part of the water has distilled over, increase the distillation rate to 200 drops per min. 

Until no more water is collected. Purge the reflux condenser during the distillation with 5-ml portions of xylene to wash down any moisture adhering to the walls of the condenser. The water in the receiver may be made to separate from the xylene by using a spiral copper or nichrome wire. Move the wire up and down in the condenser occasionally, thus causing the water to settle at the bottom of the receiver. Reflux for at least 2 hr., and shut off the heat at the end of this period.

Wash down the condenser with 1 ml of absolute ethyl ethanol. Adjust the temperature of the distillate to 20 °C and read the volume of water.

Calculations of Moisture, percentage by dean stark

Calculate the percentage of moisture as follows

Moisture, percentage = V * 0998 * 100 


V = Volume of water in milliliter at 20 °C

W = Weight of the sample in grams.

Precision for Dean-Stark apparatus

Two single determinations performed in one laboratory shall not differ by more than 0.28 at 10% level or 0.38 at 50% level.

Single determinations performed in two different laboratories shall not differ by more than 0.52 at 10% level or 1.88 at 50% level.

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