6. Apparatus
6.1 Karl Fischer Volumetric Titrator, consisting of a titration cell, dual platinum electrode, magnetic stirrer, dispensing buret and control unit. Many manufacturers of general purpose laboratory titrators offer optional accessories that allow their instrument to perform KF titrations.
7. Reagents
7.1 Purity of Reagents - Use reagent grade chemicals in all tests. Unless otherwise indicated, all reagents shall conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society where such specifications are available. Other grades may be used, provided it is first ascertained that the reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determination.
7.2 Purity of Water - Unless otherwise indicated, references to water shall mean reagent water as defined by Type II and III of Specification D1193.
7.3 Karl Fischer Reagents - Traditionally, pyridine was the organic base used in KF reagents. Pyridine-free formulations, however, are available now and are preferred by most KF instrument manufacturers for use with their equipment. These reagents are less toxic, less odorous, and more stable than those containing pyridine. The use of pyridine-free reagents is recommended whenever possible.
7.3.1 Pyridine-Free Karl Fischer Titrant - Typically consists of a mixture of an organic base, sulfur dioxide and iodine dissolved in a solvent such as methanol or 2-methoxyethanol. Reagents with titers of 1.00, 2.00, and 5.00 mg H2O/mL can be commercially obtained.
7.3.2 Pyridine-Free Karl Fischer Solvent - Anhydrous methanol is the most frequently used solvent, however, other alcohols including glycols and glycol ethers are used. Some commercially available solvents also contain an organic base and sulfur dioxide.
7.3.3 Karl Fischer Reagent Containing Pyridine - The KF reagent may be either prepared in the laboratory or purchased. Two types of reagent are commonly used. Directions for preparing these and diluting if necessary, along with commercial sources of supply, are as follows: (Warning - Follow standard precautions for handling toxic gases in preparing the reagents (1) or (2) as described in 7.3.3.1 and 7.3.3.2. Carry out all operations in a hood. Wear rubber gloves and a face shield when handling pyridine and sulfur dioxide and when mixing chemicals. Special precautions must be observed when dispensing sulfur dioxide to prevent drawback of the solution into the gas cylinder, which might cause an explosion. This is best accomplished by placing a trap in the line between the gas cylinder and absorption vessel.)
7.3.3.1 Karl Fischer Reagent (Ethylene Glycol Monomethyl Ether Solution, 1 mL = 6 mg H2O) (2) - For each litre of solution, dissolve 133 more or less 1 g iodine in 425 more or less 5 mL of pyridine in a dry glass-stoppered bottle. Add 425 more or less 5 mL of ethylene glycol monomethyl ether. Cool to below 4°C in an ice bath. Bubble 102 to 105 g of gaseous sulfur dioxide (SO2) into the cooled mixture. Determine the amount of SO2 added by the change in weight of the SO2 cylinder or the increase in volume (about 70 mL) of the reagent mixture. Alternatively, add about 70 mL of freshly drawn liquid SO2 in small increments. Mix well and set aside for at least 12 h before using. (Warning - see 7.3.3.)
7.3.3.2 Karl Fischer Reagent (Methanol Solution, 1 mL = 6 mg H2O) - For each litre of solution, dissolve 133 more or less 1 g of iodine in 425 more or less 5 mL of pyridine in a dry, glass-stoppered bottle. Add 425 more or less 5 mL of methanol. Cool the mixture in an ice bath to below 4°C. Bubble 102 to 105 g of gaseous sulfur dioxide (SO2) into the cooled mixture. Determine the amount of SO2 added by the change in weight of the SO2 cylinder or the increase in volume (about 70 mL) of the reagent mixture. Alternatively, add about 70 mL of freshly drawn liquid SO2 in small increments. Mix well and set aside for at least 12 h before using. (Warning - see 7.3.3.)
7.3.3.3 Karl Fischer Reagent (Ethylene Glycol Monomethyl Solution, Stabilized, 1 mL = 6 mg H2O).
7.3.3.4 Karl Fischer Reagent, Dilute - Prepare more dilute solutions of the KF reagent by diluting with the proper solvent as follows:
These dilute solutions cannot be prepared by simple proportion, since water added with the diluent must be accounted for. The volumes to add, indicated above, are calculated assuming the diluent contains 0.05 % water.
7.4 Methanol, Standard (1 mL = 1 mg H2O) - This solution can be stored conveniently in a bottle with rubber cap and portions removed with a hypodermic syringe.
7.5 Sodium Tartrate Dihydrate - Grind certified material (water content 15.61 to 15.71 %) to a fine powder (preferably overnight in a sealed ball mill) and store the ground material in a stoppered bottle. If doubt exists as to its water content, dry a 2 to 3-g sample in an oven at 155 more or less 5°C to constant weight (min 4 h). (See Note 16.)
7.6 Solvents:
7.6.1 Acetic Acid, glacial.
7.6.2 Ethylene Glycol Monomethyl Ether, maximum 0.1 % water. (See Note 1.)
7.6.3 Methanol, max 0.15 % water, in accordance with Specification D1152. (See Note 1.)
7.6.4 Pyridine, maximum 0.1 % water. (See Note 1.)
7.7 Solvents, Mixed - In addition to the single solvents (7.6), the following mixed solvents are useful for dissolving various samples:
7.7.1 Methanol - Chloroform (1 + 3) - Mix 1 volume of methanol with 3 volumes of chloroform. Use for liquid petroleum products.
7.7.2 Methanol - Salicyclic Acid - Dissolve 150 g of salicyclic acid in 1 L of methanol. Use for amines.
7.7.3 Pyridine - Ethylene Glycol (1 + 4) - Mix 1 volume of pyridine with 4 volumes of ethylene glycol. Use for compounds containing carbonyl groups.
7.7.4 Pyridine - Methanol (1 + 4) - Mix 1 volume of pyridine with 4 volumes of methanol. Use for organic acids.
7.8 Sulfur Dioxide, anhydrous grade. (See Note 1 and 7.3.3.)