THE KARL FISCHER method depends on the reaction of iodine with water in the presence of sulfur dioxide and pyridine to form hydriodic acid and sulfuric acid. It is one of the most accurate methods.

The seed must first be finely ground and the moisture extracted with methyl alcohol or other water solvent.

The method has not been widely used under practical conditions, since it involves rather complicated equipment and intricate technique. Its greatest usefulness in respect to determination of seed moisture seems to be in checking the reliability of oven methods and in providing fundamental data for use in devising oven procedures that will give the greatest possible accuracy in the determination of moisture content.

Data of this type have been obtained for a number of different agricultural and vegetable seeds and are published in Marketing Research Report No. 304, "Oven Methods for Precise Measurement of Moisture Content of Seeds," issued in 1959 by the Agricultural Marketing Service.

PRACTICAL METHODS for determining the moisture of seed are needed under many circumstances where the basic methods take too much time. Practical methods standardized against one or more of the basic methods therefore have been devised. The results generally are likely to be less accurate than those obtained by the basic methods, but they may be good enough for most practical purposes.

Other heating methods sometimes are used to shorten the time required by standard oven methods for determining moisture content. In general, these methods require heating of the material to considerably higher temperatures than those employed in the usual oven methods. Heating may be accomplished by ordinary electric heating coils, by radiation from infrared lamps, or by means of a high-frequency, high-voltage field. When these methods are employed, it is customary to determine in advance the time of heating and the temperature or other adjustment of the equipment required for each type of material to be tested in order to obtain results in reasonably good agreement with those obtained by one or more of the basic methods. These methods should be quite useful in certain types of practical seed-testing work because the time required to complete a test is usually considerably less than I hour (in some instances 10 to 15 minutes), a number of tests may often be made at the same time, the cost of equipment usually is low, and the accuracy may be fairly high if the proper conditions for making the test are accurately determined for each kind of seed to be tested.

THE BROWN-DUVEL DISTILLATION method was used for many years in the routine inspection of grain. It is still used when the faster electrical methods cannot be used satisfactorily. A weighed amount of unground grain is heated in oil to a definite temperature. The moisture volatilized is condensed, collected, and measured in a graduated cylinder. The apparatus must be standardized to provide a definite amount of heat in a definite period of time. The method is arbitrary, and the exact procedure to be followed for each kind of grain in order to obtain results equivalent to those obtained by an official basic oven method must be determined.

The Brown-Duvel method should be applicable to most seeds except those that are light and chaffy. Before it can be used, however, it would be necessary to determine the exact procedure for each kind of seed in order to obtain results equivalent to those obtained by an appropriate basic method. The proper procedures have been established for seeds of wheat, corn, oats, rye, grain sorghums, barley, buckwheat, flax, soybeans, emmer, rice, beans, peas, mustard, cotton, and shelled peanuts.

CALCIUM CARBIDE reacts chemically with water to produce acetylene gas. This reaction has been used for determining the moisture content of various materials by measuring the weight lost, or, in a closed system, the pressure produced by the evolution of acetylene.

A special device with which pressure is measured is available for making moisture determinations on finely ground materials by use of this principle. This device has been used in testing various kinds of seeds. Theoretically, if the same weight of seed and calcium carbide were used in all tests, the relation between moisture content and pressure developed would be the same for all kinds of seeds. In actual practice, however, no such constant relationship appears to exist, and it is necessary therefore to calibrate the equipment against some basic method for each kind of seed tested.

ELECTRIC moisture meters, used in routine work, have an advantage in speed over all other methods for determining moisture.

Most of these instruments are based on measurements of either the conductivity or the dielectric properties of the grain, both of which depend primarily on the moisture content and temperature of the seed, but they also are affected to some extent by many other variable factors. Electrical methods therefore cannot be depended on to give reliable results under all circumstances.

The moisture content of most kinds of seeds probably can be determined fairly accurately under most circumstances by means of electric moisture testers, but they have to be calibrated against an accepted basic moisture-testing method. Separate calibrations must be made for each kind of seed. For some kinds it is also necessary to make separate calibrations for individual classes, varieties, or varietal types.

Because of the errors inherent in the electric methods, each calibration should be based on the testing of a large number of samples, covering a wide range in moisture, obtained from as many different points of origin as possible, and preferably representing the crops of at least several years.

It is obvious therefore that a large amount of work is necessary before a reliable calibration can be made for use in testing seeds with any electric moisture tester. Much work has been done in calibrating certain electric moisture meters for use with the various cereal grains, but relatively little has yet been accomplished in this field for most other seeds.

Electric moisture meters have a great advantage in speed over other methods. With most of the instruments, a test may be completed in a minute or less. Disadvantages are the relatively high cost of the equipment; the need for painstaking calibration for each kind of seed; and in some instances failure of the method to give results of a sufficiently high degree of accuracy. Factors other than moisture content affect the electrical properties of seed, and a more thorough understanding of these factors will be required before any appreciable increase in accuracy of electric moisture-testing methods can be expected.

LAWRENCE ZELENY is Chief of the Standardization and Testing Branch, Grain Division, Agricultural Marketing Service.