Spectroscopic Methods for the Determination of Boron in Plant and Animal Materials

*" I ^HE Department of Chemistry at the Kentucky -L Agricultural Experiment Station has been investigating for some time the necessity of some of the minor elements in the economy of plants and animals. Boron is one of the minor elements which has received attention in our laboratory. Data have been obtained on two different branches of the subject of boron in relation to agriculture. At first we were interested in ascertaining to our own satisfaction the essential nature of boron as a plant nutrient. This information was obtained by means of purified sand cultures to which boron was added in some of the cultures, but excluded as nearly as possible in others. Lettuce plants were then grown in each of the two sets of cultures in a glass house; all other conditions were the same. After the plants had grown for several weeks those in the cultures from which boron was excluded attained conditions characteristic of boron deficiency. Similar boron deficiencies were produced with sweetpotatoes, tobacco and tomato plants when grown under similar conditions as the lettuce. These results have led to the conclusion that boron is a necessary nutrient for the growth of plants. When lettuce plants were grown in purified sand cultures a rather narrow range of boron concentration was observed between boron deficiency on the one hand and boron toxicity on the other. For example, in purified sand cultures lettuce containing about 20 p.p.m. of boron in the dry leaves attained the maximum yield and marked boron toxicity developed when the boron content reached about 60 p.p.m. in the plant material. Further work in our laboratory on the subject of boron has been concerned with methods for the determination of this element by spectroscopic procedures. We chose to make use of the spectroscopic procedure because of the minute quantity of boron to be dealt with, and furthermore, in this procedure, we are dealing with specific properties which are characteristic of the boron atom. The best make of instrument available for our use at first was a medium sized, direct vision Schmidt and Haensch spectroscope. Certain attachments have been added to this instrument and by their means it was possible to obtain quantitative data on the boron content of plants. The procedure is as follows: A definite amount of dry, finely ground plant material is weighed into a clean silica dish and ignited gently with the small flame of a Bunsen burner until most of the volatile and combustible matter is consumed. The ignition is completed in an electric furnace at a temperature of about 500 °C. After cooling, the ash from the sample is transferred to a suitable flask, methyl alcohol and a saturated solution of citric acid added and the mixture refluxed for about one hour on a hot water bath. The boron is thus converted to methyl borate by the procedure and it is distilled off, collected, made to a definite volume with methyl alcohol and a suitable aliquot taken for a boron determination by means of a spectroscope. We have recently installed in our laboratory a large size Littrow spectrograph and the necessary accessory equipment for further investigations pertaining to the necessity of minor elements in the economy of plants and animals. For a boron determination by means of the spectrograph, a suitable homogeneous portion of the carbonfree ash is weighed and placed in the crater of a boron-free carbon electrode and arced for a definite time for an adequate exposure on a spectrographic plate. The exposed plate is then developed by a standard procedure and the characteristic lines of the element identified as to wave length from a comparison with a standard iron spectrum. The percentage of the element under consideration is determined from a comparison of the density of the characteristic line