No abstract

The total nitrogen content of plant tissue may be determined by direct nesslerization of sulphuric acid-hydrogen peroxide digests with accuracy up t o 5%. Similar application to soil extracts involves somewhat greater errors.In a spectrophotometric study of the Nessler colour, a filter showing maximum transmission a t 420 mp absorbs the greatest proportion of the colour.In the sulphuric acid-hydrogen peroxide digestion of plant tissue, added ammonia is fully recovered but added nitrate suffers a loss of 24-47y0, the percentage loss decreasing as the amount of nitrate present increases.Bromothymol blue is a suitable internal indicator for the neutralization of test solutions before nesslerization. The blue colour of the indicator does not affect the absorptiometric measurement of the Nessler colour.Soil solutions and soil extracts may be decolorized satisfactorily by a specially treated activated charcoal. This charcoal does not absorb appreciable amounts of ammonia from soil extracts made with Morgan's reagent, 2.5% acetic acid or water. For more accurate determinations of ammonia in soil extracts etc., distillation of the ammonia into acid in the presence of bromothymol blue, followed by nesslerization, is preferable.A modified Nessler reagent that showed diminished susceptibility to interfering factors and could be used immediately after preparation has been described in a previous paper.l The present communication deals with the use of this reagent in the micro-determination of total nitrogen in plant tissue and of ammonia-nitrogen in soil extracts and soil solutions by direct nesslerization. Some observations on the effective use of light filters in the spectrophotometric measurement of the Nessler colour are included. Spectrophotometric measurement of Nessler colourIn previous work it was shown that for the measurement of the Nessler colour a dark-blue filter OBI (approximate maximum transmission at 435 mp) afforded about three times the sensitivity of the light-blue filter OBz (480 mp).1 A Unicam spectrophotometer, which has J. Sci. Food Agric., 5, August, 1954 * Part I : J. Sci. Fd Agric., 1952, 3 , 441 YUEN G POLLARD-DETERMINATION OF NITROGEN. 11 365 FIG. I .-Spectraphotometric study of Nessler colour since become available in this Laboratory, made possible a more detailed examination of the character of the Nessler colour with respect to the visible spectrum. To each of a series of 50-ml. graduated flasks, containing respectively 0, 20, 50, IOO or 150 pg. of ammonia-nitrogen, were added I ml. of sodium potassium tartrate solution and 2 ml. of Nessler reagen't.1 The readings at different wavelengths, recorded with the Unicam spectrophotometer, are shown in Fig. I.It was evident that the spectrophotometer readings for the Nessler colour increased steadily with decrease of wavelength from 600 to 400 mp. With light transmissions between 600 and 700 mp, the absorption of Nessler colour was apparently small. Owing to the high ' blank ' obtained with wavelengths approaching 400 mp, the absolute maximum readings...

A specific method is described for determining paraquat (1,l'-dimethyl-4,4'-bipyridylium cation) in fruits with a sensitivity of 0.01 p.p.m. It depends on measurement of the light absorption of reduced solutions of paraquat after they have been concentrated and purified -by cation-exchange chromatography. Correction for extraneous background absorption from naturally occurring plant substances is made by using a "base-line" method of calculation. The method has been used with minor modifications for determining paraquat in a wide variety of other food crops and in water.PARAQUAT is the common name for the l,l'-dimethyl-4,4'-bipyridylium cation, which is manufactured in the form of its dichloride (I) or di-(methyl sulphate) salt, and is the active ingredient of the commercial formulation, Gramoxone.

Methods are described for determining diquat and paraquat, singly and in admixture, in formulations. For determining diquat, ultraviolet absorptiometry a t 310 m p in a sodium acetate buffer solution a t pH 4.05 is adopted.Paraquat is determined in a diluted solution by measuring the optical density, a t 600 mp, of the blue free radical produced by reduction with alkaline sodium dithionite. For analysing mixtures containing both diquat andparaquat, these methods are combined and use is made of a base-line correction procedure to compensate for interference of diquat in the determination of paraquat. The error for these methods is established to be within 3 2 per cent.

1. A modified Nessler reagent in which the ratio HgI2 : KI : NaOH is 1 : 2.2 : 20 is described. It is clear and ready for immediate use and is less susceptible to interference by various cations. Distillation with phosphoric acid was satisfactory for preparing ammonia‐free water. The best range of ammonia‐nitrogen for this determination was from 0 to 100 μg. 2. The amount of the reagent used, the time and temperature of colour development, and the acidity of the test solution were critical factors in the accuracy of the determination. 3. Many cations interfered with the test to various extents. Among anions tested only arsenite, chromate, cyanide, dichromate and permanganate caused serious interference. Addition of sodium potassium tartrate, which did not alter the effects of anions, generally lessened cation interference. 4. With the modified reagent and under the conditions described quantities of ammonia‐nitrogen in the range 0–100 μg. may be determined with an accuracy of 2–3%.