Spectrophotometric Method for Determining Low Levels of Water-Soluble Boron in Fertilizers Harvey P. Peterson and David W. Zoromski Wisconsin Department of Agriculture, General Laboratory Division, Madison, Wis. 53705
VOLUMETRIC METHODOLOGYin common use (I) for determining water-soluble boron in fertilizers can not accurately detect low levels of boron. The method here proposed has increased sensitivity and rapidity for this determination. Colorimetric analyses have been developed that are capable of determining microgram quantities of boron in solution. Hatcher and Wilcox (2) proposed such a method based upon the reaction of boron with a solution of carmine in concentrated sulfuric acid. The procedure proposed utilizes the carmine color development after the water-soluble boron has been extracted and interfering impurities removed using a 2-ethyl-1,3-hexanediol extraction similar to that reported by Agazzi (3). EXPERIMENTAL Apparatus. All absorbance measurements were made on a Beckman DK2A recording spectrophotometer. The use of borosilicate glassware was avoided where possible. Reagents. A 10% solution of practical grade 2-ethyl-1,3hexanediol (Eastman Organic Chemicals, Rochester, N.Y.) in chloroform was prepared. Carmine reagent was prepared by dissolving 0.05 g of carmine No. 40 N.F. (Sargent-Welch Co.) in 100 ml of concentrated reagent grade sulfuric acid. Heating on the steam bath with occasional shaking facilitated the solution of the carmine. A 100-ppm standard boron solution was prepared by dissolving 0.5716 g of AR grade boric acid in distilled water and diluting to 1 liter. A 4-ml aliquot of this solution was diluted to 100 ml to produce the 4-ppm working standard. Norit “A” Alkaline Decolorizing Carbon (C-176) was obtained from Fisher Scientific Co. Procedure. A sample containing not more than 0.4 mg of boron was weighed into a 250-ml erlenmeyer flask and 80 ml of distilled water added. This was boiled for 15 minutes and quantitatively transferred to a 100-ml volumetric flask, cooled, diluted to mark, and filtered through Whatman No. 2 filter paper. A 50-ml aliquot was pipetted into a 125-ml standard tapered erlenmeyer flask. A 50-ml distilled water reagent blank and 50 ml of the working standard were analyzed with the sample starting at this point. Then 5 ml of 10% sulfuric acid and 20 ml of 10% 2-ethyl-1,3-hexanediol in chloroform were added by pipet. The stoppered erlenmeyer was shaken for 5 minites (3). The chloroform and aqueous phases were quantitatively transferred into a 125-ml separatory funnel with a 5-ml chloroform rinse. The chloroform layer was drained into a dry 125-ml erlenmeyer flask. Twenty milliliters of 2 % NaOH solution was added by pipet, the flask was stoppered, and shaken for 5 minutes. The contents were transferred into a dry separatory funnel and the chloroform layer was discarded. The aqueous layer was transferred into a 100-ml beaker containing 0.5 g of Norit, mixed by swirling, and filtered through Whatman No. 2 filter paper. A 2-ml aliquot was pipetted into a 25-ml erlenmeyer flask and 2 drops of HC1 were added. Exactly 10 ml of concentrated (1) “Official Methods of Analysis,” 11th ed., AOAC, Washington, D.C., 1970, sec. 2.106. (2) J. T. Hatcher and L. V. Wilcox, ANAL.CHEM., 22,567 (1950). (3) E. J. Agazzi, ibid.,39,233 (1967).
~
Fertilizer sample No. 674 674 677 677 679 679 683 683 693 693 694 694
Table I. Recovery Studies PPM PPM PPM level of Theofound found boron retical in the in added to PPM in spiked sample sample sample sample 11.3 11.3 7.7 7.7 7.7 7.7 78.6 78.6 11.3 11.3 12.3 12.3
125.0 200.0 125.0 200.0 125.0 200.0 62.5 100.0 125.0 200.0 125.0 200.0
136.3 211.3 132.7 207.7 132.7 207.7 141.1 178.6 136.3 211.3 137.3 212.3
Recovery,
z
141.4 211.2 137.0 208.5 145.8 213.8 141.4 184.6 138.6 211.2 144.1 208.5
103.7 100.0 103.2 100.4 109.9 102.9 100.2 103.4 101.7 100.0 105.0 98.2 Average recovery: 102.4
z
HzSOa was added slowly, mixed with gentle swirling, and allowed to cool. To the cooled acidified sample, exactly 10 ml of carmine solution was added. The contents of the erlenmeyer was thoroughly mixed and allowed to stand at least 45 minutes for color development. Absorbance of the solution was measured in a I-cm cell at 600 nm against the reagent blank. The boron concentration was calculated from the absorbance value found for the standard. RESULTS AND DISCUSSION
The boron in the fertilizer sample was put into solution by boiling in 80 ml of water. A boiling time of 15 minutes was sufficient. An aliquot of this water extract was acidified and the boron recovered from it by a single extraction with 10% diol solution. The boron recovery increases with a diol concentration up to 10%. Above this point, recoveries were not significantly affected by the diol concentration. This procedure gave a 90 % recovery using a 10 diol solution. A 5-minute extraction with an aqueous sodium hydroxide solution results in maximum recovery of boron. Traces of diol that are carried over were removed by Norit, since the diol chars on contact with H2SO4 producing a yellow color. Although single extractions do not result in complete recoveries of boron, in practice no error is introduced since calculations are based upon standards of aqueous boron solutions extracted by the same procedure. The HC1 is added to the final aliquot to remove interference by nitrates and nitrites (2). Six commercially available inorganic fertilizers were analyzed for boron by this method. Samples of each of these fertilizers were spiked with known amounts of boric acid and again analyzed. Average recovery for these samples was 102.4% as shown in Table I. RECEIVED for review December 2, 1971. Accepted February 11,1972. ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
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