sintcr sliould be previously wxslied with 1% (v/v) HC1 until the absorbance at 300 in? of the filtrate in 4-cm. cells exceeds that of the original acid solution by no more than 0.002.)Measure the absorbnnce at 300 mp of the filtrate from the sample using 4-cm. cells. Make a ' blank ' determination in exactly the same way, using 200 ml. de-ioniscd water, and subtract the absorbance from that of the sample to give the true absorbance clue to the sample. Determine the approximate concentration of fuliric acids by reference to a calibration curve prepared from ptire solutions of fiilvic acids. (This conccntration must be corrected by an amount dependent on the concentrations of ferric iron, silica and nitrate.) Note.-It is possible that in some types of water samples not examined here, (e.g. polluted river waters) othcr interferences may be present; for siich samplcs each case must be examined individually. Corrections for silica and nitrate are quite small; for example 20 p.p.m. silica and 5 p.p.m. nitrate give apparent fulvic ackl concentrations of 0.06 and 0.1 p.p.m., respectively. Iron is the major source of interference and is determined as described below.
Drtermiwatioiz of ironAdd three drops of thioglycollic acid to 100 nil. of tlie filtrates from the sample and ' blank,' and then 40 drops of 18 M-aq. N 1 3 , with stirring. Measure the absorbance of tlie iron-thioglycollate coniplex in both solutions using 4-cm. cells, and subtract tlie value for the ' blank ' and also tlie absorption of the fulvic acids themselves. (With a Spekker absorptiometer using Ilford 604 filters, 10 p p m . of fulvic acids give an absorbance of 0.036.) From the corrected alwrbance of the iron-thioglycollatc complex calculate tlie concentration of iron in the sample from a calibration curve prepared from solutions containing known amounts of iron, and from a second calibration curve, prepared from solutions containing known concentrations of ferric iron in 1% (v/v) IICI, the absorbance at 300 mp due to the iron in the sample. Deduct this value from the measured absorbance at 300 mp of the sample to obtain a closer approximation to the absorbance due to fulvic acids.Note.-A second estimate of the fulvic acid concentration is thus obtained, and may be nsed to repeat the calculation above in order to obtain the true fulvic acid concentration by a method of successive approximations. I n general, it is not necessary to make more than two estimates as the corrections involved are usually quite small.
