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Nu3 N in Soil . <br />(ASA Agronomy No. 9 Methczis of Soil Analysis) " <br />Page Tr,o <br />Procedure, Contd. <br />9. Dilute 1 ml of 1.000 PPM Na3 N S1D in a 100 ml wltaretric flask and <br />pipet 0.5 ml, 1.0 ml, 2.0 ml and 3.0 ml into separate 100 ml beakers <br />for 5, 10, 20 and 30 PPM standards respectively. <br />10. Pipet 1.0 ml of saturated CaCO3 into each beaker and to an additional <br />beaker for a zero standard. <br />11. Evaporate each beaker to dryness in an oven at 105°C or 55°C overnight. <br />Ib not heat past dryness: <br />Development of Nitrophenoldisulfonic Color <br />12. When the beakers cool, pipet 2.0 ml of phenoldisulfonic acid into each <br />beaker and roll beaker to be certain the acid comes into contact with <br />all residual salt. Allow 10 minutes for reaction. <br />13. Add 1.0 ml of deionized water to each beaker. <br />14. Slowly, with swirling, add con. NHaOH to each beaker (caution should be '. <br />observed in pipetting a concentrated base into a concentrated acid) until <br />the so lution is distinctly alkaline as indicated by deLmlognent of yellow <br />color, then add 3 ml, excess, but with a maxuman of no irore than 10 ml. <br />Add 10 ml NH4OH to blank and sales which develop no visible Dolor. <br />15. Quantitatively transfer the solution to a 25m1 wlumetric flask, diluting <br />to wlume with d S d H2O. <br />16. Zero the spectrophotometer on the Zero Standard at 420 Lan using a 1 czn <br />cell path, then read and record the absorbance of the standards. <br />17. Zero the spectrophotometer os1 the blank at 420 tan using a 1 can cell path, <br />then read and record the absorbance of the samples. <br />Calculations: <br />1. Obtain the slope from the standard trove by either plotting cortoetLtration <br />vs. absorbance or by - <br />/ 5~ l0us 2_ 0~ 30u~_\ <br />Slope of STD CLave =f AB55~ + AHS10~ + AB520ug + ~30ug J <br />l\ NimbPS of Standards (In this case 4) <br />the diltrticn is derived as follows: " <br />Xug/25 ml = Xug/10 ml extr. = 2.5 Xug/25 ml extr. _ .25 Xug/gND3 N <br />Concentration of N03-N in PPM = (sang~le ABS~) x .25 ug/9 x slope <br />6-67 <br />