MIL-PRF-26539F
valves. Pass several cell volumes (at least 10 mL) of sample through the cell. Make sure the cell is free
of bubbles, then close the cell outlet and inlet valves and the N2O4 cylinder valve, in that order. If
possible, purge the cell with dry nitrogen before cooling, using several cell volumes of sample, and then
checking for the absence of bubbles. Place the cell in the spectrophotometer cell compartment and
connect to an ice-water-glycol bath through suitable tubing. Allow the coolant to circulate for
approximately 5 minutes. Set the absorbance to zero (100%T) at a wave length of 700 nm. Remove the
cell to the apparatus (Figure 1) in the hood and expel the N2O4 into the scrubber with a low pressure
stream of nitrogen. Repeat the above procedure substituting the sampler. The temperature of the
sampler should be 77 ± 3.6°F (25 ± 2°C). Measure the absorbance of the sample at 32 ± 1.8°F (0 ± 1°C)
and at a wave length of 700 nm. Determine the NO percent by weight from a calibration curve prepared
according to 4.3.2.2.3.
4.3.2.2.3 Calibration curve. A calibration curve shall be prepared as follows: Weigh accurately
58.164 g of ACS reagent grade or equivalent nickel nitrate hexahydrate [Ni(NO3)2 · 6H20], to make
100 mL of a 2.0M stock solution. In a similar manner weigh out 26.008 g of sodium azide (NaN3) to make
100 mL of a 4M stock solution. Filter or decant the latter after allowing it to stand overnight. Standard A,
equivalent to 0.5 wt% NO, is prepared by placing exactly 1.00 mL of the 2M Ni2+ solution in a 50 mL
beaker. Add 4.00 mL of the 4M NaN3 solution and 15.0 mL distilled water and mix. Standard B,
equivalent to 0.86 wt% NO, is prepared by mixing 1.00 mL of the 2M Ni2+ solution with 4.0 mL of the 4M
NaN3 solution, and 8.34 mL of distilled water in a second 50 mL beaker. Fill a 5.0 mm absorption cell with
distilled water as a blank. Set the spectrophotometer at 700 nm. Set the blank at 100%T (0 absorbance).
Determine the absorbance of standards A and B in the same 5.0 mm cell at 700 nm. Plot the absorbance
of Standards A and B versus the equivalent wt% NO (x-axis) and draw a straight line through the origin
(0 wt% NO). The mixed synthetic standard solutions are stable for 48 hours. The sodium azide stock
solution is stable for 30 days in the dark. The nickel nitrate stock solution is stable indefinitely.
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