Labconco Rapid Distillation Apparatus 65000 - Manual (Page 14)

NORMAL OPERATION
10
Determination
(1)
Samples are digested utilizing the Labconco Rapid Digestor 5 place of the 25 place units
equipped with 250 ml volumetric digestion tubes (refer to digestion methods available
upon request from Labconco). After digestion, remove rack of tubes from digestor and
allow to cool for 8 – 10 minutes (cooling time depends upon air flow around tubes). Direct
rapid spray of water (kitchen sink dish rinsing sprayer works well) to bottom of each tube
to dissolve acid digest completely. If precipitate forms, dissolve by manual mixing or by
placing tubes in ultrasonic bath. Low recovery of nitrogen will result if precipitate is not
completely dissolved. Let sample cool, dilute to the mark and mix thoroughly by inversion
20 times. Using a 25 ml buret, prepare the sulfuric acid receiving solution by adding the
calculated amount of 0.05N sulfuric acid standard solution to a 125 ml Erlenmeyer flask.
Add 5 – 7 drops of indicator (see calculating section for amount of standard solution to
add). Dilute volume with water to approximately 70 – 75 ml.
Pipet ml aliquot of sample solution to sample addition funnel on the Rapid Distillation
Apparatus. Distillation rate must be between 4 – 5 ml minute. Adjust water flow control
valve to keep water level in the steam reservoir approximately 2/3 full. Introduce sample
into sample chamber. Rinse funnel with 3 – 4 ml portions of water. Raise 125 ml
Erlenmeyer flask, containing the receiving solution, into position under the condensate
outlet tube of the still. Correct position is where the top of the outlet tube is completely
submerged below the receiving solution liquid level. Add 10 ml + 1 ml of sodium
hydroxide/potassium sulfide to the addition funnel. Slowly add alkali to the sample
chamber. (It is very important to add the caustic slowly, while visually monitoring the
standard acid solution to see that the solution does not siphon back through the condenser
and into the steam reservoir.) Leave column of caustic in the funnel stem to act as a liquid
seal. Distill for 3 – 5 minutes, lower receiving flask and allow distillation to continue for
approximately one minute. Titrate excess standard acid with 0.01N sodium hydroxide
standard solution. Correct for blank determination on reagents.