Velp Scientifica
– UDK139 Operating Manual
10004762/B5
65
Catalysts
Mercury is considered the best catalyst for Kjeldahl oxidative digestion, but its toxicity
for man and the environment brought to dismiss its use. The same can be said for
Selenium. To-day Copper sulphate (CuSO
4
• 5 H
2
O) with lower environmental
damage is commonly used. It is previously mixed with anhydrous Potassium
sulphate (K
2
SO
4
) with a weight ratio of 9 K
2
SO
4
to 1 CuSO
4
• 5 H
2
O. Each sample is
added with 7.5 g of this mixture (Missouri catalyst). There are commercially available
tablets containing anhydrous Potassium sulphate and different types of catalysts in
pre-dosed amounts, easily used (for example packing of 1000 tablets by Velp
Scientifica).
Other additions
A rise of boiling temperature of the digestion acid is obtained by adding mineral salts,
mostly Potassium sulphate (K
2
SO
4
). The use of Hydrogen peroxide (H
2
O
2
)
35% (130 volumes) is aimed to assure a complete mineralization of samples.
This reagent is to be added to the cold mixture (sample + acid) before digestion in
order to avoid violent reaction and dangerous splashing.
Do not ever start heating if no acid is still added, because Hydrogen peroxide in
contact with a sample can give rise to explosions.
Digestion temperatures and times
Normal digestion temperature is 420°C. Lower temperatures increase digestion time.
At 420°C digestion is completed in 20-45 minutes.
A completed digestion gives raise to a clear colourless solution that crystallizes on
cooling. If this crystallization is not wanted it is enough to lower a little the amount of
the salt added for raising the boiling point of the acid.
Special samples as steel or coal may require a digestion time of many hours (up to
24).
Other reagents
An alkaline reaction of digested samples, before distillation, is obtained by adding a
concentrated (32-35%) solution of Sodium hydroxide (NaOH), ammonia free (50 ml
of NaOH for 7 ml of concentrated H
2
SO
4
).
1 ml of concentrated sulfuric acid 98% d. 1.84 contains 36.6 milliequivalents of acid.
1 ml of sodium hydroxide solution 32% w/w d.1.35 contains 10.8 milliequivalents of
alkali. Thus 1 ml of concentrated sulphuric acid is stoichiometrically neutralized by
3.39 ml of sodium hydroxide 32% solution. The distillation of ammonia requires a
highly alkaline environment which means that ammonia is completely in the so called
"free" form. This is obtained adding a 100% excess of alkali.
We suggest to use 50 ml of sodium hydroxide 32% solution for a sample digested
with 7 ml of concentrated sulphuric acid (70 ml for 10 ml or 100 ml for 15 ml acid).
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