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 Frequently Asked Questions
Last update:
June 4, 2008 |
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What are the main applications for the block digestors?
It is designed for Kjeldahl digestions of total nitrogen and total phosphorus.
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What is a Kjeldahl digestion for nitrogen determination?
Samples are digested in a solution of sulfuric acid, potassium sulfate and a catalyst
(mercuric oxide or cupric sulfate). Organic nitrogen compounds are converted into ammonium.
Kjeldahl nitrogen is the sum of organic nitrogen and ammonia nitrogen. Nitrogen can be determined
by using an FIA system, where the sample pH is raised to a basic pH. The in-line neutralization of
the sample converts the ammonium to ammonia, which reacts with a series of reagents to produce a
reaction product, the absorption of which is directly proportional to TKN concentration.
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What is a Kjeldahl digestion for phosphorus determination?
This digestion is used when the same samples are being analyzed for Kjeldahl nitrogen as well.
Samples are digested in a solution of sulfuric acid, potassium sulfate and a catalyst (mercuric oxide).
Polyphosphates and organic phosphorus are converted into orthophosphate. Lachat Instruments typically
refers to the resulting analyte as Total Kjeldahl Phosphorus (TKP) in order to specify the procedure
used for digesting the samples. The resulting orthophosphate can be determined by using an FIA system,
where the orthophosphate ion reacts with reagents in order to form a colored complex.
The absorption of which is directly proportional to the TKP concentration.
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What are the advantages of the Lachat BD-46 over other blocks in the market?
Maximize accuracy and precision
Because the BD-46 has 46 tube positions rather than the usual 40, calibration standards can now be
carried through the same digestion process as the samples. This maximizes accuracy and precision since
the standards and samples are treated exactly the same.
Reduce overflow of samples with surfactants
Many discharge water samples contain varying levels of surfactants, which will cause foaming of the
sample during digestion. The foam may then overflow from the tube onto the heated block. The special
design of the BD-46 digestion tube incorporates a bulb towards the top, which breaks up most foam and
reduces overflow.
Eliminate the need for expensive acid scrubbers
Until the BD-46, acid fumes have presented a serious problem. Either an expensive and often
ineffective acid scrubber needed to be purchased or the unit needed to be in a hood where corrosive
fumes would attack the seals and significantly shorten the life of the hood. Now, with the addition
of an inexpensive glass cold finger to each digestion tube, the acid fumes are cooled, condensed and
returned to the tube.
Increased reliability and visibility of controller
The BD-46 programmable controller is attached to the block on a flexible arm, which keeps it high
and dry away from spillage, and makes it easily adjustable to vary viewing angles. Other block digestors
have controllers separate from the block and sit on the bench where they are liable to spillage and are
difficult to view.
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What digestion procedure should be used for total phosphorus determination: Kjeldahl or persulfate?
If the samples are going to be analyzed for Kjeldahl nitrogen as well, the same digestion would take
of the total phosphorus. The persulfate digestion (autocalve) is usually preferred if the only analyte
of interest is phosphorus.
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What catalyst is best for digesting samples: copper or mercury?
Mercury (mercuric oxide) has been found to be the most effective catalyst for digesting nitrogen,
but it is also an element of considerable environmental concern. Copper (copper sulfate) is suitable for
a wide variety of samples and it has become the catalyst of choice in recent years. To test completeness
of digestion, it is recommended to determine nitrogen recovery from samples that have been fortified
(spiked) with nicotinic acid. Ammonium chloride is usually used as a fortifying (spiking) solution to test
for loss of nitrogen during the digestion.
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What is the recommended procedure for storing samples that will be digested?
Samples should be analyzed as soon as possible after collection. If storage is required, preserve
samples by acidifying to pH 1.5 to 2.0 with concentrated sulfuric acid (H2SO4) and store at 4°C.
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Do I need to place the block in a hood?
For safety reasons, any digestor block should always be placed in a hood. The design of the BD blocks
minimizes the loss of acid fumes, which are hazardous.
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What are the power and wiring requirements for the block?
208 - 240 VAC, 50/60 Hz. Requires outlet for a three-conductor twist lock plug, NEMA L6-15P.
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What is the typical maintenance that the block requires?
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Cleaning up any spills onto the block |
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Cleaning the glassware |
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Cleaning the racks if any spills occurs |
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Is there any test that can be performed to ensure the block is working properly at any given time?
The User manual includes a simple procedure for making sure the block is working properly.
The block also has an orifice in which a thermocouple can be inserted in order to certify the
block temperature, which should match the reading on the controller display.
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What kind of warranty does the block have?
The blocks have a one-year limited warranty. Lachat Instruments has a loaner program that is free-of-charge
during the warranty period. All servicing is performed at the factory.
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How long does it take to do a digestion?
The typical Kjeldahl digestion takes about two hours.
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