Notas de Aplicaciones


Kjeldahl method is used in analytical chemistry for the determination of nitrogen content in organic samples which is of great interest in areas such important today as food and environmental.

Since 1883 when John Kjeldahl presented his work, his method has gained wide acceptance and is applied in a wide variety of jobs for food analysis of food, drinks, feed, grain, meat, wastewater, soils for crops and others. Today is the most widely used method for protein analysis and is performed by determining organic nitrogen. This is because different types of proteins coincide all in a similar proportion of such organic nitrogen. In most cases the following calculation factor is used:

Protein content = organic nitrogen content x 6.25

In this technique, proteins and other organic compounds in a food mixture are digested with sulfuric acid in the presence of catalysts. The total organic nitrogen is converted into ammonium sulfate through the digestion. The resulting mixture is neutralized with a base and distilled. The distillate is collected in a solution of boric acid. Borate anions thus formed are titrated with standardized HCL to determine the nitrogen content in the sample.

Generally, the Kjeldahl method has the advantage of being running by unsophisticated equipments and can be performed by less experienced technicians.


The Kjeldahl method has been officially recognized by a large number of government agencies and associations such as: the International AOAC, EPA, AACC, AOCS, ISO, USDA and others.


The method consists of three stages: DIGESTION - DISTILLATION - TITRATION.

DIGESTION occurs in the nitrogen decomposition contained in organic samples by using a concentrated acid solution. This is obtained by boiling the sample in a sulfuric acid concentration. The result is an ammonium sulfate solution.

Ammonia is liberated in the DISTILLATION stage, which is retained in a solution with a known amount of boric acid. Initially a steam distillation is performed by the water steam distillation method, by which distillation obtainment is accelerated.

TITRATION is used in the end to finally assess the amount of ammonium present in the distilled sample.



(1) n - C -NH2 + mH2SO4 → CO2 + (NH4)2 SO4 + SO2
protein heat→


(2) (NH4)2SO4 + 2 NaOH → 2NH3 + Na2SO4+ 2H2O
(3) NH3 + H3BO3 (boric acid) → NH4 + H2BO3- (borate ions)


The borate anion (proportional to the amount of nitrogen) is titrated with standardized HCl (or H2SO4):

(4) H2BO3- + H+ → H3BO3


In recent years, new equipments are being developed and improving technologies to implement these analytical techniques.

J.P. Selecta, aware of these laboratories’ needs, has devoted a considerable effort to put on the market a new range of equipments, as complete as possible, in order to help the work of developing the Kjeldahl method with the speed, accuracy and reproducibility of results.

The equipments for organic nitrogen determination are composed of three basic elements:

- Bloc-Digest digestion unit.
- Tools for handling (Macro or Micro).
- Pro-Nitro “M”, Pro-Nitro “S” (semiautomatic) and Pro-Nitro “A” (automatic) distillers.

Recently, the new automatic system Auto Digest 20 has been added, which optimizes speed and reliability of laboratory professionals.


A number of interrelated conditions in the digestion process determine the speed of reaction and the decomposition of nitrogen into ammonium sulfate, such as the amount of heat transferred, the quantity of salts to raise the acid boiling temperature, the catalyst employed and the time of digestion. Adjustment of any of these parameters influences the rest. There are studies to determine the parameters needed to obtain optimum conditions depending on the samples matrix. For example, the amount of acid required varies depending on the fat present in the sample. The more quantity of fat, the more acid is required. It also varies with the time of digestion. The longer time, the more acid lost by evaporation.

Digestion time should be determined depending on the amount of recovery by using known matrix samples.

Salts addition is helpful to raise the boiling temperature of H2SO4. Depending on the kind of salts used, the temperature may go from 330°C being just the sulfuric acid, to a one of 400°C, thereby accelerating the rate of decomposition and considerably shortening the digestion time.

To perform digestion, a heating block made ​​of aluminium is normally used, surrounded by a thick layer of thermal insulation and assembled on a stainless steel frame. There are different block sizes for 6, 12 and 20 samples. The heating element is a high electrical resistance which is controlled by an electronic device incorporating a microprocessor which allows the user to choose and memorize several programs working with fully programmable ramps and times. Such programming capability optimizes digestions according to the material used.


  1. Depending on the sample water content, begin the digestion by evaporating the water at 150°C during 15 and 30 minutes.
  2. Perform a second step between 270 and 300°C within a period of 15 to 30 minutes in order to reduce the production of white fumes.
  3. Continue the digestion at 400ºC during 60 and 90 minutes.

Visual Control: The result is a clear transparent liquid with light blue colour, green or yellow depending on the catalyst used. No black residues should remain attached to the tube wall.


Cheese or meat:
Step 1: 150ºC / 30’ Step 2 : 270ºC / 30’ Step 3: 400ºC / 90’

Step 1: 150ºC / 15’ Step 2 : 300ºC / 15’ Step 3: 400ºC / 60’



  • Temperature independent control.
  • Bidirectional serial RS-232 connection for temperature recording and digestion program edition with RAT connected to a computer.
  • A software CD is included in the digestion unit.
  • Less sample handling.
  • Uniform heating of the aluminium block.
  • Temperature range from 45 to 450°C.
  • 4 steps memory for 20 programs.
  • Maximum time per step: 600 minutes.
  • Acoustic indication for end of digestion program.
  • Two selectable temperature gradients: Kjeldahl / D.Q.O.
  • Alarm for temperature sensor breakage.
  • Independent temperature control.
  • Bidirectional RS-232 serial connection for temperature recording and digestion program edition with RAT connected to a computer.

A software CD is included in the digestion unit. The software facilitates the digestion editing programs and allows you to track and record the digestor temperature.


Specially designed to absorb and neutralize acid gases generated in Kjeldahl digestion processes.

It consists of a "Scrubber" unit that blocks and neutralizes the acidic condensations, and a recirculating water pump that provides a great volume of vacuum for gases suction.

It is essential to place the "Scrubber" unit with the neutralizing solution between the digester and the recirculation pump.


The equipment carries out the digestion process fully automated, with rise and fall of the rack holder.

Equipment with metallic structure and automatic sample holder epoxy covered. Tube support rack made ​​of a special dur-al plate chemically treated.


  • Automatic sample manipulation.
  • Uniform heating.
  • Control automatic control for up to 20 programs of temperature, time, and sample’s elevation after digestion and start/stop of the "Scrubber".
  • RS-232 port for temperature recording and digestion programming from computer.
  • Gas collection system which could be used without extracting cabinets.

It is completely supplied with:

  • 1 metal heating block of 20 positions.
  • 1 automatic lifting system for samples.
  • 1 “Rat-2” programmer for time/temperature processes.
  • 1 tube support rack.
  • 1 gas collector.
  • 20 digestion tubes of 250 ml capacity.


The digestion product is usually diluted with ammonia-free water to minimize the effects of mixtures containing high proportions of acid / salts.

Most of the NH3 is distilled and caught in the acidic solution during the first 5 to 10 minutes of boiling, but depending on the volume of digestion mixture and on the method followed, from 20 to 140ml of condensate can be collected to obtain a complete nitrogen collection. Sometimes distillation is required to be extended, which produces more water but this does not alter the results when making the titration.

The distillation speed varies with the condenser cooling capacity and with the capacity of generating heat from the heater. The system of heating by water steam accelerates the obtaining of distillation. Using a recipient solution made of boric acid, dosing is not required with accuracy, as titration measures exactly the amount of ammonia by neutralizing 1:1 the complex formed by ammonia and boric acid. In fact, quite boric can be added to ensure the complete absorption of ammonia.

The reception solution must remain at 45°C to avoid loss of ammonia.


Pronitro M is a Kjeldahl with an automation level that provides a simple and safety operation. It is suitable for a laboratory with a small or medium samples volume.


  • Steam distillation unit.
  • Compact steam generator with safety overtemperature thermostat and overpressure protection pressure switch.
  • Safety door that avoid distillation with door open.
  • Presence detection of digestion/distillation tube. This device avoids NaOH dosing if there’s no tube.
  • Universal adapter for digestion/distillation tubes MACRO (Ø 42 mm) and MICRO (Ø 26 mm).
  • H2O and NaOH reservoirs are placed inside the equipment, which saves space in the laboratory.
  • Stainless steel case and ABS plastic front.
  • Automatic titration adapter kit.


  • Measuring range: from 0,2 to 200 mg of Kjeldahl nitrogen.
  • Programmable distillation time.
  • Nitrogen recovery: > 99,5%
  • Distillation speed: from 35 to 40 ml/minute.
  • Typical distillation time: from 7 to 10 minutes.
  • Water consumption rate: from 80 to 100 litres/h.
  • Steam generator water consumption: 2,5 litres/h.
  • Water reservoir capacity for steam generator: 6 litres.
  • NaOH reservoir capacity: 2 litres.


Boric acid captures ammonia gas and forms a boric ammonia complex. When ammonia is captured, colour of the recipient solution changes. Proceed as follows:

• Titrate the distillate with HCl or H2SO4 till colour changes. (End point: pH 4.65)
HCl moles = NH3 moles= N moles in the sample
H2SO4 moles = 2 NH3 moles = 2 N moles in the sample

Actually, different indicators can be used to get a turn as clean and sharp as possible. If it becomes difficult to detect the turning point, it may be useful to use a white reference solution.


When making calculations, we must take into account the recipient solution and the dilution factors used in the distillation process. References may be taken in the published reference methods.

• Make the calculation:

mg N = N x V x 14


N = Titration acid normality.
V = Consumed acid volume.
14 = Nitrogen atomic weight.

• To switch to protein content correct it by the appropriate factor depending on the nature of the sample (6.25 by default).
• Periodically perform a blank test and subtract it from the result.

% Proteins = P2/P0 x 100 x F


P2: Nitrogen (mg).
P0: Sample weight (mg).
F: Protein factor.
(6.25 by default)


Kjeldahl distiller is fully automatic and with a titration system "on-line" (real-time titration). Equipment for a systematic analysis, highly accurate, and with minimal staff, easy and safe. Suitable for laboratory with medium or large sample volume.

Kjeldahl distiller PRO-NITRO “A” titrates distillation while it is obtained (“On-Line” titration), and therefore distillation and titration become a single operation, drastically shortening analysis time.

This type of titration provides an additional advantage: it detects the point at which the sample no longer produces nitrogen, this property is used for stopping distillation at the right moment and thus ensuring that the distillation time is always optimal for a maximum nitrogen recovery and extend distillation no longer than necessary.

The colorimetric evaluation is accepted by the AOAC and needs no periodic calibration.


  • Steam distillation unit.
  • Automatic «On-line» colorimetric evaluation.
  • Steam generator with overtemperature safety thermostat and overpressure protection pressure switch.
  • Safety door that prevents distillation with door open.
  • Presence detection of digestion/distillation tube. This device avoids NaOH dosing if there’s no tube.
  • Universal adapter for digestion/distillation tubes MACRO (Ø 42 mm) and MICRO (Ø 26 mm).
  • H2O and NaOH, boric acid and HCl reservoirs are placed inside the equipment, which saves space in the laboratory.
  • Drain system of digestion /distillation tube and collector.
  • Automatic distillation stop.
  • Large LCD display of 20 x 4 characters.
  • RS-232 output to print the results.
  • Stainless steel case and ABS plastic front.


  • Measuring range: from 0,2 to 200 mg nitrogen.
  • Nitrogen recovery: > 99,5%.
  • Distillation speed: from 35 to 40 ml/minute.
  • Water consumption rate: from 80 to 100 litres/h.
  • Steam generator water consumption: 2,5 litres/h.
  • Water reservoir capacity for steam generator: 6 litres.
  • NaOH reservoir capacity: 2 litres.
  • Boric acid reservoir capacity: 2 litres.
  • Tritant reservoir capacity: 2 litres.
  • Titrator accuracy: 1,5%.
  • Titrator minimum dose: 0,01 ml.



1. Principle:

The method consists of mineralizing the sample with a concentrated sulfuric acid and alkalinizing with sodium hydroxide. The ammonia released is carried by distillation and collected on boric acid. Subsequent titration with hydrochloric acid allows calculation of the amount of protein initially present in the sample.

2. Reagents needed:

  • 96% sulfuric acid (d = 1.84).
  • NaOH, 35% solution (w / v).
  • Mixed indicator, especially for ammonia titrations.
  • Catalyst Kjeldahl.
  • 4% boric acid (w / v).
  • 0.25N HCl.
  • Distilled water.
  • Pumice stone in grains.

Note: It is important that all reagents are completely free from nitrogen.

3. Material needed:

  • Resolution balance 0.1 mg.
  • Digestor unit (Bloc-Digest).
  • RAT process programmer.
  • Collector / Extractor fan.
  • Pro-Nitro “M” or Pro-Nitro “A” Distiller.
  • Burette for titration.

4. Digestion:

  • Weigh about 1 gram of sample perfectly ground and homogenized in a nitrogen-free paper and insert it into a digestion tube.
  • Add 10 g of Kjeldahl catalyst, 25 ml of 96% sulfuric acid (d = 1.84), and some pumice granules treated to the sample tube.
  • Place the digestion tubes with the samples in the Bloc-Digest with the flume collector running.
  • Digestion can be performed at a temperature between 350 ... 420°C and at a time which can vary between 1 and 2h.
  • At the end, the resulting liquid is transparent green or blue depending on the catalyst used.
  • Let the sample cool down to room temperature.
  • Avoid precipitation by stirring occasionally.
  • Slowly dose 50 ml of distilled water in each sample tube (be careful of the reaction violence).
  • Let the sample cool down to room temperature.
  • If precipitation occurs, gently stirring or heat.

5. Distillation

Dose 50 ml of boric acid in an Erlenmeyer flask and some drops of mixed indicator. Place the Erlenmeyer flask in the refrigerant extension taking care it has to be submerged in the boric acid.

Once the sample tube and the Erlenmeyer with boric acid are placed, dose 50ml of NaOH and start distillation.

Distillation should be extended long enough to distillate at least 150 ml, approximately from 5 to 10 minutes.

6. Blank test

After distillation of a sample, perform a blank test by using the above described method, but using 5 ml of distilled water.

7. Titration

Titrate the obtained distillate with 0.25N hydrochloric acid until the solution turns from green to violet.

Calculate the amount of nitrogen detected.

% Nitrogen = 14 x (V1-V0) x N / P

% Protein = % Nitrogen x F


P = Sample weight in g.
V1 = Volume of HCl consumed by titration (ml).
N = HCl normality.
V0 = Volume of HCl consumed by blank titration (ml).
F = Conversion factor to pass from nitrogen content to protein content. For crude protein a value of 6.25 is normally used. For greater accuracy, other conversion factors can be used by distinguishing the protein quality according to the sample nature.


Aspectos relevantes de la tecnología Peltier aplicada a las estufas incubadoras refrigeradas Selecta / Relevant aspects of Peltier technology applied to Selecta refrigerated incubators

From the first day of its activity, J.P. Selecta has tried to be aware of the latest technologies and methodologies in the field of laboratory equipments. Continuing with this innovative eagerness, as soon as the new electronic elements to produce both heat and cold through the principle called Peltier were known, a series of research projects aimed at creating a new refrigerated incubator based on this system started. Early work with Peltier elements for knowing technology started in the early 80's but it was not until 1990 when the first incubator finally hit the market as a world premiere, running only by Peltier elements.

Over these 32 years that we have had our incubators cooled with Peltier system on the market, effectiveness and utility have been proven, being this line of ovens the most prestigious of our brand.

Particularly it is worth mentioning a long list of advantages if compared with traditional ovens made with conventional heating elements and cold groups:

With Peltier technology, heating and cooling processes are conducted into one compact system. Both the excellent adjustment level and the oscillation reduced to the minimum of the temperature obtained occur thanks to the perfect development of refrigeration and heating technique, with a considerable energy saving and environmentally friendly.


Biotechnology, Bacteriology, Plasma fractions, Biology, Enzymatic test, Research, Serum studies, Metrology, Botany, Phytopharmacy, Cosmetics, Water analysis, Industry, Agriculture.


at 5ºC at 37ºC at 60ºC
Stability ±0,05 °C ±0,05 °C ±0,05 °C
Homogeneity ±0,35 ±0,30 ±0,75 °C
Set point error ±0,25 °C ±0,20 °C ±0,40 °C

Most important aspects:

  • The energy consumption reduction is up to 90%, when working with an ambient temperature of 22 °C.
  • No air exchange with the environment. Closed refrigeration system.
  • Hermetic refrigeration system.
  • Optimum temperature distribution.
  • Silent.
  • Stable.
  • Vibration-free.
  • Condensations free.
  • High accuracy.
  • Tempered glass inner door.

Peltier System Background
The discovery of thermoelectric phenomenon two centuries ago, and the search for new alternative energy generation, has allowed continued progress in thermoelectric technology in recent years. Since 1834 this is known as the Peltier effect; however, its practical application needed the development of semiconductor materials. The Peltier effect is characterized by the occurrence of a temperature difference between two sides of a semiconductor when a current flows through it. Generally such cells are made with Bismuth for the P type semiconductor face and with Tellurium for the type N face.

Operation Theory

The establishment of a heat flow, opposite to the thermal diffusion, when a material subjected to a temperature gradient is crossed by an electric current, suggests thermoelectric cooling applications. This alternative solution to the traditional refrigeration that uses compression-expansion cycles does not need moving parts, which increases its reliability. Besides being totally quiet, they have a very small size and weight, and they can easily handle bumps and vibrations, they can be used in any position, vertical, horizontal, inclined and also, thanks to them, cooling capacity can be adjusted by simply varying the feed stream.

What makes them even more interesting is the fact that, when reversing the supply polarity, their operation is also reversed, that is to say: the surface that formerly generated cold begins generating heat and the one generating heat, starts generating cold. These properties are essential in applications where the temperature must be controlled very precisely and reliably, such as in the containers used in transporting organs for transplantation or in those in which vibrations are serious disadvantages, for example: guide systems that employ laser, or integrated circuits. Furthermore, the possibility of creating a thermal flow directly from an electrical current makes the use of gases such as freon useless, as they are harmful to the ozone layer.

Moreover, the high reliability and durability of these systems (due to the absence of moving parts) have motivated their use in spacecraft electrical power, as in the Voyager spacecraft, launched into space in 1977. Here the heat flow established between the fissile material PuO2 (the PuO2 is radioactive and disintegrates, and so constitutes a heat source) and the outside passes through a thermoelectric conversion system based on SiGe (silicon and germanium thermocouple), thereby allowing electrical power of the probe (space probes cannot be powered by solar panels beyond Mars, as the solar flux is too weak).

As discussed below, conversion systems that use the thermoelectric effect have a very small output. Currently, their applications are limited to commercial sectors where reliability and durability are more important than price.

Some Typical Uses of Thermoelectric Cooling and Heating


Electronics, laser diodes, temperature control plates, and climate process chambers, freezing point reference baths, constant temperature baths, dew point hygrometers, osmometers, microscope stages.


Moving Boxes, cabinets or containers for food distribution, medical equipments, and pharmaceuticals, by land, sea or air.


Mobile or stationary storage of blood or pharmaceutical instruments, hypothermia blankets, coolers, ophthalmic cornea freezers, blood analyzers, preparation and storage of tissues.


Constant temperature portable cases for blood distribution and pharmaceutical supplies, electronic devices, inertial guidance systems for cooling and heating, parametric amplifiers and other equipment on ships, submarines, trucks, aircrafts and spacecrafts.

Ricard Cardus

R & D Department




Sterilization is the process by which destruction or elimination of all forms of microbial life, including bacteria and their structures, is achieved.

Laboratory autoclaves can perform sterilization by using steam at a higher pressure than atmospheric, and thus accumulating the steam temperature and reaching from 105ºC to 134ºC, according to the microorganisms to be destroyed. The steam penetrates the sterilization chamber at the set pressure; when condensating, it releases damp heat and simultaneously heating the material inside.

J.P. SELECTA offers a wide range of desktop, for dentistry and medicine, laboratory autoclaves being able to choose between manual purge, atmospheric or vacuum models, from which we highlight the following laboratory autoclaves:

"PRESOCLAVE II" for capacities of 50 and 80 litres, with electronic temperature, time and atmospheric purge regulation.

"AUTESTER ST DRY PV II" for capacities of 50, 80 and 150 litres, with microprocessor control of processes, drying system and vacuum fractional automatic purge.

This equipment has a RS232 output available for computer parameters printing, USB module for parameters storage and thermal printer with temperature, pressure, time and mode indications.

Only for information, please find below a relation of the different programs used for the different materials to be sterilized:

J.P. Selecta autoclaves meet with the Quality Management System Certificate UNE-EN ISO 9001, AENOR body accredited by ENAC with nº 01/C-SC003 and Safety Standards: EN 61010-1, EN 61010-2-040 and EN 61326.