Complete Guide to Kjeldahl Analyzer Operation and Maintenance

Automatic Kjeldahl analyzers determine protein content in dairy and other products. They offer high sensitivity, fast analysis, and broad application with minimal sample requirements. Proper operation and maintenance ensure accurate results and prolong the life of the equipment. This guide explains the correct procedures for laboratory personnel.

1. Proper Operation Procedures

The Kjeldahl analysis system consists of four main parts: the distillation unit, titration unit, detection unit, and waste discharge unit. Here is how to operate them correctly.

1.1 Pre-Startup Checks

Check the levels of deionized water, alkali solution, and receiving solution. Empty the waste barrel. Liquid level sensors monitor these containers. The instrument triggers an alarm and stops the current test if solution levels fall below the sensors or if the waste level rises above them.

1.2 Cooling Water Inspection

Turn on the cooling water switch before startup. High temperatures occur in the distillation unit during operation. Running the machine without cooling water triggers an alarm. Operating without cooling water damages the distillation unit.

1.3 Startup and Bubble Removal

Turn on the machine after confirming water and solution levels. The instrument performs a self-check. Enter manual mode to remove bubbles from the titrator. Bubbles occupy space intended for the standard titrating acid. This reduces the actual acid volume delivered. Lower acid volume causes low measurement results.

Steps to remove bubbles:

Open the front cover. The instrument sounds a safety alarm.
Place a small magnet on the front contact point to bypass the alarm. Press the enter button.
Select the titrator fill function. Wait for the piston to stop moving upward.
Pinch the plastic hose of the titrator.
Select the empty function. Release the hose after 3 to 5 seconds. Bubbles float to the top.
Select the fill function again to expel the bubbles. Repeat this process 2 to 3 times.

1.4 Blank Testing

Test a water blank first to check instrument stability. Stability is confirmed when two consecutive water blanks differ by less than 0.05 and the total value is below 0.2. Test the reagent blank next. Input this value into the instrument. Testing a reagent blank on an unstable machine yields artificially high blank values. This leads to low final sample results.

1.5 Sample Measurement

Weighing:

Mix milk samples thoroughly. Fat easily floats to the surface in milk. Taking unmixed samples from the top layer causes low results. Weigh milk powder on weighing paper. Insert the powder directly to the bottom of the digestion tube. Wash any residue on the tube walls to the bottom using sulfuric acid. Residue left on the walls leads to low measurement results.

Digestion:

Adjust the acid volume based on the sample composition. Insufficient acid causes incomplete digestion and low results. Excess acid consumes too much alkali during the later neutralization phase. This prevents the release of free ammonia and ruins the test.

Set the initial temperature to 280°C for 1 hour of pre-digestion. Increase the temperature to 420°C afterward. Pre-digestion allows a slow, controlled reaction between the sample and sulfuric acid.

The primary digestion reaction is: $Sample + H_2SO_4 \rightarrow (NH_4)_2SO_4 + CO_2 + SO_2 + H_2O$

Skipping pre-digestion causes violent reactions and carbonization at high temperatures. Carbonization causes low results. Carbon particles also block the waste lines and stop the instrument from functioning.

1.6 Recovery Rate Measurement

Test the recovery rate using ammonium sulfate or urea. Ammonium sulfate provides a simple and fast check of the instrument’s measurement accuracy. It does not require digestion. Urea requires full digestion. Urea testing guarantees quality control for both the sample preparation process and the instrument measurement.

2. Maintenance and Care

2.1 Distillation Unit

Cleaning: Add 80 ml of water using the manual function after completing all sample tests. Run steam distillation for 5 minutes to clean the unit. Remove the digestion tube afterward. Discard the tube contents. Wipe the safety door and drip tray to remove residual alkali.

Rubber Seal Replacement: Hot alkali corrodes the rubber seal during distillation. Corrosion breaks the seal between the rubber and the digestion tube. This causes ammonia leaks and incorrect results. Inspect the seal regularly. Replace it immediately upon finding signs of corrosion.

Replacement steps: Turn off the power. Open the front cover. Remove the safety door. Grip one end of the seal with pliers and pull downward. Soak the new seal in hot water for 5 minutes before installation. Hot water softens the rubber for easier fitting.

2.2 Titration Unit

Cleaning: Clean the titration cylinder and liquid level probes after testing. This prevents receiving solution residue from drying and affecting future tests.

Standard Acid Replacement: Clear old acid from the titrator when changing standard acid concentrations. Open the front cover. Bypass the door alarm using a magnet. Select the fill function. Wait for the piston to rise. Select the empty function. Repeat this 3 to 5 times. The new acid safely flushes out the old acid.

2.3 Waste Discharge Unit

Cleaning: Clean the waste unit regularly. Neglecting this blocks the internal pipelines. Measure 25 ml of acetic acid and 5 ml of water. Run manual distillation for 30 minutes. The volatilized acetic acid removes residual alkali from the inner walls.

Repair: Carbonized particles often block the solenoid valve located under the waste cylinder. Turn off the power if a blockage occurs. Open the back cover. Remove the solenoid valve. Disassemble it to clear the internal residue. Reinstall the valve.

2.4 Important Precautions

Test the recovery rate after replacing standard acid, receiving solution, or performing maintenance. Use ammonium sulfate for this check. A recovery rate between 99.5% and 100.5% indicates normal operation. You can proceed with sample testing once this rate is achieved.

3. Final Recommendations

Keep all components clean to maintain the sensitivity and stability of your Kjeldahl analyzer. Inspect the machine thoroughly before turning it on. Read the user manual carefully. Understand the functions of the distillation, titration, detection, and waste units. Follow standard operating procedures exactly.

To learn everything about Kjeldahl Nitrogen Analyzers, visit our page: What Is a Kjeldahl Nitrogen Analyzer?

To purchase our Kjeldahl Nitrogen Analyzer, go to: Kjeldahl Nitrogen Analyzer