Overview
The automatic Kjeldahl nitrogen analyzer has the characteristics of high sensitivity, fast analysis speed, wide application range, less sample required, simple equipment and operation. It is the most widely used instrument for measuring protein in milk and dairy products.
The simple analytical device flow of the fully automated Kjeldahl nitrogen analyzer consists of four basic components: (1) distillation unit; (2) titration unit; (3) detection unit; and (4) waste disposal unit. This article mainly describes its maintenance and maintenance methods.
First, prepare to start work
1.1 Before starting the machine, ensure that each solution can meet the test determination, check the use of deionized water, lye and receiving liquid, and the waste liquid in the waste liquid tank is cleaned in time.
If the liquid level of the lye and the receiving liquid is lower than the liquid level sensor in the deionized water, or the liquid level of the waste liquid is higher than the liquid level sensor, the instrument will alarm and the ongoing test will be stopped, affecting the measurement work. Work properly.
1.2 Before starting the machine, ensure that the cooling water switch is turned on.
If the cooling water switch is turned off, the instrument can pass the self-test, but during the distillation process, because the temperature of the distillation unit is too high, the instrument will alarm and prompt to open the cooling water switch. The instrument will continue to operate when the cooling water switch is turned on. However, this test was stopped, affecting the normal measurement, and the distillation device was damaged without cooling water protection.
1.3, etc. 1.1 and 1.2 after confirming the error, the instrument is self-tested. After the self-test is passed, the instrument enters the waiting for selection process.
First enter the manual mode to eliminate the bubbles in the titrator. The presence of bubbles affects the measurement results, because after the bubbles enter, they occupy the volume of the standard titration acid, reducing the volume of the standard titration acid, and the measurement results are low.
The method of eliminating air bubbles is as follows: first open the front cover of the instrument, the instrument alarms, prompting the front cover of the instrument to be opened, and finding a small magnet placed on the contact on the front of the instrument, press the Enter key on the panel, and the instrument defaults the front cover to close. Select the titrator of the instrument function to fill the liquid. After the titrator piston moves up, stop and hold the plastic hose of the titrator. Select the titrator to empty, the titrator piston moves downward, and then let go immediately after moving for 3~5 seconds. The bubble in the titrator will float up to the upper end of the titrator, select the titrator to fill the liquid, the bubble is removed from the titrator, repeat 2~3 times, and the bubble is drained and the measurement is started.
Second, measurement considerations
2.1 Determination of blanks
In the blank measurement, the water blank is first measured, and the water blank is measured to check the stability of the instrument. The instrument is stable when the last two blanks are less than 0.05 and the water blank is less than 0.2. The reagent blank is measured and the blank value is entered into the instrument. If the reagent blank is directly measured without first measuring the water blank, the reagent blank is measured when the instrument is unstable, and a high reagent blank value will be obtained, and the measurement result will be low.
2.2 Sample determination
a.
The milk sample needs to be mixed and sampled. Because the milk sample is easy to float up and is directly sampled from the upper layer without mixing, the measurement result will be low. The milk powder samples are evenly mixed and weighed with a weighing paper and sent to the bottom of the digestive tract with tweezers. The sample remaining on the tube wall due to improper operation is flushed into the bottom of the digestive tube with sulfuric acid when sulfuric acid is added. If it remains on the tube wall without sulfuric acid, it will make the measurement result low.
b. digestion
In the process of adding acid, the influence of the composition of the sample on the amount of acid consumed should be considered. The amount of acid is small, the sample is incompletely digested, and the measurement result is low; the amount of acid is large, and the acid is excessively mixed with the alkali in the process of the machine, and the free amine cannot be distilled out, which seriously affects the measurement result.
During the digestion, pre-digestion was first carried out at 220 ° C for 1 hour and then the temperature was raised to 420 ° C. The purpose of pre-digestion is to allow the sample to react slowly with sulfuric acid. Without pre-digestion, concentrated sulfuric acid will react violently with the sample at high temperature, causing carbonization, and the measurement result will be low. The particles caused by carbonization will also block the pipeline during the waste discharge process, resulting in the instrument not working properly.
2.3 Determination of recovery rate
The recovery was measured using ammonium sulfate and urea, respectively. The simple use of ammonium sulfate is simple and fast, and should not be digested, but only the part measured by the instrument can be guaranteed, and the quality system of the digestion process of the pretreatment cannot be guaranteed. Urea needs to be digested for measurement, and the quality of the pre-treatment and instrument measurement parts is guaranteed.
Third, maintenance methods
3.1 distillation unit
3.1.1 Cleaning
After the sample was measured, 80 ml of water was added by manual function, and steam distillation was performed for 5 minutes to clean the distillation apparatus. After washing, remove the digestive tract, pour out the contents, and wipe the safety door and drip tray to remove the residual lye during the measurement.
3.1.2 Replacement of the rubber head
During the distillation process, the hot lye has a corrosive effect on the rubber head of the distillation apparatus. After a long time of use, the contact between the rubber head and the digestive tube is not closed, resulting in leakage of free amine, which affects the measurement result. The rubber head is inspected regularly (usually at approximately the cycle) and found to be severely corroded and replaced immediately.
The replacement method is as follows: Turn off the power switch of the instrument, open the front cover of the instrument, remove the safety door, clamp the end of the rubber head with pliers, and pull it down with force. When installing a new rubber head, first soak it in hot water for 5 minutes. The hot water makes the rubber head soft and easy to install.
3.2 titration device
3.2.1 Cleaning
After the test is completed, the titration cylinder and the liquid level probe are cleaned to prevent the residual of the receiving liquid from adhering to the inner surface of the titration cylinder, which affects the next measurement.
3.2.2 Replacement of standard acid solution
When using standard acid solutions with different concentrations, the acid remaining in the instrument titrator should be removed.
The method of removing the acid solution; first open the front cover of the instrument, the instrument alarms, prompting that the front cover of the instrument is opened, find a small magnet placed on the contact on the front of the instrument, press the Enter key on the panel, and the instrument defaults the front cover to close. The instrument function selection titrator is filled, the titrator piston moves up and stops; then the titrator is selected to empty, the titrator piston moves downward, repeated 3~5 times, the old acid solution will be replaced by the new acid solution.
3.3 Discharge device
3.3.1 Cleaning
The waste disposal device is cleaned regularly (usually in a cycle), and the pipe is blocked if it is not cleaned for a long time. The 25 ml of acetic acid solution and 5 ml of water were weighed and manually distilled for half an hour, and the volatilized acetic acid removed the residual lye on the inner wall of the waste discharge device.
3.3.2 Maintenance
When the waste disposal device is clogged, it is generally blocked by the carbonized particles in the solenoid valve below the waste liquid tank. Turn off the power switch of the instrument, open the back cover of the instrument, remove the solenoid valve, remove the internal waste residue after disassembly, and install it.
3.4 Notes
The recovery rate of the instrument is determined after each replacement of the standard titration acid or receiver solution and after repair of the instrument. Generally, ammonium sulfate is used for recovery analysis to ensure that the recovery rate is within the range of 99.5% to 100.5%, indicating that the instrument is normal and the sample can be measured.
Fourth, the conclusion
In order to maintain good sensitivity and stability of Kjeldahl, it is possible to analyze samples quickly and accurately, and obtain accurate analysis results. When using it normally, care should be taken to keep the various components of the instrument normal. Check carefully before starting, confirm that it is correct. After the start measurement.
Special note: Read the instruction manual of the instrument repeatedly and carefully, so that the functions, characteristics and related matching of the main components of the instrument such as distillation device, titration device, detection device and waste disposal device are familiar, understood and integrated. Strictly follow the instructions and the operating procedures of the instrument, and carry out the standard operation, which is the prerequisite for the correct use and scientific maintenance of the instrument.
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