Distillation and absorption is carried out in the micro Kjeldahl nitrogen analyzer. Kjeldahl nitrogen distillation device is very diverse, generally by the vapor generation, distillation of ammonia and ammonia absorption of three parts.
1, the instrument washing
Instrumentation before the installation, the components need to be washed by the general method of washing clean, the rubber tube used, the plug shall be immersed in 10% NaOH solution, boiled for about 10min, washed, boiled for 10min, and then washed several times, and then mounted and fixed in an iron frame on the table. Instruments before use, micro-all pipelines must be steam washed to remove possible residual ammonia in the pipeline, the instrument is being used, each time before the measurement of samples, steam washing 5min can be. If the instrument has not been used for a long time, repeat the steam washing, not less than three times, and check whether the instrument is normal. Check each connection carefully to ensure that there is no air leakage. First add about 2/3 volume of distilled water to the steam generator, add a few drops of sulfuric acid to keep it acidic to avoid the ammonia in the water being vaporized and affecting the results, and put in a little zeolite (or capillary tube, etc.) to prevent the explosion from boiling. Along the wall of the small glass cup add distilled water about 20mL let the water flow into the reaction chamber through the cannula, but the water in the glass cup do not drain, plug the stick glass plug to keep the water seal to prevent leakage. Immediately after the steam occurs, close the switch on the waste liquid discharge tube so that the steam can only enter the reaction chamber, resulting in rapid boiling of the water in the reaction chamber, the vapor out of the steam from the upper port of the reaction chamber through the nitrogen-determining sphere into the condenser tube to cool it down, and at the lower end of the condenser tube, a conical flask is placed to receive the condensate. Starting from the nitrogen fixing ball hot start timing, continuous steaming for 5min, and then remove the gas lamp. After rinsing, clamp the connecting rubber tube between the vapor generator and the collector, due to the reduction of gas cooling pressure, the waste liquid in the reaction chamber is automatically pumped into the reaction chamber shell, and the waste liquid discharge outlet clamp is opened to release the waste liquid. So cleaning 2 ~ 3 times, and then in the condenser tube under a conical flask containing a mixture of boric acid - indicator so that the condenser tube under the mouth of the solution is completely submerged, distillation 1 ~ 2min, observe the conical flask of the solution whether or not the color change. If it does not change color, it means that the distillation device has been cleaned internally. Remove the conical flask, and then distilled for 1 ~ 2min, rinse the condenser with distilled water under the mouth, turn off the gas lamp, the instrument can be used for measuring samples.
2, inorganic nitrogen standard sample distillation absorption
Because the nitrogen determination operation is cumbersome, in order to familiarize with the operation of distillation and titration techniques, beginners should be used to repeat the practice of inorganic nitrogen standard samples, and then for the determination of organic nitrogen samples of unknown. Commonly known concentration of standard ammonium sulfate test three times. Take five clean 100mL conical flasks, add 2% boric acid solution 20mL, hypomethyl blue-methyl red indicator (violet-red) 3 to 4 drops, cover the mouth of the bottle for use. Take one of the conical flasks to be connected to the lower end of the condenser tube, and make the outlet of the condenser tube submerged in the solution. Note: The collector piston must be opened before this operation to avoid backsiphoning of liquid from the conical flask. Accurately aspirate 2mL of ammonium sulfate standard solution into a glass cup, carefully lift the rod stopper so that the ammonium sulfate solution slowly flows into the distillation flask, rinse the small glass cup with a small amount of distilled water for 3 times, and put it into the distillation flask. Then use a measuring cylinder to add 10 mL of 30% NaOH solution to the small glass, so that the alkali solution slowly flowed into the distillation flask, in the alkali solution has not yet completely flowed into the rod glass stopper tightly. Add about 5 mL of distilled water to the small glass cup, and then slowly open the glass stopper so that half of the water flows into the distillation flask and half remains in the small glass cup as a water seal. Close the collector piston and heat the vapor generator for distillation. The boric acid-indicator mixture in the conical flask changes from purplish red to green due to absorption of ammonia. Since the color change, and then distilled for 3 to 5min, move the conical flask so that the liquid level in the flask away from the lower mouth of the condenser tube about lcm, and a small amount of distilled water to rinse the lower mouth of the condenser tube, and then continue to distill for 1min, remove the conical flask, cover it, ready for titration. After a distillation is completed, remove the gas lamp, clamp the rubber tube between the vapor generator and the collector, exclude the waste liquid of the reaction is completed, rinse the small glass cup with water several times, and exclude the waste liquid. So repeatedly rinse clean, you can carry out the next sample distillation. Do it twice more with standard ammonium sulfate according to the above method. Another 2mL of distilled water instead of standard ammonium sulfate for the blank determination of the second time. Titrate the conical flask of each distillation together.
3, unknown samples and blank distillation absorption
Three digested protein samples, three blank control solution, in turn for distillation absorption. Add 5mL of hot distilled water to the digested samples or blank control solution, through the small glass added to the reaction chamber, and then wash the small glass with hot distilled water three times, each time the amount of water is about 3mL, and the washing solution was poured into the reaction chamber. The rest of the operation was carried out according to the distillation of standard ammonium sulfate. Due to the high concentration of potassium sulfate in the digestive solution was viscous, it is not easy to pour out of the Kjeldahl flask, it must be added to the hot distilled water 5 mL diluted, if there are crystals precipitated, it must be dissolved in microthermal heat, and added to the glass cup while it is still hot to make it flow into the reaction chamber. In addition, care should be taken to add the sample or blank control solution immediately while the instrument wash is not yet completely cooled, otherwise the digestive solution will easily precipitate crystals through the cooled pipeline, resulting in clogging. After the sample and blank have been distilled, titrate them together. Open the cap of the receiving bottle and titrate with 0.0100 mol/L standard hydrochloric acid solution using an acid microburette. When the solution in the bottle is dark gray, the inner wall of the conical flask is rinsed once around with distilled water. If the shaking resurfaced green, should be careful to drop into the standard hydrochloric acid solution half a drop, shaking to observe the color change of the solution in the bottle, dark gray in one or two minutes unchanged, when considered to reach the end point of the titration. If pink, indicating that the titration has exceeded the end point, can be titrated in the amount of standard hydrochloric acid solution used to subtract 0.02mL, each group of samples nitrogen end point color must be completely consistent. The color of the solution in the blank control solution receiving bottle is unchanged or slightly changed not yet appeared green, can not be titrated. Record the number of milliliters of standard hydrochloric acid solution consumed for each titration for calculation.