Hello, all! We are using the Kjeldahl method to determine nitrogen content of fish samples. We do not have an automatic Kjeldahl analyzer, so we are using basic glassware. Regarding the second step of the Kjeldahl method, distillation: how long does this step typically take? How can we tell when our solution (now mixed with NaOH) has been distilled to completion?
Thank you in advance for your help!
You need to standardize this and once you standardize with specified level 9f distillate , repeat the other samples at similar level of distillate.....its a matter of your skill...
This depends on the N concentration, in most cases it takes from 5 to 10 min
Anoop's advice is correct. You need to follow his recommendation by distillation of NH3 standards of a range of relevant concentrations. May I also suggest that you also need to test the digestion efficiency by recovering N from TRIS base. TRIS is sold in high purity. TRIS requires about the same severity of digestion to recover the N as many plants and soil.
The amount of samples taken decided by the N content. One needs to standardize the time and known strength. Also can have a litmus test of the distillate. Skill vis-a-vis your expertise with time will also solve the issue.
The time varies with the type of distiller, temperature, among other factors. It is best to use a known sample, the TRIS sample was already suggested. Add it so that it has a quantity close to N of your samples and proceed with the process.
The titration conditions of TRIS should be the same as your sample!
At the end you will do the recovery calculation, use the time you have maximum recovery and standardize your samples with this time.
Normally for plants 4 min.
The time taken for the completion of distillation depends on nature of the sample to be tested. Since you are using fish sample which is organic in nature, it may take little bit more time than inorganic samples. Any way the completion of distillation can be known by placing wet red litmus paper at the end of distillation tube. If the red litmus paper changes its colour to blue, the distillation has to be continued for some more time. If there is no colour change you can finish the distillation process. However, the distillation may be finished within 30 minutes per sample.
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