My Own Definition

According to Wikipedia, Johan Kjeldahl was a Danish chemist internationally recognized for the invention of a simple yet clever laboratory technique in 1883. His technique, named after him, is of an immense value to scientists because it is one of the few ways to determines the nitrogen content in certain substances using basic principles of chemistry. Since there are many obvious applications, Kjeldahl's method is therefore widely used in various areas of science such as nutrition, geology, and forensic. For instance, it is important to know what kind of things that reside in water reservoir that we all drink from. Nitrogen, being one of such things, is dangerous in high concentration in the reservoir. However, there are several disadvantages that exist for this technique. The method is, in terms of usefulness, limited by the fact that it does measure the nonessential nitrogen compounds, giving the inaccurate picture of nitrogen purity. The milk manufacturers and their schemes were a part of notorious scandals that eventually made to the headlines in China because they took advantage of the fact that such technique is not completely foolproof. They basically added methonines, a nitrogenous amino acid, to the milks in order to mask their undesirable products so that these would be passed by the inspectors. Although there are other methods that measure the particular content, but Kjeldahl's method is often favored due to its relatively simple procedure and inexpensive tools required to perform.


Figure 1: Apparatus for Kjeldhal’s method

In this laboratory, the goal is to use the compounds experimentally made in previous lab weeks - [Co(NH3)5Cl]Cl2 and [Co(NH3)6]Cl3 - and determine their nitrogen contents using the Kjeldhal's method. There are two general procedures before we complete the works. First thing is to standardize the HCL. The purpose for doing so is to know the molarity of HCL used for Kjeldahl's method so that we can calculate the nitrogen content. Second thing is to learn the technique itself by using boric acid to react with two Cobalt coordination compounds by boiling for a long period of time. Zn is also used because it is a catalyst that speeds up the reaction, as well as bromocresol green indicator for determining whether the compound is ready for titration or not.


Figure 2: Top chemical equation is for standardizing, and
both two bottom equations are for Kjeldhal’s method

The chemical theory behind Kjeldahl's method is based on the nature of acid and base chemistry. When a compound is inheritably acidic, it means that it would lose a proton, often hydrogen, and become negatively charged, when the solvent reaches to a certain pH. The base is the compound that also loses the proton, but it would become neutral unlike acidic counterpart. To exploit this characteristic, the method utilizes the idea that nitrogen content comes in the form of NH4+, which is basic, in this laboratory. The form of NH4+ does not have to be always NH4+ in other cases, however. When a substance is identified to be littered with such nitrogen compound. The general procedure is three steps where it begins with decomposing the substance, breaking its bonds with nitrogen molecules apart, and ends with titrating by reacting with freed basic nitrogen molecules with an acid. The titration curve is a diagram that basically shows how much of the acids convert to their conjugate bases or vice versa upon adding the chemicals that neutralize. The most important part about titration curve is that it can and do tell you how much nitrogen content there is.


Figure 3: Equivalence point at curve is Ve.p

The simplified formula does exist in regard to the curve. This equation is to determine the mol of ammonia, NH4+. The reason is that it is indirect in measuring the nitrogen content because it is directly measuring the amount of HCL. Based on a chemical equation and its ratio, we infer the amount of nitrogen content by comparing the amount of HCL used in titration.


Figure 4: Top equation is for standardizing HCL, and
the bottom is for Kjeldhal’s method