Biochemistry and biology (001-050)

The NIO electrode is characterized by the fact that it is selective for both sodium tetraphenylborate (= STPB) and oleophilic molecules. This means that it can be used successfully for the potentiometric indication of titrations in aqueous solution in which STPB is used as the titrant and the species to be titrated has oleophilic properties. This results in ideal S-shaped titration curves.

This Bulletin describes the determination of active pharmaceutical ingredients in raw materials and formulations (tablets, powders, gels, creams, syrups, drops). These titrations are a valuable, environment-friendly alternative to other analytical methods.

Non-aqueous titrations, particularly those in nonpolar solvents with a low conductivity, are strongly affected by electrostatic charges to the extent that the evaluation of the titration curves may be extremely difficult or even impossible. With the 6.2129.000* Differential Amplifier Metrohm has created a useful tool with which easily evaluable titration curves can be obtained even under difficult conditions.

* In Titrinos the differential amplifier is already built in.

Molybdenum is an essential trace element for plant growth. Since it occurs in natural waters only in trace amounts, a very sensitive method of determination is needed. With the aid of the following polarographic method it is possible to determine 5x10^-10 mol/L resp. 50 ng/L.

The principle of the method is based on the reaction between the molybdate ion MoO₄^2- and the complexing agent 8-hydroxy-7-iodo-quinoline-5-sulfonic acid (H₂L) to form an MoO₂L₂^2- complex, which is adsorbed on the mercury electrode. The adsorbed Mo(VI) is reduced electrochemically to the Mo(V) complex. The hydrogen ions present in the solution oxidize the Mo(V) again, giving rise to the Mo(VI) complex, which is thus newly available for electrochemical reduction. This catalytic reaction is the reason for the high sensitivity of the method.

This Bulletin describes three potentiometric and one photometric titration method for the determination of sulfate. Which indication method is the most suitable depends above all on the sample matrix and is illustrated with examples.

Method 1: Precipitation as barium sulfate and back-titration of the Ba²⁺ excess with EGTA. The ion-selective calcium electrode is used as indicator electrode.
Method 2: As in method 1, but with the electrode combination tungsten/platinum.
Method 3: Precipitation titration in semi-aqueous solution with lead perchlorate using the ion-selective lead electrode as indicator electrode.
Method 4: Photometric titration with barium perchlorate, thorin indicator and the 662 Photometer or 525 nm Spectrode. Particularly suitable for micro determinations!

Although the known photometric methods for the determination of ammonia are accurate, they require a considerable amount of time (Nessler method 30 min, indophenol method 90 min reaction time). A further disadvantage of these methods is that only clear solutions can be processed. Cloudy solutions must first be clarified by time-consuming procedures. These problems do not exist with the ion-selective ammonia electrode. Measurements can be easily performed in drinking water, groundwater and surface water as well as in wastewater, soil extracts and Kjeldahl digestion solutions (without distillation). The determination of ammonia in ammonia salts, of the nitric acid content in nitrates and of the nitrogen content of organic compounds is based on the principle that the ammonia ion is released as ammonia gas upon addition of excess sodium hydroxide:

NH₄⁺ + OH^- -> NH₃ + H₂O

The outer membrane of the electrode allows the ammonia to diffuse through it. The change in the pH value of the inner electrolyte solution is monitored by a combined glass electrode. If the substance to be measured is not present in the form of an ammonia salt, it must first be converted into one. Organic nitrogen compounds, especially amino compounds, are digested according to Kjeldahl by heating with concentrated sulfuric acid. The carbon is oxidized to carbon dioxide in the process while the organic nitrogen is transformed quantitatively into ammonium sulfate. Numerous examples are given. In the appendix various sample preparation methods and an example of a Kjeldahl digestion are described.

Very sensitive methods of determining iron and manganese are described. They are primarily suitable for the investigation of ground, drinking and surface waters, in which the concentration of these metals is important. The methods can naturally also be used for trace analysis in other matrices.

Iron is determined as its catechol complex by AdSV. The limit of determination lies at ρ(Fe) = 5 μg/L.

Manganese is determined in an alkaline borate buffer by the ASV method. Interference by intermetallic compounds is prevented by the addition of zinc ions to the sample. The limit of determination lies at ρ(Mn) = 2 μg/L.

Ascorbic acid as well as its salts and esters can be determined by titration with 2,6-dichlorophenolindophenol (DPIP) or by polarography, with the ascorbic acid being oxidized to dehydroascorbic acid:

C₆H₈O₆ - 2 H⁺ – 2 e > C₆H₆O₆

Ascorbic acid -> Dehydroascorbic acid

If 2,6-dichlorophenolindophenol is used as the titrant it is reduced by ascorbic acid:

C₁₂H₇Cl₂NO₂ + 2 H⁺ + 2 e -> C₁₂H₉Cl₂NO₂

In the titrimetric determination a bi-voltametric or photometric endpoint indication can be used. However, please remember that only the bi-voltametric indication is independent of the self-coloration of the sample.

Polarography is the most selective of the methods descibed here, as other reducing or oxidizing substances are not determined.