Quantitative detection of trace amounts of water in solids

Water is one of the most important raw materials in the production process. It can facilitate or even play a direct role in chemical reactions, and affect material properties such as mouldability and shelf life. For this reason, precise, quantitative data on the water content of solids is of major significance both in production and in the development of new products.

Thermogravimetric methods that utilise drying ovens or infrared or microwave dryers are practical only when the sample material does not contain volatile components besides water that contribute to weight loss in the sample upon evaporation and thus skew the measurement results. Moreover, such methods can only detect relatively high water contents.

One technique that is well suited to the quantitative detection of trace amounts of water - within the range of just a few parts per million (ppm) - is coulometric Karl Fischer titration, named for the coulomb, which is a unit of electric charge. The method, however, is limited by the complexity of the equipment required and the use of chemicals that are subject to strict disposal regulations.

The phosphorus pentoxide method is also based on the proven principle of coulometry, but does not call for the use of wet chemical reagents. This technique was developed about 25 years ago for the detection of trace amounts of water in gases. Today, it is increasingly used in moisture analysis in such instruments as the WDS 400 water detection system (Sartorius AG, Goettingen, Germany), shown in Figure 1.

Types of bonds

Technically, analysis by the water detection system (WDS) is comparable to coulometric Karl Fischer titration, but is performed in just a few steps. One advantage is that basic knowledge of chemistry is not required to operate the WDS 400. The operator need only ensure that the sample is weighed-in correctly. The preselection of suitable reagents required for titration is not necessary.

After weighing-in the sample on a microbalance or semimicrobalance, a nickel scoop is used to transfer the sample to the stainless steel oven in the system (Figure 2). Using the principle of thermoanalysis, the sample can be heated in stages at user-defined temperatures to allow a distinction between the various forms of water bond: surface water, capillary water, and the more tightly bound water of crystallisation. This method relies on physical forces such as van der Waals forces, hydrogen bonds, and dipolar and interactive electrostatic forces.

Selective detection of water

The water vapour produced during the heating stage is transferred to an electrolytic cell using N2 (Class 5.0) as the carrier gas (Figure 3). Alternatively, oil-free, dried compressed air of the same grade can be used as the carrier gas, which also reduces operating costs.

The electrolytic cell is an electrochemical sensor in which the selective detection of water by coulometry takes place. The sensor consists of a ceramic disk with a thin coating of phosphorus pentoxide between two parallel electrodes, applied using a special coating procedure. When the carrier gas enters the electrolytic cell, a chemical reaction occurs between the water carried by the gas and the extremely hygroscopic phosphorus pentoxide, resulting in the electrolytic dissociation of the water molecules. Each electrolysed water molecule causes two electrons to be displaced, which generates a measurable electric current.

The electrical charge required for the complete dissociation of water molecules is quantitatively measured, and then Faraday's law is applied (Figure 4) to convert it to the amount of water initially contained in the sample. Unlike the thermogravimetric measurement techniques, other substances that evaporate during heating are not detected.

A major advantage of the phosphorus pentoxide method is its wear-resistant electrolytic cell. Due to its catalytic property, the sensor regenerates itself during measurement and can be re-used soon after an analysis has been completed. Furthermore, this technology reduces operating costs by saving on expendable materials and their disposal.

Low detection limit

The measuring range spans from 15% water content down to just a few ppm, with a detection limit of approximately 100 ng of water. By virtue of its high-resolution sensor technology, the WDS 400 can be used to analyse sample quantities under 15 mg. This feature is particularly important in development projects in which sample materials are sometimes available only in minute amounts. The typical sample quantity for measurement with the system ranges from 25 to 2000 mg.

Users have the option of performing a tare measurement before beginning the actual analysis to measure the effect of ambient moisture on the measurement process. Disturbance factors such as surface moisture on the sample scoop or penetration of moisture when the oven door is opened are quantitatively detected and taken into consideration in the evaluation of results.

An interfaced PC allows control of all technical and operational sequences and convenient evaluation of the analysis (Figure 5). Assigning start and end markers for individual, quantified perspectives can fractionate the individual peaks recorded during measurements on each phase. This enables the user to make very precise statements about the drying characteristics, for example, at what temperature and with what level of intensity a particular sample releases capillary water or water of crystallisation. The analysis is recorded and displayed graphically on the PC monitor. In addition, the analysis includes a quantitative evaluation of the forms of water bond.

Flexibility

Another important feature of the system is the universal applicability of the electrolytic cell with respect to sample materials. Whether testing foods, chemicals, chemical products, or the active ingredients in pharmaceutical products, the electrolytic cell does not need to be changed.

Samples of widely varying composition can be analysed one after another in rapid series with no additional set-up time. Because the sample does not come into direct contact with the electrolytic cell, the possibility of chemical cross-reactions that could negatively affect the accuracy of results is virtually ruled out. Known restrictions on the type of materials that can be analysed include ammonium compounds, which reduce the sensitivity of the sensor, and compounds such as alcohols, which decompose into water molecules and thus increase the apparent water content of the sample. Table 1 provides an overview of several different products and the time required for their analysis.

As part of its Application Support program, the manufacturer is offering customers the opportunity to have a sample tested using the phosphorus pentoxide method to test its suitability for use with particular products.