Content, water

Water content affects the proton conductivity and the mechanical strength of the PEM significantly. A PEM with a smaller EW can take in more water. The water content X, which is defined as the number of water molecules held by each -SO3H group, can be determined by the weight difference between dry and wet PEM  

The water content is determined by azeotropic distillation in the Dean-Stark apparatus.  

The water content [23] is expressed as the percentage of free, nonchemically bound water in the oligo-polyol. The water content is determined by the classical Karl-Fischer method, described, in detail, for oligo-polyols in ASTM D4672 [23] and ISO 14897 [24], As a general rule, the water content which is acceptable for the majority of oligo-polyols is between 0.05-0.1%. 

The water content of polyols can be determined by the Karl Fischer method which is based on the reduction of iodine by sulphur dioxide in the presence of water. The reaction occurs quantitatively only in the presence of pyridine and methanol, which both take part in the reaction. Standard Karl Fischer reagent consisting of iodine, pyridine and sulphur dioxide in ethylene glycol monomethyl ether is available commercially and it is recommended that this be used. The polyol is mixed with anhydrous methanol and the dissolved water titrated with the Karl Fischer reagent. The colour change from yellow to brown is not easy to determine accurately and potentio-metric titrations are common. 

In most cases the amount of water present is small and less than 0 5% by weight of the polyol. In such cases the accuracy of the method is such that a reproducibility of less than 0-01% absolute water content should be obtained (ASTM D1638). 

As already mentioned, the amount of water present is fundamental to the stability of confectionery products. Not surprisingly, the measuring 

In this system, a reagent prepared by reacting sulfur dioxide with iodine dissolved in pyridine and methanol is used  

Water determinations tend to work well on instrumental analysis probably because water is radically different from other substances. Methods such as NMR and near infra-red are both applied to confectionery products. 

Proton NMR is obviously likely to give an enormous range of signals from a typical confectionery product. For the analysis of water in confectionery, the NMR instrument used must be of low resolution whether it is of the original continuous form or of the later pulsed type. The aim of the exercise is to discriminate between the protons in water and those in other molecules. Fortunately, this is not too difficult. 

It might be expected that measuring the moisture content of sweets being dried is easy. This is obviously a useful control measure in a factory where gums or pastilles are being stoved (see also Chapter 10). The problem with this measurement is that the sweet is not homogeneous. It is entirely possible to have a dried sweet where the outside has a solids content of 92% but the middle with a solids content of 86%. Any technique that is surface-biased can produce any value between 92 and 86% on a cross-section of the same sweet. 

The amount of water present is fundamental to the stability of confectionery products. Unsurprisingly, therefore, measuring water content is an important exercise. Various methods are used. Some oven drying moisture content determinations are still carried out. This sort of work is difficult since the moisture contents are normally low and the samples can only be dried with difficulty. In particular, there are problems in drying the product in a reasonable time without charring it. Various other methods of water content determination are in use. One such is the Karl Fischer titration. 

Near-infrared Spectroscopy. Near-infrared spectroscopy (NIRS) uses that part of the electromagnetic spectrum between the visible and the infrared. This region has the advantage that the instrumentation is nearest to visible instrumentation. Signals in the near-infrared come not from the fundamental vibrations of molecules but from overtones. As 

Nuclear logging using gamma radiation, to measure the bulk density, and neutron radiation, to measure water content, are techniques used both at sea and ashore. Preiss (1968) and Richards and Chaney (1997) describe how these techniques may be used for marine sediments both on core samples and in situ. These in-situ methods are particularly apph-cable to near-surface sediments which can be extremely porous and tend to suffer the greatest amount of disturbance when sampled. It is difficult using these techniques to obtain accuracies better than 1%, due to the problems of cahbrating the instruments with specimens of different chemical and mineralogical compositions. 

Water content, or more correctly pore water content, is important in the calculation of most material phase relationships. Water content can be expressed in two ways as the ratio of pore water mass to total sample mass 

The term solid particles as used in geotechnical or sedimentological work is typically assumed to mean naturally occurring mineral particles that are not readily soluble in water. Therefore, the water content of materials containing extraneous matter (such as shells), water-soluble matter (such as salt), and highly organic matter typically require special treatment or a qualified definition of water content. 

A number of laboratory methods to dry soils have been developed of late. In the following discussion, two standard methods (mechanical convection oven and microwave) will be presented along with one nonstandard method (freeze drying). 

Early protein crystallographers, proceeding by analogy with studies of other crystalline substances, examined dried protein crystals and obtained no diffraction patterns. Thus X-ray diffraction did not appear to be a promising tool for analyzing proteins. In 1934, J. D. Bernal and Dorothy Crowfoot (later Hodgkin) measured diffraction from pepsin crystals still in the mother liquor. Bernal and Crowfoot recorded sharp diffraction patterns, with reflections out to distances in reciprocal space that correspond in real space to the distances between atoms. The announcement of their success was, in effect, a birth announcement for protein crystallography. 

Careful analysis of electron-density maps usually reveals many ordered water molecules on the surface of crystalline proteins (Plate 4). Additional disordered water is presumed to occupy regions of low density between the ordered particles. The quantity of water varies among proteins and even among different crystal forms of the same protein. The number of detectable ordered water molecules averages about one per amino-acid residue in the protein. Both the ordered and disordered water are essential to crystal integrity, and drying destroys the crystal structure. For this reason, protein crystals are subjected to X-ray analysis in a very humid atmosphere or in a solution that will not dissolve them, such as the mother liquor. 

NMR analysis of protein structure suggests that the ordered water molecules seen by X-ray diffraction on protein surfaces have very short residence times in solution. Thus most of these molecules may be of little importance to an understanding of protein function. However, ordered water is of great importance to the crystallographer. As the structure determination progresses, ordered water becomes visible in the electron-density map. For example, in Plate 2, water molecules are implied by small regions of disconnected density. 

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The tendency to separate is expressed most often by the cloud point, the temperature at which the fuei-alcohol mixture loses its clarity, the first symptom of insolubility. Figure 5.17 gives an example of how the cloud-point temperature changes with the water content for different mixtures of gasoline and methanol. It appears that for a total water content of 500 ppm, that which can be easily observed considering the hydroscopic character of methanol, instability arrives when the temperature approaches 0°C. This situation is unacceptable and is the reason that incorporating methanol in a fuel implies that it be accompanied by a cosolvent. One of the most effective in this domain is tertiary butyl alcohol, TBA. Thus a mixture of 3% methanol and 2% TBA has been used for several years in Germany without noticeable incident. 

Water content No decanted water Non-detectable by cobalt bromide test (NF M 41-004) Pass valve freezing test (ASTM D 2713, ISO 13758)... 

Water content Karl Fisher (NF T 60-154) or Azeotropic distiilation (NFT 60113)... 

The water content of crude oils is determined by a standardized method whose procedure is to cause the water to form an azeotrope with an aromatic (generally industrial xylene). Brought to ambient temperature, this azeotrope separates into two phases water and xylene. The volume of water is then measured and compared with the total volume of treated crude. 

The water content of crude oils at the wellhead is usually small as shown in Table 8.12 it generally increases during transport and storage and can attain 3%. 

Crude oil name Country of origin Water content in volume %... 

The water and sediment contents of crude oils is measured according to the standard methods NF M 07-020, ASTM D 96 and D 1796, which determine the volume of water and sediments separated from the crude by centrifuging in the presence of a solvent (toluene) and of a demulsifylng agent Table 8.13 gives the bottom sediment and water content of a few crude oils. 

Table 8.13 Bottom sediments and water content of some crude oils. i 1 i ...

Water (content) NFT 60-154 ISO/DIS 6296 ASTM D 1744 Karl Fischer Method (electrometric, alter addition of KF reagent)... 

Produced water has to be separated from oil for two main reasons, firstly because the customer is buying oil not water, and secondly to minimise costs associated with evacuation (e.g., volume pumped, corrosion protection for pipelines). A water content of less than 0.5% is a typical specification for sales crude. 

A knock out vessel may on the other hand be followed by a variety of dehydrating systems depending upon the space available and the characteristics of the mixture. On land a continuous dehydration tank such as a wash tank may be employed. In this type of vessel crude oil enters the tank via an inlet spreader and water droplets fall out of the oil as it rises to the top of the tank. Such devices can reduce the water content to less than 2%. 

To ensure disposal water quality is in line with regulatory requirements (usually 40 ppm), the oil content in water is monitored by solvent extraction and infrared spectroscopy. The specification of 40 ppm refers to an oil in water content typically averaged over a one month period. 

Increasingly, dielectric measurements are being used to characterize the water content of emulsions. One model for the dielectric constant of a suspension, ... 

In a substance such as a salt hydrate (for example BaCl2.2H20) water can be determined by heating until it is all driven off. Provided that only water is evolved on heating, the difference in weight gives the water content. If water is mixed with other decomposition... 

The hydration shell is formed with the increasing of the water content of the sample and the NA transforms from the unordered to A- and then to B form, in the case of DNA and DNA-like polynucleotides and salt concentrations similar to in vivo conditions. The reverse process, dehydration of NA, results in the reverse conformational transitions but they take place at the values of relative humidity (r.h.) less than the forward direction [12]. Thus, there is a conformational hysteresis over the hydration-dehydration loop. The adsorption isotherms of the NAs, i.e. the plots of the number of the adsorbed water molecules versus the r.h. of the sample at constant temperature, also demonstrate the hysteresis phenomena [13]. The hysteresis is i( producible and its value does not decrease for at least a week. 

Taking into account the hydration shell of the NA and the possibility of the water content changing we are forced to consider the water -I- nucleic acid as an open system. In the present study a phenomenological model taking into account the interdependence of hydration and the NA conformation transition processes is offered. In accordance with the algorithm described above we consider two types of the basic processes in the system and thus two time intervals the water adsorption and the conformational transitions of the NA, times of the conformational transitions being much more greater... 

Wetzel, R., Zirwer, D., Becker, M. Optical anisotropy of oriented deoxyribonucleic acid films of diffi-rcrit water content. Biopolymers 8 (1969) 391-401... 

An important application of the critical solution temperature is to the determination of the water content in such substances as methyl and ethyl alcohols. Here the system is usually the alcohol and a hydro carbon, such as -hexane or dicyclohexyl the water is, of course, insoluble in the hydrocarbon. Thus, the methyl alcohol - cyclohexane system has a C.S.T. of 45 -5° and even 0 01 per cent, of water produces a rise of 0-15° in the C.S.T. The experimental details are given below. 

For ethyl alcohol, two volumes of dicycZohexyl are mixed with one volume of the alcohol, a thermometer is introduced, and the mixture heated until it becomes clear. The solution is then slowly cooled, with constant stirring, and the temperature is determined at which the opalescent solution suddenly becomes turbid so that the immersed portion of the mercury thread of the thermometer is no longer clearly visible. This is the C.S.T. The water content may then be evaluated by reference to the following table. 

As a rough approximation it may be assumed that one mg. of water contained in one litre of gas at 25-30° exerts a 1 mm. partial vapour pressure. Obviously, the lower the residual water content or the vapour pressure, the more intense is the ultimate drying capacity of the substance. 

Reaction (1) usually proceeds readily provided the magnesium is activated with iodine and the water content does not exceed one per cent. Subsequent interaction between the magnesium ethoxide and water gives the highly insoluble magnesium hydroxide only a slight excess of magnesium is therefore necessary. 

Absolute methyl alcohol. The synthetic methanol now available is suitable for most purposes without purification indeed, some manufacturers claim a purity of 99 85 per cent, with not more than 0 1 per cent, by weight of water and not more than 0 02 per cent, by weight of acetone. Frequently, however, the acetone content may be as high as 0 1 per cent, and the water content 0-5-1 per cent. 

Acetone. The analytical reagent grade contains about 1 per cent, of water and meets most requirements. Synthetic acetone of a high degree of purity (water content about 1 per cent.) is also available as a technical product. 

If the approximate water content of commercial glycerol is known, the above dehydration may be avoided by adding sufficient SO, in the form of oleum to the concentrated sulphuric acid employed in the Skraup reaction to combine with all the water present. 

There are some alternatives to this HCI generator type of crystallization. There are, of course, canisters of HCI gas that can be purchased. Also, one can crystallize with very concentrated (fuming) HCI by pouring the stuff directly into the ether/freebase [26]. Regular 35% HCI can do this too, but the water content may dissolve the MDA.HCl or make the crystals sticky which means that the chemist will have to dry the solution by removing the water. 

The amount of metal required gives an indication of the water content. note 3. If the conversion takes longer, add some liquid ammonia to keep the volume of the suspension between 500 and 800 ml. iinte 4. The conversion of lithium and potassium into the alkali amides has never given problems. 

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