ISOTOPES OF HYDROGEN AND OXYGEN BASIC INFORMATION AND TUTORIALS

The heaviest water must fall first. Aristotle

In ordinary natural water the hydrogen and oxygen each consist of three different isotopic forms with different masses; this fact is of some geochemical significance.

The isotopes present in water are:
1H, 2Hor D, 3Hor T, 16O, 17O, 18O:

The first isotope of hydrogen has a nucleus consisting of only a single proton, and accordingly the mass number is 1. The second isotope has both one proton and one neutron in the nucleus, and the mass number is 2; this isotope has a special name: “deuterium,” hence the common use of the letter “D” to represent the isotope.

The third isotope has one proton and two neutrons, a mass number of 3, and also has a special name: “tritium.” Tritium is radioactive.

The nuclei of oxygen have 8 protons, and 8, 9, or 10 neutrons. These isotopes are present in all possible combinations, so that (not including molecules with tritium) all natural water contains nine kinds of water molecules.

The ratios of the different isotopes, one to another, can be measured with remarkably high precision using an isotope ratio mass spectrometer.

The different isotopic forms of water each have different vapor pressures and freezing points; these physical differences are important because they make it possible to use the isotopes as tracers of geochemical processes.

The differences in vapor pressure lead to a fractionation of the isotopes whenever water evaporates or condenses. The heavier isotopes are, in every case, concentrated into the liquid phase. This leads to differing isotopic compositions in water from different sources.

When water freezes, there is also a fractionation such that the heavier isotopes are concentrated into the solid phase, although the effect is less than in the
gas–liquid exchange processes.

CYANIDES - DANGER OF CYANIDES BASIC INFORMATION AND TUTORIALS

What are the dangers of cyanide?

As a group, the cyanides are among the most toxic and fast-acting poisons. (This is due to the cyanide ion which interferes with cellular oxidation.)

Hydrogen cyanide (prussic acid) is a liquid with a boiling point of 26°C. Its vapour is flammable and extremely toxic. This material is a basic building block for the manufacture of a range of chemical products such as sodium, iron or potassium cyanide, methyl methacrylate, adiponitrile, triazines, chelates.

Toxic effects of hydrogen cyanide 
Concentration in air Effect (ppm)
2–5 Odour detectable by trained individual
10 (UK MEL 10 mg/m3 STEL (SK))
18–36 Slight symptoms after several hours
45–54 Tolerated for 3–60 min without immediate or late effects
100 Toxic amount of vapours can be absorbed through skin
110–135 Fatal after 30–60 min, or dangerous to life
135 Fatal after 30 min
181 Fatal after 10 min
270 Immediately fatal

Although organocyanides (alkyl cyanides, nitriles or carbonitriles), in which the cyanide group is covalently bonded, tend as a class to be less toxic than hydrogen cyanide, many are toxic in their own right by inhalation, ingestion or skin absorption. Some generate hydrogen cyanide under certain conditions, e.g. on thermal degradation.

Depending upon scale of operation, precautions for cyanides include:
• techniques to contain substances and avoid dust formation (solid cyanides), aerosol formation (aqueous solutions), and leakages (gas);
• gloves, face and hand protection;
• high standards of personal hygiene;
• ventilation and respiratory protection (dust or gaseous forms);
• environmental monitoring for routine processes;
• health surveillance.