separated ions are then detected using a conductivity detector and the peak areas reported are directly related to the concentrations. The peak areas of the individual anions (i.e. chloride) are compared to standard solutions and the concentration in the actual sample calculated. Examples of a typical anion chromatogram and standard are shown on the previous page. The determination of the cations (i.e. sodium) is done separately using the same instrument but with a different eluent and column system.


Determination of cations using ICPES or AAS


These two analytical techniques will determine the concentrations of various metals/cations, such as sodium, calcium, magnesium, potassium, strontium etc in an aqueous solution. The two techniques can be described as follows:


●


AAS is an analytical technique in which the sample to be analysed is converted into an atomic vapour by spraying the sample or solution of the sample into a flame. The absorbence at a selected wavelength for each individual metal/cation is measured and compared to the absorbence measurements of the sample and standard.


●


ICPES is an analytical technique where the sample solution is introduced into an inductively coupled argon plasma at a temperature of approximately 8000°C. At this temperature the individual elements (cations) become thermally excited and emit light at their characteristic wavelengths. This light is collected by the spectrometer and amplified to yield an intensity measurement that can be converted to an elemental concentration by comparison with known standards.


Dry cargoes (spot test)


As noted before, for many dry cargoes such as steel, for example, an indication of seawater contamination can be achieved on site by the ‘silver nitrate spot test’.


The basis of the test is that addition of acidified silver nitrate to any solution containing chlorides will produce a white precipitate of insoluble silver chloride. The chloride content of most seawaters would be expected to be around the 20,000 ppm level. Therefore, the addition of acidified silver nitrate to seawater will produce a dense white precipitate making the solution appear ‘milky’. The test is performed by the attending surveyor by adding a small amount of acidified silver nitrate to any contaminating water found on the dry cargo. If the resulting solution appears ‘milky’ then it is possible that seawater contamination has occurred. However, as stated previously, to confirm the presence


28


of seawater further laboratory analysis will be required. An alternative to the silver nitrate spot test is to take swabs of the contaminating water, which should then be placed in a suitable airtight container and submitted to the laboratory for further analysis.


It is important to note that certain other water sources, such as brackish or river water, will contain chlorides and would also produce a ‘milky’ solution with the addition of acidified silver nitrate. Even domestic tap water contains enough chlorides to give a slightly ‘milky’ solution when tested with acidified silver nitrate.


The silver nitrate spot test should be applied to galvanised goods with caution as it gives a black precipitate due to interference from the zinc present. In addition, the silver nitrate spot test can be misleading when applied to certain types of packaging, leather goods etc, which may, themselves, contain chlorides. Control samples of uncontaminated cargo should, where possible, always be obtained and/or tested in situ.


Liquid petroleum cargoes (laboratory test)


Unlike dry cargoes, any water contamination of a liquid petroleum cargo will become either suspended within the cargo and/or percolate through and settle on the tank bottoms. The nature of the petroleum cargo will dictate whether the contaminating water will settle to the bottom of the cargo tanks. For example, a viscous fuel oil cargo is more likely to retain the water in suspension than a gasoline or kerosene cargo.


Unfortunately, water contamination of a petroleum cargo will almost certainly not be spread homogeneously throughout the entire cargo, which makes representative sampling extremely difficult.


However, the identification of the type of water present can be achieved using either the AAS or ICPES techniques described previously. Prior to the use of either of these techniques, the petroleum/water sample will have to be ignited and burnt until only an ash remains. This ash is then dissolved in either an acid or a flux and an aqueous solution is prepared, which is then examined using either AAS or ICPES to determine the concentrations of the metals/cations present. The analysis of samples taken from various stages of the voyage (i.e. loadport shore tanks, after loading and before discharge) can be determined. In conjunction with the actual water content, it is then possible to produce a ‘cation fingerprint’ of the water present in the samples taken at various stages of the voyage, which can then be used to try and identify the source of the contamination.


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34