1.1 Brief summary of methodologies 1.1.1 Titration procedures
Anionic and cationic surface active agents, aliphatic amines, nitriles, amines, nucleic acid, nitriloacetic acid, mercaptans, disulphides, humic acid and fulvic acids, and also bromine, chlorine, iodine and organic nitrogen have been determined in a variety of water samples by titration procedures. As titration procedures are relatively insensitive compared to some other procedures, it is likely that they would be applied only to those types of water sample where the concentration of the determinand is relatively high, e.g. sewage and trade effluents, and not, for example, to potable water samples.
The titration process has been automated so that batches of samples can be titrated non-manually and the data processed and reported via printouts and screens. One such instrument is the Metrohm 670 titroprocessor. This incorporates a built-in control unit and sample changer so that up to nine samples can be automatically titrated. The 670 titroprocessor offers incremental titrations with variable or constant volume steps (dynamic or mono tonic titration). The measured value transfer in these titrations is either drift controlled (equilibrium titration) or effected after a fixed waiting time; pK determinations and fixed end points (e.g. for specified standard procedures) are naturally included. End-point titrations can also be carried out.
Sixteen freely programmable computational formulae with assignment of the calculation parameters and units, mean-value calculations and arithmetic of one titration to another (via common variables) are available. Results can be calculated without any limitations.
The 670 titroprocessor can also be used to solve complex analytical tasks. In addition to various auxiliary functions, which can be freely programmed, up to four different titrations can be performed on a single sample.
In addition to the fully automated 670 system, Metrohm also supply simpler units with more limited facilities which nevertheless are suitable for more simple titrations. Thus the model 682 titroprocessor is recommended for routine titrations with automatic equivalence pointer cognition or to preset end points. The 686 titroprocessor is a lower-cost version of the above instrument, again with automatic equivalence point recognition and titration to preset end points.
Mettler produce two automatic titrators, the DL 40 GP memotitrator and the lower-cost DL 20 compact titrator. Features available on the DL 40 GP include absolute and relative end-point titrations, equivalence-point titrations, back-titration techniques, multimethod applications, dual titration, pH stating, automatic learn titrations, automatic determination of standard deviation and means, series titrations, correction to printer, acid balance analogue output for recorder and correction to the laboratory information system. Up to 40 freely definable methods can be handled and up to 20 reagents held in store. Six control principles can be invoked. The DL 20 can carry out absolute (not relative) end-point titrations and equivalence-point titrations, back-titration, series titrations, and correction to printer and balance and the laboratory information system. Only one freely definable method is available. Four control principles can be invoked.
The DL 40 GP can handle potentiometric, voltammetric or photometric titrations.
This technique has been applied to the following determinations in water, all of which are capable of undergoing an oxidation reduction (i.e. redox): polyaromatic hydrocarbons, aldehydes, amides, esters, lactams, quinones, anionic and non-ionic surface active agents, polychlorobiphenyls, chlorophenols, nitrophenols, ethylenediamine tetraacetic acid, nitriloacetic acid, hydroxymethylene sulphide salts, phosphorus-containing insecticides, humic acid, fulvic acids and organocompounds of lead, antimony and arsenic.
Differential pulse polarography has found limited applications (carboxylic acids and nitriloacetic acid), as has scanning voltammetry (chlorinated insecticides, polychlorobiphenols and organocadmium compounds).
Three basic techniques of polarography are of interest and the basic principles of these are outlined below.
Universal: differential pulse (DPN, DPI, DPR) In this technique a voltage pulse is superimposed on the voltage ramp during the last 40ms of controlled drop growth with the standard dropping mercury electrode; the drop surface is then constant. The pulse amplitude can be preselected. The current is measured by integration over a 20ms period immediately before the start of the pulse and again for 20ms as the pulse nears completion. The difference between the two current integrals (12 - is recorded and this gives a peak-shaped curve. If the pulse amplitude is increased, the peak current value is raised but the peak is broadened at the same time.
Classical direct current (DCT) In this direct current method, integration is performed over the last 20ms of the controlled drop growth (Tast procedure): during this time, the drop surface is constant in the case of the dropping mercury electrode. The resulting polarogram is step-shaped. Compared with classical DC polarography according to Heyrovsky, i.e. with the free-dropping mercury electrode, the DCT method offers great advantages: considerably shorter analysis times, no disturbance due to current oscillations, simpler evaluation and larger diffusion-controlled limiting current.
Rapid square wave (SQW) Five square-wave oscillations of frequency around 125Hz are superimposed on the voltage ramp during the last 40ms of controlled drop growth - with the dropping mercury electrode the drop surface is then constant. The oscillation amplitude can be preselected. Measurements are performed in the second, third and fourth square-wave oscillation; the current is integrated over 2ms at the end of the first and at the end of the second half of each oscillation. The three differences of the six integrals (11 - 12, 13 - 14, 15 - 16) are averaged arithmetically and recorded as one current value. The resulting polarogram is peak shaped.
Metrohm are leading suppliers of polarographic equipment. They supply three main pieces of equipment: the Metrohm 646 VA processor, the 647 VA stand (for single determinations) and the 675 VA sample changer for a series of determinations). Some features of the 646 VA processor are listed below:
• Optimized data acquisition and data processing
• High-grade electronics for a better signal-to-noise ratio
• Automatic curve evaluation as well as automated standard addition for greater accuracy and smaller standard deviation
• Large, non-volatile methods memory for the library of fully developed analytical procedures
• Connection of the 675 VA sample changer for greater sample throughout
• Connection of an electronic balance
• Simple, perfectly clear operation principle via guidance in the dialogue mode yet at the same time high application flexibility thanks to the visual display and alphanumeric keyboard
• Complete and convenient result recording with built-in thermal recorder/printer
The 675 VA sample changer is controlled by the 646 VA processor on which the user enters the few control commands necessary. The 646 VA processor also controls the 677 drive unit and the 683 pumps. With these auxiliary units, the instrument combination becomes a polarographic analysis station which can be used to carry out on-line measurements.
The 646 VA processor is conceived as a central, compact component for automated polarographic and voltammetric systems. Thus, two independent 647 VA stands or a 675 VA sample changer can be added. Up to four multidosimats of the 665 type for automated standard additions and/or addition of auxiliary solutions can be connected to each of these wet-chemical workstations. Connection of an electronic balance for direct transfer of data is also possible.
Program-controlled automatic switching and mixing of these three electrode configurations during a single analysis via software commands occur. The complete electrode is pneumatically controlled. A hermetically sealed mercury reservoir of only a few millilitres suffices for approximately 200 000 drops. The mercury drops are small and stable; consequently, there is a good signal-to-noise ratio. Mercury comes into contact only with the purest inert gas and plastic free of metal traces. Filling is seldom required and very simple to carry out. The system uses glass capillaries which can be exchanged simply and rapidly.
Up to 30 complete analytical methods (including all detailed information and instructions) can be filed in a non-volatile memory and called up. Consequently, a large extensive and correspondingly efficient library of analytical methods can be built up, comprehensive enough to carry out all routine determinations conveniently via call-up of a stored method.
The standard addition method (SAM) is the procedure generally employed to calculate the analyte content from the signal of the sample solution. The SAM is coupled directly to the determination of the sample solution so that all factors which influence the measurement remain constant. There would be no doubt that the SAM provides results that have proved to be accurate and precise in virtually every case.
The addition of standard solutions can be performed several times if need be (multiple standard addition) to raise the level of quality of the results still further.
Normally, a real sample solution contains the substances to be analysed in widely different concentrations. In a single multi-element analysis, however, all components must be determined simultaneously. The superiority of the facilities offered by segmented data acquisition in this respect is clear when a comparison is made with previous solutions. The analytical conditions were inevitably a compromise; no matter what type of analytical conditions were selected, such large differences could rarely be reconciled. In the recording, either the peaks of some of the components were shown meaningfully - each of the other two were either no longer recognizable - or led to gigantic signals with cut-off peak tips. And all too often the differences were still too large even within the two concentration ranges. Since the recorder sensitivity and also all other instrument and electrode functions could only be set and adjusted for a single substance, even automatic range switching of the recorder was of very little use.
The dilemma is solved with the 646VA processor: the freedom to divide the voltage sweep into substance-specific segments and to adjust all conditions individually and independently of one and another within these segments opens up quite new and, to date, unknown analytical possibilities. Furthermore, it allows optimum evaluation of the experimental data.
Various suppliers of polarographs are summarized in Table 1.1.
1.1.3 Spectrophotometric methods 22.214.171.124 Visible spectrometry
This technique is only of value when the identity of the compound to be determined is known. There are also limitations on the sensitivity that can be achieved, usually milligrams per litre or occasionally, micrograms per litre.
The following types of compounds have been determined in water samples: aliphatic and aromatic hydrocarbons, carboxylic acids, alcohols, aldehydes, carbohydrates, dioxans, phenols, anionic, cationic and non-ionic surface active agents, haloforms, chlorophenols, chlorinated isocyanurates, aliphatic and aromatic amines, nitro compounds, nitriles, amides, nucleic acids, nitriloacetic acid, benethionium salts, Thiram insecticide, carbamate insecticides and herbicides, chlorophyll and other plant pigments, organocompounds of arsenic, lead, mercury, tin,
Table 1.1 Suppliers of polarographs
Metro h m
RDT Analytical Ltd
EDT Analytical Ltd
Direct current Square wave
Direct current, normal pulse, differential pulse, 1st harmonic ac., 2nd harmonic ac„ Kalousek Direct current, sampled differential pulse
On-line voltammetric analyser for metals in effluents and field work Differential pulse anodic-stripping online voltammetric analyser for metals in effluents and field work
On-line voltammetric analyser for continuous measurement of metals in effluents and water Cyclic voltammetry, differential pulse voltammetry, linear scan voltammetry, square-wave voltammetry, singie-and double-step c h ro n opotentiometry and chronocoulometry
646 VA processor
647 VA stand
675 VA sample changer 665 Dosimat (motor driven piston burettes for standard additions) 506 Polarecord
2-IOfig L"' quoted for nitriloacetate
DC 626 Polarecord
Was this article helpful?
Thousands Have Used Chemicals To Improve Their Medical Condition. This Book Is one Of The Most Valuable Resources In The World When It Comes To Chemicals. Not All Chemicals Are Harmful For Your Body – Find Out Those That Helps To Maintain Your Health.