Water chemistry analysis

Water analyzes are used to determine chemical, physical and microbiological parameters that describe the nature of the particular sample. Such analyzes are carried out for example in the assessment of ground water, medicinal water, spring water and the like. Of particular importance are water analysis when drinking or raw water. Here are legally required number and scope of analysis in many countries, in Germany by the German Drinking Water Ordinance. Another important application is fishing.

Most often water is examined for chloride, sulfate, nitrate, nitrite, ammonium, pH, oxygen content, electrical conductivity, phosphate content and the water hardness. Frequently investigated parameters are also the organic load, ie TOC, chemical oxygen demand, or the oxidizability by potassium permanganate and the Biological Oxygen Demand.

Sampling

The type of sampling of water depends in general on the requirements of the sample, ie on the parameters to be determined dependent. Some of them have to be determined during or at least immediately after sampling, since the values ​​would change the course of storage. These include temperature, pH, oxygen content, as well as odor, turbidity and the like. In spring waters may churning sediment, but also on the surface floating particles do not get into the sample vessel. At the same time it should be avoided to retain or reduce the source water level.

When vessels are clean colorless glass or polyethylene bottles to be several times rinsed prior to analysis with the water to be tested. For transport, the bottles are sealed with plugs or screw caps of the same material. If organic loads such as PAHs (polycyclic aromatic hydrocarbons) are studied, including metal containers are suitable, where chemical reactions between the bottle and the water sample must be excluded.

The removal of the sample begins when the water is not affected by external influences, and can therefore be considered as representative of the water body to be examined. In general, this state is reached when the electrical conductivity is constant.

In areas of drinking water analysis so-called stagnation samples were taken for determination of heavy metal pollution. Here the line is first rinsed for about 5 minutes at the tap turned on. Thereafter, no further withdrawal of water, ideally throughout the building. After a waiting time of an average of 4 hours, the samples of a given water connection be removed. The first fraction 1L is in the analysis of information on the fixtures released by the heavy metals. Here are found usually increased nickel and lead concentrations. In the following second 1L fraction of the content of heavy metals is investigated, which are delivered through water pipes into the drinking water. Due to the stagnation of the water has the opportunity to react with the metals. The concentration of heavy metals is here information about the state of corrosion and material properties of the fittings and pipes used. By this method, for example, examined whether lead installations are laid without having to perform wall openings or cellar visits.

Field analysis

Organoleptic and physico-chemical parameters are determined locally. To include organoleptic odor, color, turbidity, sediment, and for samples with drinking water quality of the taste. The physicochemical parameters of water temperature, pH, oxygen content and redox potential, and the total gas saturation can only be measured in situ correct, because they depend not only on the ingredients of the sample, but also on the environment. So if it is at all possible, these values ​​should be determined on site at the time of sampling. In addition to these parameters, the determination of carbonic acid must (especially required for this purpose pH measurement) as soon as possible after sampling done.

Water temperature, electrical conductivity and pH

These three parameters are determined for example using a multimeter. To the respective measurement sensors are mounted and calibrated prior to measurement, where appropriate, on the device.

The electrical conductivity or specific indicates how much electricity is transported in a solution by anions and cations is thus a measure of the ions dissolved in the water. The measurement is performed by means of a normalized conductivity detector and a conductivity meter, and is reported in units of ĩS / cm. As a rule of thumb one can say that 90 ĩS / cm (at 25 ° C) of a carbonate hardness of 1 meq / l ( 2.8 ° dH ), where back usually go about 80 -60 ĩS / cm to other ions. Otherwise, the measured value is a measure of the burden of dissolved salts. For surface waters, often considered a value between 500 and 1000 ĩS / cm. Higher values ​​indicate an excessive burden (eg sodium chloride).

The pH - value is defined as the negative decadic logarithm of the activity (in mol / l) of H ions and is a measure of the acidity or alkalinity of solutions. If the pH value determines electrometrically, this means that using a combination electrode of a pH meter measured a potential difference and it automatically becomes the pH is calculated. Before the measurement, the device must be defined by means of buffer solutions pH values ​​(eg, 4, 7, 9 ) to be calibrated. The pH should be in natural waters around 7. Smaller values ​​(eg 3-6 ) indicate an exposure to acids, higher values ​​(more than 8 ) to a load with bases out.

Analysis of chloride ion

Normal surface water and groundwater contain 10-30 mg Cl / l The value can be much greater near coasts. Also near salt deposits of chloride content may be sharply increased. Likewise, have a higher chloride content by the potash industry there the wastewaters and thus the rivers. A chloride content greater than 250 mg Cl / l gives the water a salt-like taste. The limit for the edibility is about 400 mg Cl / l, but this content is physiologically harmless. A high chloride content also promotes the corrosion of iron pipes and fittings. For domestic water here is to think of a desalination. Usually the chloride is sodium chloride zurückzuführen.400 mg Cl / l correspond to about 660 mg / L of dissolved sodium chloride.

The determination of chloride ion can be carried out by the method of titrimetry argentometry. For small amounts of chloride of mercury (II ) nitrate method is in question: Here, the water sample (NO3 ) 2 solution of concentration 0.01 mol / l and the indicator is titrated with a Hg diphenylcarbazone. This forms the chloride ions undissoziertes mercury (II ) chloride. At the end point of the titration of the indicator from the Hg ( NO3 ) 2 solution reacts with excess Hg2 ions to a violet colored complex compound.

Analysis of sulfates

Normal water have a sulfate content of up to 50 mg SO42 - / l. In certain areas (eg brown coal fields ) may be substantially higher the sulphate content. Likewise, polluted waters have a significantly higher sulfate content. The sulfate content has essentially its cause in the dissolved gypsum. Sulfate -rich water is harmful for concrete structures. Water with a high sulfate content ( greater than 250 mg / l ), in particular due to sodium sulfate, has a laxative effect. Sulfate is the main component of the non-carbonate.

According to the following method, sulfate ions can be determined with sufficient accuracy: Here, the turbidity after the addition of barium chloride solution ( precipitation of barium sulfate) is determined photometrically. If no photometer is available, can also be a visual determination, but with a larger uncertainty occur. For comparison, falls from a barium sulfate solution of known concentration.

For water with a sulfate content greater than 100 mg / l, provides the gravimetric method after precipitation as barium sulfate.

Titrimetric can sulfates with Bariumperchloratlösung and determine Thorin as indicator: When the titration as long as barium sulfate as sulfate ions are present forms. At the end point Thorin reacts with excess barium ions to form a red-orange complex compound. First, however, metal cations must be removed using an ion exchanger, because the latter would react with Thorin.

Analysis of oxygen

Surface water contains oxygen often to saturation ( 9.2 mg / l at 20 oC, 14.5 mg / l at 0 oC ). It can also come by plankton activity for oversaturation occasionally. On the other hand, the oxygen may decrease by anaerobic processes. For fish at least an oxygen content of 5 mg / l should be present. Pure oxygen-free groundwater is usually.

The determination is carried out according to the method of Winkler or electrochemically with an oxygen electrode.

Analysis of bicarbonate

For the determination of the bicarbonate ion (HCO3- ), the acid capacity must be determined in the first rule. The acid capacity ( KS4, 3 ​​or alkalinity " SBV " ) is defined as the ratio of the amount of hydronium ions n ( H3O ), which can accommodate a corresponding amount of water until the pH of the volume V ( H2O). As a unit different from the SI unit [mol / m³] is usually chosen [ mmol / l] or as an equivalent amount of [ meq / l].

Determining the acid capacity of the hydrochloric acid concentration of 0.1 mol / l in a particular volume of sample, usually 100 ml, titrated after it was treated with a few drops of Cooper indicator. The change in color from steel blue to yellow sun occurs when the fixed pH of 4.3. Conventionally, as an indicator methyl orange used, which also turns at pH 4.3. Therefore this acid capacity was referred to as "m - value ". On this basis the acid capacity is also referred to as carbonate hardness and converted by a factor of 2.8 meq / l in " German hardness degrees " (° dH).

The calculation of the acid capacity using the formula

KS4, 3 ​​[mol / l] = V [ ml] · c (hydrochloric acid) [mol / l ] / sample volume [ml ]

From the amount of the consumed amount of hydrochloric acid, the concentration of hydrogen is calculated in the terminal. For this purpose, one must first be removed amount of hydrochloric acid that is responsible for the pH - value of 4.3, that is 0.05 mmol / l To calculate the formula used

HCO3- [mg / l] = KS4, 3 x 61.017 x 1000 calculated, where 61.017 is the molar mass of bicarbonate (HCO3-).

Alternatively, the concentration of the bicarbonate also be determined by ion - HPLC or capillary electrophoresis.

Analysis of free carbonic acid

The determination of total free carbon dioxide (CO2) a water sample by titration to a pH value of 8.2 [ KB8.2 ], the colorless to pink visible, for example, by a color change of phenolphthalein ( indicator This method is very inaccurate; better pH meter ). At this point, the previously free carbonic acid is converted according to the dissociation of carbonic acid completely in bicarbonate (HCO3-).

In the water technology a positive or a negative p- value in meq / l or mmol / l for the concentrations of carbonic acid and carbonate ( or bases ) is indicated, depending on whether you look at the lower pH 8.2 by or of higher initial pH values ​​her ( and accordingly with alkali or acid as Titrand ) must converge. ( The "p " is derived from the indicator phenolphthalein. ) A negative p- value indicates the level of a water free of CO2, while a positive p- value of the content of carbonate (or OH - ions possibly from other free bases ) indicates.

More specifically, the analysis of the most low CO2 concentrations, however. Usually by an accurate determination of the acid capacity and the pH of the water sample, with subsequent calculation of the CO2 after the dissociation of carbonic acid The temperature and the electrical conductivity are to be considered as representative of the ionic strength.

Analysis in the treatment of water for industrial purposes

In contrast to the analysis of drinking water are just some of the parameters for the assessment of water of importance in the treatment of water for industrial purposes. The knowledge of the contents of trace elements in the raw water is usually unimportant.

For the design, control and monitoring of technical systems for de-acidification, decarbonization, softening or partial and full desalination only the values ​​for the various positive and negative m-and p- values ​​( named after the used indicator of methyl red / methyl orange and phenolphthalein), the total hardness, the content of silicon dioxide (SiO2) and the oxidizability ( potassium permanganate consumption) of water needed.

Note: In the technology is still largely the unit mEq / l instead of mmol / l, as can thereby be avoided precise knowledge of the valence of the ions. M- and p- values ​​are, therefore, often expressed in meq / l. In the summation of ions must either be calculated only with meq / l or mmol / l but then.

The following table shows details of the individual values ​​, their respective importance, and a calculation of the commonly used methods of analysis are listed below:

M - value

• Is the content of the carbonate to

• Meq / l or mmol / l
• Is measured directly

• Analysis of HCO3 and
• Analysis of CO2

- P- value

• Are the content of free carbon dioxide in

• Meq / l or mmol / l
• Is measured directly

• Analysis of CO2

Non-carbonate

• Distribution Total hardness in non- carbonate and

• Meq / l or mmol / l
• Total hardness minus m - value

• Total hardness ( sum of calcium and magnesium ions) is determined with EDTA
• M values ​​see above

- M- value

• Meq / l or mmol / l

• If not computed over Bestimmumg of - m- value from a water sample was passed through a strongly acidic cation exchanger

Sodium and potassium

• Determining the total alkalis

• Meq / l or mmol / l
• ( - M value plus m ) minus total hardness

• If not calculated by direct determination with flame photometer

KMnO4 value

• Is the content of the org. Substances (for example, humic acids )

• Mg KMnO4 / l
• Is measured directly

• Boiling the water sample with potassium permanganate, for more details see Manganometry.

SiO2 - value

• Shows the content of compounds of silica as SiO2 at

• Mg / l
• Is measured directly

• SiO2 forms with ammonium heptamolybdate a yellow heteropoly acid formula H4 [ SiO4 ( Mo3O9 ) 4], which is evaluated photometrically.

With the given values ​​of the count of natural water for all the different ingredients is captured. The unlisted p- value indicates the amount of OH - ions in the water. This is for example the excess of calcium hydroxide in a decarbonated water. With the exception of the rare soft water with a sodium content, this value can only occur with chemically treated and not in a natural water.

The total amount of cations and anions in the respective water is calculated with these values ​​as follows:

• Sum of cations = m value plus - m- value
• Sum of anions = m value plus - p- value plus - m value plus SiO2 content

For the design and operation monitoring of partial and full desalination plants, the levels of carbonates and weak acid anions are calculated separately often still. This is also possible with the above values.

Knowledge of KMnO4 and SiO2 contents are important for the design and choice of ion exchange resins of a desalination plant.