Titrating soil

Because of the complex nature of soil and soil solutions, it is rarely possible to directly determine specific soil constituents by titrating soil or soil solutions using a pH meter, ion-specific electrode, or a platinum electrode (with appropriate reference electrode) [3],... 

Stevenson, F. J., and Chen, Y. (1991). Stability constants of copper(II)-humate complexes determined by modified potentiometric titration. Soil Sci. Soc. Am. I. 55,1586-1591. 

A primary goal of this chapter is to learn how to achieve control over the pH of solutions of acids, bases, and their salts. The control of pH is crucial for the ability of organisms—including ourselves—to survive, because even minor drifts from the optimum value of the pH can cause enzymes to change their shape and cease to function. The information in this chapter is used in industry to control the pH of reaction mixtures and to purify water. In agriculture it is used to maintain the soil at an optimal pH. In the laboratory it is used to interpret the change in pH of a solution during a titration, one of the most common quantitative analytical technique. It also helps us appreciate the basis of qualitative analysis, the identification of the substances and ions present in a sample. 

In many titrations, one solution—either the analyte or the titrant—contains a weak acid or base and the other solution contains a strong base or acid. For example, if we want to know the concentration of formic acid, the weak acid found in ant venom (1), we can titrate it with sodium hydroxide, a strong base. Alternatively, to find the concentration of ammonia, a weak base, in a soil sample, titrate it with hydrochloric acid, a strong acid. Weak acids are not normally titrated with weak bases, because the stoichiometric point is too difficult to locate. 

At the end of this period the solution was removed from the condenser while still hot and titrated immediately with 0.002500 N sodium thiosulfate before any appreciable oxygen could be absorbed and oxidize iodide ion to triiodide ion. The disappearance of the yellow color of triiodide ion against a white background was used for the end point. These solutions usually had a slight brown tint at the end point, which was assumed to be organic matter distilled over from the soil. Accordingly, the blank was usually titrated first and its final color was used as a standard end point color for the other three solutions run with it. 

Stevenson F.J. Nature of divalent transition metal complexes of humic acids as revealed by a modified potentiometric titration method. Soil Sci 1977 123 10-17. 

Potentially mineralizable C and N are often measured by incubating a sample of field-moist soil at a known temperature in a sealed chamber containing an alkali trap. The C02-C accumulated in the trap is measured by acid titration and this represents the quantity of C mineralized. Alternatively, C02 in the headspace of the incubation chamber can be measured using a C02 analyser. The amount of N mineralized during incubation is calculated as the difference in extractable NH4+ - andNCV-N measured in the soil before and after incubation. Mineralizable N can also be measured in an open incubation system where the soil is leached periodically and NH4+- andNCV-N in leachates is measured (Stanford 1982). 

Double-cell system, in soil and ground water treatment, 25 835 Double crucible technique, 11 135-136 Double-deck kiln, 15 530 Double-drum dryers, 9 134 Double-drum separator, 15 446 Double-effect distillation, 10 153 Double end point titration method,... 

Soil solutions can also be titrated to obtain information about both their pH status and their buffering capacity. Chapters 9 and 10 give a more detailed discussion of electrical and titration methods applied to soil. 

Titration is a general word used in many different disciplines. Any time a solution of known concentration is used to find the amount of an unknown component in another solution, it can be called a titration. Although this type of analysis is very old, it still finds widespread used in chemical analysis. Titrations are used in soil analysis to measure soil acidity, soil organic matter content, and various constituents isolated from soil, particularly ammonia. 

Soil and soil suspensions are colored and hard to see through. Thus, it is hard to directly titrate them using colored indicators. There are typically only two cases where direct titrations of soil are carried out. The first is to determine the amount of amendment needed to bring the soil to a desired pH. The second is in the determination of soil organic matter where organic matter is oxidized with chromate and the unreacted chromate is titrated (actually called a back titration) to determine, by subtraction, the amount of dichromate reduced and thus the amount of organic matter present. 

In other titrations, the component to be titrated is separated from soil and subsequently titrated. The simplest of these is the determination of soil ammonia. However, all forms of nitrogen in soil are important, so methods of converting other nitrogen-containing compounds to ammonia, distilling it, and determining its concentration by titration are important. 

Typically, acid soils are titrated with a sodium or calcium hydroxide [NaOH or Ca(OH)2] solution and basic soils with hydrochloric acid (HC1), and pH changes are most commonly followed using a pH meter. Carbonates in basic soils release C02 during treatment with HC1, thus making the titration more difficult. For this reason, carbonates are often determined by other methods. It is important to keep in mind that basic solutions react with carbon dioxide in air and form insoluble carbonates. This means that either the basic titrant is standardized each day before use or the solution is protected from exposure to carbon dioxide in air. Specific descriptions of titrant preparation, primary standards, and the use of indicators and pH meters in titrations can be found in Harris [1] and in Skoog et al. [2],... 

Most oxidation reactions are between specific metal cations or metal oxy-anions and cations. The problem that arises when applying oxidation-reduction reactions to soils is that all soils contain a complex mixture of oxidizable and reducible cations, anions, and organic matter, which means that it is impossible to determine which is being titrated. An exception to this is the oxidation of organic matter where an oxidation-reduction titration is routinely carried out. Organic matter determination will be discussed in Section 10.3. 

Precipitation titrations are typified by the titration of chloride with silver or vice versa. In this case, interferences with the precipitation reaction may occur because of components in the soil, and the soil itself may interfere with detection of the end point. Thus, complexation reactions are rarely applied directly to soil however, they can be applied to soil extracts. Common environmental titration methods described in the United States Environmental Protection Agency (USEPA) methods are summarized in Table 10.1 [1,2],... 

Back titrations are common in soil analysis, as they are used in both nitrogen and organic matter determinations. Back titrations are highly valuable analytical techniques and are applicable to other environmental analyses as well. 

Titration of soil pH is an old method that is not widely used today. Basically, an acid soil suspension is prepared and titrated with a standardized base, often sodium hydroxide, although various basic calcium compounds such as calcium oxide (CaO) and calcium hydroxide [Ca(OH)2] can also be used. Because of the dark color of many soils, they are often titrated using a pH meter as the indicator of the end point. A setup for the titration of soil is shown in Figure 10.1. Titration is slow in that it takes some time after the addition of titrant for some semblance of equilibrium to be reached. Once this happens, a reading can be made or simply another addition of titrant made. 

A titration curve for an acid soil suspension to which 1 mL of a calcium hydroxide titrant is added and the change in pH followed for 2.3 minutes is shown in Figure 10.3. As can be seen, the pH initially increases and then falls back toward the original pH. The curve not only has a sawtooth pattern but is also curved in the reverse direction from a standard titration of an acid with a basic solution. 

Figure 10.3. Stepwise titration of acid soil with calcium hydroxide.

Figure 10.5. Titration of 50 g of soil suspended in 50 mL of distilled water with 0.1 M NaOH using a pH meter. The titrant was added slowly and continuously with stirring. 

If a slow continuous addition of base is made to the same soil used in Figure 10.3, a similar titration curve without the sawtooth pattern is seen. Figure 10.5 shows the titration curve obtained by the continuous slow addition of 0.1 M NaOH. Again, the curve is not a smooth line, and irregularities seen in this titration are seen in other titrations of this same soil. Note that no distinct titration end point is seen here as there is in Figure 10.2. However, it is possible to determine the amount of base needed to bring this soil to pH 6.5, which is a typical pH desired for crop production. 

This is the basis for a common method for the determination of ammonia in soil.1 Soil is suspended in water and placed in a Kjeldahl flask. The suspension is made basic by the addition of a strong (5-50%) sodium hydroxide solution, and the flask is immediately attached to a steam distillation setup. Steam distillation of the suspension carries the released ammonia to an Erlenmeyer flask, catching the distillate in a standardized acid solution that is subsequently back titrated via acid-base titration. The amount of ammonia in soil can be calculated from the end point of the titration. This procedure is similar to a standard Kjeldahl determination and can be carried out using the same equipment, although no digestion is needed. 

Both nitrite and nitrate are highly mobile in soil and easy to extract. However, it is also possible to reduce each individually to ammonia, steam-distill the ammonia, capture it, and titrate it as described earlier for ammonia. If this... 

The titrimetric determination of soil constituents is most commonly applied to a limited number of soil analyses, namely, organic carbon, nitrogen compounds, carbonates, and chlorides. Determination of acid content by titration is generally not done because the titration curves are not amenable to typical titration analysis. Because of the color of soil and the fact that it is a suspension when stirred, it is often necessary to remove the constituent of interest before titration. In other cases, it is possible to do a direct titration using an appropriate indicator. However, even in these cases, detection of the end point is difficult. 

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