Types of Saturation Indices: Calcium Carbonate

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Table of Contents

Abstract
Types of Saturation Indices
Variations between the Different Types of Saturation Indices
Conclusion
References

Abstract

There are different types of saturation indices used in chemistry. The measures are used to determine the levels or ‘saturation’ of calcium carbonate compounds in water. They include, among others, Langelier saturation index (LSI), Ryznar stability index (RSI), and Puckorius scaling index (PSI). In this paper, the author analyzed the major differences between the various indices. Finally, the measures that are commonly used were identified. The reasons for their popularity were reviewed. Langelier, Ryznar, and Puckorius were found to be some of the commonly used indices. Their applications in different settings, including in industrial water companies, were reviewed. In addition, the link between these measures and scaling in water was analyzed.

Types of Saturation Indices

The concept ‘saturation’ has different definitions in the chemistry field. For the purposes of this paper, however, the author will focus more on surface processes of liquids and gas mixtures (Han & Alzamora, 2011). To this end, saturation refers to the degree of full occupation of a binding site. It is noted that saturation indices are used to measure the degree of this form of occupation. In water, when the binding sites are fully filled, saturation is said to be balanced. Failure to inhabit the sites creates an ‘appetite’ for water. In this case, the substance is said to have a ‘craving’ for the said water. It is also aggressive for the ions missing from it.

The right amount of saturation helps prevent precipitation of compounds. Precipitation creates the problem of clogging of pipes and cementing of filters, which reduces the efficiency of these elements. In this paper, the author will analyze different types of saturation indices. The researcher will focus more on Langelier index. The major differences between the indices will be reviewed. Finally, the measure that is mostly used will be highlighted.

There are different types of saturation indices. They include Langelier saturation index (LSI), Ryznar Stability Index (RSI), and Puckorius Scaling Index (PSI). Other indices include the Stiff-Davis Index, Oddo-Tomson Index, and the Larson-Skold Index (Laier, 2003). The measures mainly focus on the description of how calcium carbonate behaves in different mediums, such as water, gas, and oil.

As stated above, all the saturation indices seek to describe the conduct of calcium carbonate in fluids. It is noted that LSI, RSI, and PSI are the most commonly used measures. Each of these indices is described in detail in this section.

Langelier Saturation Index (LSI)

The measure is also commonly referred to as Langelier stability index. According to Stein (2008), the value of the index describes the stability of the calcium carbonate (CaCO₃) contained in water. In addition, the value helps predetermine the likelihood of water dissolving, precipitating, or staying at equilibrium with the compound in question (Papavinasam, 2013). The index was developed by Wilfred Langelier in 1936. He predicted the pH level of water that would ensure it is fully saturated with calcium carbonate. The pH is normally referred to as pHs. The LSI is regarded as the difference between the actual pH and the saturation pH. As such, the computation below is used:

LSI= pH (the measured pH) – pHs.

When the LSI is greater than zero, meaning that it is a positive value, then the water is said to be super- saturated. In addition, such water tends to precipitate a layer of CaCO₃. The layer floats on the surface of the water, forming scales (Stein, 2008). In the event that the value of LSI is at zero, then water is said to be saturated. At this point, the medium is in equilibrium with the CaCO₃ in it.

As such, there is no precipitation of this calcium compound. As a result, no scale layer is formed. Equilibrium also means that the compound is not dissolved. On the contrary it is well balanced with water. For LSI values that are less than zero (negative LSI values), water is said to be under saturated. In this case, it is viewed as being CaCO₃ deficient. Solid CaCO₃ contained in the water is dissolved. In addition, the liquid has a very low potential to form scales. It is important to note that the scaling potential of a fluid increases with a rise in LSI (Stein, 2008).

It is important to note that LSI is temperature sensitive. To this end, the value of the index is known to increase with a rise in temperatures. The effect of temperature is clearly discernible when water drawn from a well is used. When the liquid is pumped from the well, it has lower temperatures compared to the laboratory where it is to be tested. It is noted that equilibrium may have existed previously. However, scaling is likely to start as the LSI begins to increase as the temperatures begin to rise (Stein, 2008). Water heating systems will also lead to further precipitation of such liquid, leading to more scaling. Such water does not have an appealing appearance. Furthermore, it causes irritations to the skin of its user.

Ryznar Stability Index (RSI)

Unlike LSI, RSI uses a database that contains different values of scale thickness. The databases are used to predict the effects of water chemistry, especially for companies supplying this commodity to major municipalities in the world. The index was developed after it was observed that water with a high scaling potential often caused films in steel pipes. In addition, the water was seen to corrode such pipes (Papavinasam, 2013).

The value of RSI is obtained using the following formula:

RSI = 2 pHs- PH (Value of the pH measured).

For water to be at equilibrium with CaCO₃, the value of RSI should range between 6.5 < RSI < 7 (RSI greater than 6.5, but less than 7). Within this range, no scaling will occur (Papavinasam, 2013). The CaCO₃ contained in the water neither precipitates nor forms scales. When the value of RSI is greater than 8 (RSI > 8), it is said to be under saturated. The CaCO₃ contained in the water at this point tends to dissolve as a result of its deficiency. There are lower chances of scaling at this RSI values. Water with an RSI value that is lower than 6.5 is said to be super- saturated with CaCO₃. Such medium tends to precipitate the compound, leading to scaling (Papavinasam, 2013).

Puckorius Scaling Index (PSI)

The PSI measurement is also referred to as the Practical Scaling Index. The index was formulated to form a highly accurate indication of the potential of water to form scales. It puts into consideration the buffering capacity of water (Laier, 2003). The value of PSI can be used to determine the amount of CaCO₃ that can be precipitated in order to stop the formation of scales.

According to the index, water that has a high calcium deposition, but which is low in buffering capacity and alkalinity, is likely to be highly saturated with CaCO₃. Such liquid is likely to precipitate the compound, which then floats to the surface, leading to scaling. The PSI value is calculated using a formula that is similar to the one used for RSI. However, PSI uses equilibrium pH, unlike RSI, which utilizes the actual system pH. The formula below is used:

PSI = 2 (pH_s) – Ph_eq.

In the formula, pH_s is the pH of CaCO₃ at saturation. Ph_eq, on the other hand, is calculated as Ph_eq =1.465 * log 10[Alkalinity] + 4.54. In this case, alkalinity is calculated using the formula given below:

Alkalinity = [HCO₃^–] + 2 [CO₃^2-] + [OH^–].

Variations between the Different Types of Saturation Indices

The table below illustrates the differences between the various types of saturation measures:

Table 1. Saturation indices.

Aspect.	Langelier saturation index (LSI).	Ryznar Stability Index (RSI).	Puckorius Scaling Index (PSI).
Formula.	LSI= pH (the measured pH) – pHs.	RSI=2 pHs- PH (Value of the pH measured).	PSI = 2 (pH_s) – Ph_eq.
Value of index at equilibrium.	0.	6.5 < RSI < 7.	Unspecified.
Temperature sensitivity of the index.	Temperature sensitive.	Not temperature sensitive.	Not temperature sensitive.
Consideration of the buffering capacity.	Does not.	Does not.	Does.

The The Langelier saturation index (LSI) is the most commonly used index. The reason for this is the ease associated with its computation. The index also shows all the relationships that may exist between water and calcium carbonate. The relationships are dissolving, precipitating or being at equilibrium. The index also allows for the estimation of the pH at which the equilibrium will occur. The property allows experts in the field to be in a position to regulate this pH (Laier, 2003).

Ease in regulation of pH ensures that the users of the water are not affected by problems associated with precipitation and scaling of CaCO₃. The LSI values are also easy to interpret (Papavinasam, 2013). The equilibrium exists at zero. Values that are less than zero are used to depict under saturation while those that are higher than zero show over saturation. Unlike other indices, LSI is also temperature sensitive. As such, experts are in a position to predict the effect of change in temperatures and make the necessary adjustments in the regulation of CaCO₃ in water. Temperatures can also be regulated to reduce the expenses associated with the regulation of CaCO₃.

Conclusion

Saturation is the term used to describe the capacity of binding sites that is fully occupied in liquids and gasses. Liquid and gasses are said to have an appetite when these sites are not completely filled. The paper has mostly concentrated on water since it is widespread in nature and is an essential need for human dwelling (Stein, 2008). Several types of saturation indices have been discovered today.

The indices are Langelier saturation index (LSI), Ryznar Stability Index (RSI), Puckorius Scaling Index (PSI), the Stiff-Davis Index, Oddo-Tomson Index, and the Larson-Skold Index. Although these indices vary in terms of their computation and interpretation of the index value, they are all efficient in describing the relationship that occurs between calcium carbonate and water (Stein, 2008). The LSI is the most commonly used of these indices. The reason behind its widespread use is its ease in computation and interpretation. All relationships that can occur between water and calcium carbonate are also well outlined. The index also puts into consideration the effect of temperature on saturation.

References

Han, J., & Alzamora, D. (2011). Geo-Frontiers 2011: Advances in geotechnical engineering. Reston, VA: American Society of Civil Engineers.

Laier, T. (2003). Prediction of scaling problems likely to occur during geothermal heat production using the FFC-01 well: Estimation of saturation indices for saline formation waters. Copenhagen: GEUS, Geological Survey of Denmark and Greenland.

Papavinasam, S. (2013). Corrosion control in the oil and gas industry. Burlington: Elsevier Science.

Stein, R. (2008). Water supply. New York: H.W. Wilson Co.

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