Online analysis of calcium hydroxide concentration

According to chemical principles, the solubility of calcium hydroxide changes with temperature; at room temperature it is around 0.16–0.17 g per 100 mL, which corresponds to a concentration of about 0.15%, far below 15%. Therefore, it is necessary to clarify the definition of “15%” (whether it refers to the proportion of the total mass of the solid-liquid mixture), and to distinguish between “solutions” and “suspensions” in order to avoid conceptual confusion. If the purity of this sample is satisfactory, the percentage content of calcium hydroxide can be calculated by determining the calcium content (using the EDTA titration method) after appropriate pretreatment.

This post was last edited by qqgd on 2025-4-10 at 12:34. The sample mentioned by the original poster is not a solution, but rather something similar to lime milk – 15% calcium hydroxide, likely in the form of a suspension of free, undissolved calcium hydroxide. It is very difficult to find suitable instruments and methods for directly measuring such a sample using chemical analysis techniques. First, the representativeness of samples containing large amounts of suspended substances is limited; second, the reliability of direct detection for suspension samples is also limited. You can try the following method: if the sample under test contains only calcium hydroxide suspensions and no other components that affect alkalinity, the concentration of the suspensions can be measured to indirectly indicate the concentration of calcium hydroxide. However, it is essential to maintain sample homogeneity throughout the measurement process. Another method is to measure its density, which can be done using a mass flow meter, or by detecting the differential pressure associated with a fixed liquid level/level of substance in order to convert it into the sample density; this value can then be correlated with the concentration of calcium hydroxide in the suspension. The refractometry method may not be applicable, as the sample is not uniformly transparent; furthermore, the refractive index at the sample interface with suspended particles can severely affect the detection process due to excessive scattering. Another relatively reliable testing method is costly, as the analyzers used for this purpose are expensive; it involves X-ray fluorescence analysis. Such instruments can conduct direct testing without the need for sample preprocessing, and they offer high reliability in detection. However, they are much more expensive, and their use may require compliance with regulations regarding radiation equipment. Judging from the posts the original poster has shared in the past, they are an online analyst working on the front line, as your questions are all very practical and specific. Is this lime milk used for the absorption of fluorine-containing exhaust gases in the production process? Because the formed calcium fluoride can be recycled. Could you please tell me the name of the analyzer or manufacturer that provided the analysis and solution? Since a rough plan can be devised, it may be necessary to have some knowledge of online analysis; however, for such special samples or media, it requires more experience beyond standard analytical methods as well as specific technical skills to address the issues.

Titration methods may have limitations related to the uniformity of sampling; calcium hydroxide suspensions have a high particle density and settle rapidly, which can lead to blockages in the sampling pipelines as well as related valves and fittings during sampling and sample transfer. Using physical methods may make online analysis easier to achieve.

It’s a suspension; I didn’t explain it clearly enough

It is indeed for the absorption of fluorine-containing exhaust gases; we contacted an agent, who said that the density method is relatively cheaper while the refractometry method is more expensive. At present, the instruments used for refractometry are imported; domestic ones also exist, but they are not widely used.

For the online analysis of calcium hydroxide (Ca(OH)₂) solutions with a concentration of around 15%, factors such as its tendency to precipitate, its alkaline corrosiveness, and the appropriate concentration range need to be taken into account. The following are the recommended methods and equipment for online analysis: 1. Conductivity method (recommended as the preferred option). Principle: The conductivity of calcium hydroxide solutions is directly proportional to their concentration; at a concentration of 15%, the conductivity is relatively high, around several dozen mS/cm. A conductivity-concentration model can be established through calibration. Equipment: Online conductivity sensors (from brands such as Mettler-Toledo, E+H, Hach, etc.), with alkali-resistant materials required (such as titanium alloy electrodes and polytetrafluoroethylene housings). Integrated temperature sensor (automatic temperature compensation, as conductivity is affected by temperature). Advantages: low cost, fast response, and simple maintenance. Note: Regular calibration is required (especially when impurities in the solution or CO₂ absorption cause calcium carbonate precipitation). 2. pH method (for auxiliary verification) Principle: The pH of a saturated calcium hydroxide solution is approximately 12.4 (it is nearly saturated at a 15% concentration), but the pH does not change significantly at higher concentrations; therefore, it needs to be used in conjunction with conductivity measurements. Equipment: Strong-base resistant pH electrode (such as a glass electrode with an anti-fouling coating). Limitations: Suitable only for rough monitoring, requiring frequent calibration. 3. Density method (optional) Principle: The density of a 15% Ca(OH)₂ solution is approximately 1.15 g/cm³ (at 20°C), and the concentration can be determined by using this density value. Equipment: Online densitometer (such as a vibrating densitometer; brands: Anton Paar, E+H). Note: Settlement blockage should be avoided; it is suitable for use with a mixing system. 4. Optical method (for suspension adjustment): Turbidity/scattered light method: If the solution contains undissolved particles, a turbidimeter can be used to monitor changes in suspended matter (such as a Hach online turbidimeter). Near-infrared (NIR) spectroscopy: Requires the development of specific models, is suitable for complex operating conditions, but is costly. Equipment selection recommendations: Core solution – conductivity + temperature compensation + automatic cleaning (e.g., a conductivity probe with ultrasonic cleaning). Alternative approach: Combine pH or density measurement to improve reliability. Anti-settling design: Install it near the flow point or add a stirring mechanism to prevent the probe from being covered by deposits. Precautions: Regular maintenance: Calcium hydroxide tends to form scale, so the probe needs to be cleaned regularly (for example, by acid cleaning to remove calcium carbonate). Calibration frequency: Calibration is recommended weekly (adjust according to operating conditions). Equipment material: Choose alkali-resistant materials (such as PTFE, PEEK, titanium alloy). Based on the actual budget and operating conditions, it is advisable to first test the applicability of the conductivity method before considering the addition of other parameters.

Thank you. This isn’t a critical point, so I don’t want to spend too much money; I plan to consider the density method

It seems you don’t really have a clear concept of 15% lime emulsion – it’s not a solution but rather lime slurry. How can one measure the conductivity to determine the concentration of large amounts of undissolved substances? It must be dissolved to contribute to conductivity; if it is not dissolved, it contributes nothing. The same issues as mentioned above apply to pH measurement. Additionally, calcium hydroxide, as well as any remaining calcium carbonate and calcium fluoride, can deposit on the surface of the electrode, severely affecting pH measurement. Moreover, how can the alkali bias caused by high pH values on the pH electrode be overcome? In the density method, the vibration tube used in the vibrating densitometer has an excessively small diameter; the deposition of this type of emulsion during the measurement process can severely affect the proper functioning of the detector. Unless the diameter of the detection tube is increased to a level that does not interfere with the measurements – probably to at least 15 millimeters – it is better to use a mass flow meter based on the Coriolis principle. In optical methods, the probe of the sludge concentration meter might perform slightly better, but do you think the price of the near-infrared analyzer is reasonable? For a specific issue, it is necessary to take into account the actual conditions of the specific customer as well as the possible practical investments; relying solely on responses obtained through AI searches online would be highly unprofessional.