Table of Contents

• Objective
• Audience
• Disclaimer
• Let's get started
  • Creating our input streams
  • Creating the Titration (Mixer)
• Supplemental Material
  • A Titration Experiment
    • HF Titration with CaCl₂
    • Stream Composition Note
  • Calculating the pH of the individual streams
  • Results – pH of the individual streams
  • Calculating the pH of the Mixture
  • Titrating HF with CaCl₂
  • Titration Results and Plotting
• Conclusion

Objective:

Often, it is beneficial to combine two or more streams to achieve a desired endpoint. One such is a titration. We use titrations to find end-points for alkalinity measurements and to find the pKa values of acids.  

Sometimes our results from a titration can be confusing (at first), but with some analysis, we can use OLI Studio to figure out what is going on.

Audience:

Intermediate OLI Studio users (those who already have basic competency)

Disclaimer:

We used OLI Studio version 12.0 (build 11) for this demonstration. Earlier and later versions may have slightly different answers. The case file will be included.

Let's get started

Creating our input streams

We need two input streams for this calculation. Since you already know how to use OLI Studio: Stream Analyzer, please create these two streams:

The video we will show you uses these two streams, which are named in the above table:

Creating the Titration (Mixer)

Supplemental Material

A Titration Experiment

In this example, you will explore the volume mixing option to recreate a titration experiment. As we go through the example, different functionalities, reporting, and plotting for the Mixer tool will be introduced.

HF Titration with CaCl₂

In this example, we will first create two new streams, a 0.1 m HF solution and a 0.1 m CaCl₂ solution. What is the pH of these individual streams?

Second, we will mix these two streams in equal amounts. What is the final pH of the mixture?

And finally, the 0.1 m HF solution will be titrated with a 0.1 m CaCl₂ solution. What will the titration curve look like?

See above for the stream composition.

Calculating the pH of the individual streams

• Add a Single Point – Isothermal calculation for each stream
    
• Change the name to HF-Isothermal and CaCl2-Isothermal, respectively, using the <F2> key
  
• Calculate the isothermal calculation for each stream
  
• Check the pH values of the individual streams in the Summary Box
  
  
  

Results – pH of the individual streams

Calculating the pH of the Mixture

• After creating the streams, Add a Mixer. The Mixer object can be accessed from the Menu Bar by selecting Calculations > Add Mixer or by selecting the Add Mixer in the Actions Pane.
  
      
• Press <F2> to rename the mixer to Titration

Note: The Available Streams window displays all available streams for the mixing calculation. Also, note that this window displays both Stream and Single Point calculations. Additionally, the available streams’ thermodynamic framework appears within brackets, e.g., [AQ], [MSE], or [MSE-SRK]. This is important because to carry out mixing calculations, the selected streams must use the same thermodynamic framework.

• Select HF – [MSE] stream. Use the >> button to move it to the Selected window
  
• Select the CaCl2 – [MSE] stream. Use the >> button to move it to the Selected window
  
• Select Single Point Mix as the Mixing Method and Isothermal as the Type of Calculation
  
• Leave the default values for the Multiplier (1.0) and change the Temperature at which the mixture takes place to 30 ⁰C.
  
• Now, we are ready to perform the calculation. Click on the Calculate button or press the <F9> key
  
• At this point, it is time to save your file (File > Save as…) or use the save icon in the toolbar. You can save under the same file that we created before, named Mixer Calculations.
    
• Check the pH values of the individual streams in the Summary Box
  
  
  
  
  

The resultant pH is 1.44. How can this be? An acid stream (the HF stream had a pH of approximately 2.13) and a nominally basic stream (the CaCl₂ stream had a pH of 6.82) are mixed, and the pH is outside the value of either stream.

We will continue using OLI Studio: Stream Analyzer to further examine the chemistry in more detail. We will titrate the HF stream with CaCl₂.

Titrating HF with CaCl₂

• Change the Single Point Mix as the Mixing Method to the Multiplier option.
  

 Note: You will receive a warning message: Need 1 stream selected as the variable stream (Specs...)

• Click on the Specs button. This will open a new window, indicating that you should select the Stream to vary.
  
  
  
• In the Select Stream tab, select the CaCl2 stream
  
• Select the Survey Range tab. Change the Multiplier Range from 0 to 2.0, with an increment of 0.1
  
• Now, we are ready to perform the calculation. Click on the Calculate button or press the <F9> key
  
• It is time to save your file (File >Save as…) or use the save icon in the toolbar. You can save under the same file that we created before, named Mixer Calculations.

Titration Results and Plotting

Click on the Plot tab. Note: You will see an empty plot, since we need to define the variables that we want to report.

Click on the Variables button, expand the Additional Stream Parameters, and move pH to the Y1 Axis.

Why is there a dramatic drop in pH when adding a more basic stream?

Let’s add some variables.

Select Variables and then expand the Liquid-1 section. Select and put the following species in the Y1 axis using the >> button:

HF-Liq1
H3O+1
F-1
Ca+2

Expand the Solid section and select CaF2(s).

Move the variable, pH, and put it in the Y2 Axis using the >> button. Then click OK.

As you can see, adding CaCl₂ to this solution lowers the pH to values as low as 1.33. The pH begins to slowly increase after 0.5 (50%) of the CaCl₂ solution has been added.

Why the unusual pH behavior? You can see that a small amount of Ca²⁺ in solution causes the formation of the solid. This effectively removes the fluoride from the solution according to the following equation:

Ca²⁺ + 2F^- → CaF₂

As we add more CaCl₂ to the solution, more CaF₂ is formed. As a result, the molecular HF species decreases because it dissociates to maintain the equilibrium by producing more ions. This shifts the following equilibrium to the right:

HF^o + H₂O → H₃O⁺ + F^-

This causes an increase in H₃O⁺ in solution and, as a result, a decrease in the pH. At some point, at around 0.5 multiplier (50%) of 0.1 m CaCl₂, the solid reaches a steady value, indicating that the solid has reached its saturation (maximum formation) value. 

Conclusion

This example demonstrates how OLI Studio: Stream Analyzer's Mixer tool can be leveraged to model titrations, interpret unexpected pH results, and explore the underlying chemistry driving those changes. By systematically calculating the pH of individual streams, mixing them, and performing a stepwise titration, we can uncover hidden reactions (such as CaF₂ precipitation) that significantly impact solution chemistry. The exercise highlights the importance of considering both aqueous equilibria and solid formation when interpreting titration data, as these phenomena can produce counterintuitive results. Equipped with these tools and insights, intermediate OLI Studio users can more confidently design, analyze, and troubleshoot titration experiments in both research and industrial applications.