The pH issue of liquid caustic and sodium sulfide solutions.
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Sodium sulfide solution is obtained by using liquid caustic to absorb hydrogen sulfide gas. They want to use a pH meter to monitor the absorption process, but the project owner has raised some issues: 1. They tested it using pH test strips, and the pH value did not change much during the absorption process. Sodium sulfide solution is also alkaline. 2. To prevent the precipitation of sodium sulfide, it is necessary to provide heat tracing for the container; high temperatures are not favorable for pH meters. Can’t the pH meter be used at all?
It is recommended to have it tested in the laboratory. First, take an appropriate amount of analytically pure sodium sulfide sample and prepare it into an aqueous solution of a certain concentration; then measure the corresponding pH value and plot a curve showing the relationship between pH value and concentration ； If it is suspected that temperature is also a factor affecting the pH value, conduct test data at different temperatures. Effect of excess alkalinity on pH value: A small amount of caustic soda solution can be added dropwise to the sodium sulfide sample solution mentioned earlier, in order to determine the effect of different levels of excess alkalinity on the pH value of the sodium sulfide solution. If the effect of excess alkalinity on the pH value is significant, an online pH meter can be used to control the excess alkalinity and the flow rate of the alkali solution during the absorption process.
When it comes to high-temperature environments, there are indeed pH electrodes suitable for use in such conditions. Even when choosing an online pH analyzer, there are special materials that are appropriate for use with high-temperature solutions. The reason your construction party says it’s not suitable is probably that the current pH meters cannot withstand high temperatures. Ordinary pH electrodes are prone to damage at high temperatures, which affects their service life.
If there is an excess of liquid alkali, the pH approaches 14, with little change thereafter
Since the reaction between hydrogen sulfide and sodium hydroxide is a reaction between a weak acid and a strong base, the pH does not change much during the reaction. Additionally, in order to prevent the hydrolysis of the sodium sulfide that is formed, an excess of base is necessary. The presence of this excess base is what contributes to the stability of the entire system, as well as to maintaining its strongly alkaline nature. Using pH to monitor the alkalinity changes in hydrogen sulfide absorption solutions is not very effective; moreover, pH electrodes need to be those capable of withstanding sulfide interference, and they require certain maintenance. Temperature is not the main factor that has an impact, as many pH electrodes these days can withstand higher temperatures; however, their service life is affected by temperature. The poster can provide the process conditions for experienced online analysis technicians to review, which may yield some suggestions or solutions for improvement. Let me remind you that attempting to solve some tricky problems related to online process analysis using simple methods and low procurement costs will only result in half the effort yielding half the results.
What kind of heating is used to heat sodium sulfide solutions? Does heating release hydrogen sulfide gas, and how can crystallization be avoided?