Massachusetts Wastewater Practice Exam

To preserve samples for cyanide analysis, the pH should be adjusted to what level?

1. Less than 2.5.

2. Less than 4.7.

3. Greater than 8.3.

4. Greater than 12.0.

The correct answer is: Greater than 12.0.

To preserve samples for cyanide analysis, it is crucial to adjust the pH level to greater than 12.0. At this high pH, cyanide exists primarily in the non-toxic cyanate form, which helps in preserving the integrity of the sample and minimizing the loss of cyanide due to volatilization or degradation. In acidic conditions, cyanide can convert to hydrogen cyanide gas, which is volatile and can escape from the sample, leading to inaccurate results in subsequent analyses. Maintaining a high pH ensures that most of the cyanide remains in a form that can be reliably measured and analyzed, resulting in more accurate assessments of cyanide levels in wastewater samples. Choosing a lower pH would risk the transformation of cyanide into other forms that are not stable or detectable in their original form, thus preventing reliable results. This ensures that laboratory practices align with standards for accurate environmental monitoring and compliance with regulatory requirements.