What is the primary fixing agent used in radiography?

The primary fixing agent used in radiography is ammonium thiosulfate. This compound plays a crucial role in the development of radiographic films by removing unexposed silver halide crystals from the emulsion layer. During the film development process, after the film has been exposed to radiation and developed, the fixing stage is essential to ensure that the image is stable and won't be affected by light.

Ammonium thiosulfate effectively dissolves excess silver halide, leaving behind only the developed silver image. This not only preserves the image but also prevents fogging and degradation over time. Because of its effectiveness and efficiency in performing these tasks, ammonium thiosulfate has become the standard fixing agent in the radiographic film development process.

In contrast, other substances listed do not serve the same function or are not commonly used in the fixation process in radiography.