Continue heating the mixture until it boils. The acid mixture will bubble as you add the water and urea. Add the mixture slowly so that it doesn’t cause the acid to bubble out of its container. The water/urea mixture neutralizes the nitric acid but not hydrochloric acid in your solution. In general, you will add 1 ounce of precipitant per ounce of gold that you are refining. Avoid putting your face near the container’s opening. The smell is very strong and pungent. The acid will turn a muddy brown color, which is actually caused by the separation of the gold particles. Wait 30 minutes to allow the precipitant solution to work on the gold particles.

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Write a summary of the previous text in Chinese:
加入946毫升水，并在水中加入453克尿素。 慢慢将尿素和水的混合物加入酸中。 根据制造商提供的说明，在946毫升沸腾的水中加入针对黄金的选择性沉淀剂。 加入混合有沉淀剂的溶液时要慢。