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Rules of the Pinyin (phonetics)<4>

5. Generally speaking, each Chinese character has only one tone but there are a few exceptions.

A frequently used word “一” (meaning “One”) has three tones:

A. When “一” is used independently it pronounces as “yī” (the first tone).

For example:

之一 --- zhī yī (meaning “one of”)     

第一 --- dì yī (meaning “first”)     

B. When it is followed by a word with the first, second or third tone, it pronounces as “yì” (the fourth tone).

For example:

一些 –- yì-xiē (meaning “some”)     

一年 –- yì-nián (meaning “one year”)     

一起 –- yì-qǐ (meaning “together”)     

C. when it is followed by a word with the fourth tone, it pronounces as “yí” (the second tone).

For example:

一个 – yí-gè (meaning “a”)