问题 问答题 简答题

用氢化物-非色散原子荧光法测定土壤中砷时,样品用硝酸-高氯酸消解后为何加入硫脲和盐酸?

答案

参考答案:

土壤样品经硝酸-高氯酸消解后,砷一般以高价态[As(Ⅴ)]存在。因为只有低价砷[As(Ⅲ)]才能有效地生成砷化氢,所以要加入硫脲和盐酸进行预还原,将As(Ⅴ)还原为As(Ⅲ)。

单项选择题
阅读理解

阅读理解。

     Silence is unnatural to man. He begins life with a cry and ends it in stillness. In between be does all he can

to make a noise in the world, and be fears silence more than anything else. Even his conversation is an attempt

to prevent a fearful silence. If he is introduced to another person, and is number of pauses occur in the

conversation, he regards him-self as a failure,a worthless person, and is full of envy of the emptiest- headed

chatterbox (喋喋不休的人).He knows that ninety nine percent of human conversation means no more than the

buzzing of a fly, but he is anxious to join in the buzz and to prove that he is a man and not a waxwork figure

(蜡塑人像).

     The aim of conversation is not. For the most part, to communicate ideas; it is to keep up the buzzing sound. There are, it must be admitted, different qualities of buzz; there is even a buzz that is as annoying as the

continuous noise made by a mosquito (蚊子). But at a dinner party one would rather be a mosquito than a quiet

person. Most buzzing, fortunately, is pleasant to the ear, and some of it is pleasant even to the mind. He would

be a foolish man if he waited until he had a wise thought to take part in the buzzing with his neighbors.

     Those who hate to pick up the weather as a conversational opening seem to me not to know the reason

why human beings wish to talk. Very few human beings join in a conversation in the hope of learning anything

new. Some of them are content if they are merely allowed to go on making a noise into other people's ears,

though they have nothing to tell them except that they have seen two or three new plays or that they had food

in a Swiss hotel. At the end of an evening, during which they have said nothing meaningful for a long time.

They just prove themselves to be successful conservationists.

1. According to the author, people make conversation to _____.

A. exchange ideas

B. prove their value

C. achieve success in life

D. overcome their fear of silence

2.  By "the buzzing of a fly " (Para.1),the author means"_____ ".

A. the noise of an insect

B. a low whispering sound

C. meaningless talks

D. the voice of a chatterbox

3.  According to the passage, people usually talk to their neighbors _____.

A. about whatever they have prepared

B. about whatever they want do

C. in the hope of learning something new

D. in the hope of getting on well

4.  What is the author's purpose in writing the passage?

A. To discuss why people like talking about weather.

B. To encourage people to join in conversations.

C. To persuade people to stop making noises.

D. To explain why people keep talking.