厦门大学化学化工学院
作业20090916
A类
1. ACT p.55-56:
2.12, 2.13, 2.14
2. MAC p.98: 2
3. You have obtained the following data for the alcohol content of a sample of blood:
%C2H5OH: 0.084, 0.089, and 0.079. Calculate the 95% confidence limits for the mean assuming (a) that you know nothing about the precision of the method and (b) that on the basis of previous experience, you know that s= 0.005 % C2H5OH a nd that s is a good estimate of σ.
B类
1. 阅读: MAC pp. 82-85 Statistical analysis of data.
2. 阅读How many samples do I need？(课程网站）
3．An analytical protocol exhibits a 95% confidence interval of ±0.06. If a 90% confidence limit of ±0.06 is required by regulations, could the protocol still be used?
C类
1. The determination of nickel in a stainless steel specimen can be carried out gravimetrically using dimethylglyoxime as precipitation agent in a slightly alcaline solution; tartaric acid is used to mask Fe (III) which is the major interferent. The procedure is essentially composed of the following steps: The sample containing Ni is weighted and dissolved in a suitable volume of 6 mol/L (M) HCl followed by the addition of a suitable volume of 6 mol/L (M) HNO3 and boiling;
•A suitable volume of an aqueous solution of tartaric acid, the marking agent, is then added;
•A weighted amount of dimethyglyoxime, the precipitating agent, is dissolved in ethanol and a suitable volume of the resulting solution is added, followed by
addition of 6 mol/L (M) NH3 until a slight excess is present;
•The precipitated is digested for a convenient time and the solution allowed to cool;
•The precipitate so formed is filtered trough a filtering crucible (previously heated at 110 o C to a constant weight) and until the washing solution is free from Cl- ion;
•The washed precipitated is dried at 110o C to a constant weight and the necessary calculations, to find out the Ni content in the analyzed sample, are performed.
Try to identify the category of errors which are likely to be introduced by each of the steps above described. Based on your personal knowledge (theoretical and, hopefully, also practical) of gravimetric methods of analysis try to identify those steps introducing the most significant error. (参见厦门大学出版社《分析化学基础实验》p.125)。