Unit 3: Chemical Analysis of Primary Nutrients in Organic Manures - Practice Quiz

1 Which method is commonly used to determine total nitrogen in organic manures?

2 Which acid is commonly used to digest an organic manure sample in the Kjeldahl method?

3 Which gas is released during the distillation stage of the Kjeldahl method?

4 Which solution is commonly used to collect ammonia during Kjeldahl distillation?

5 Which instrument is commonly used for the colorimetric determination of phosphorus?

6 Why is an organic manure sample digested before total phosphorus is measured?

7 Which colour is commonly produced in the molybdenum blue method for phosphorus?

8 In colorimetric phosphorus analysis, a deeper colour generally indicates what?

9 Which instrument is commonly used to determine potassium in a digested manure sample?

10 What is measured by a flame photometer during potassium analysis?

11 What is the main purpose of a potassium standard solution in flame photometry?

12 Before instrumental measurement, potassium in digested manure should mainly be present in which form?

13 In the turbidimetric method, sulphur is commonly precipitated as which compound?

14 Which reagent supplies barium ions in the turbidimetric determination of sulphur?

15 What does increased turbidity generally indicate during sulphur determination?

16 Why is a reagent blank used during the determination of sulphur?

17 Which reagent is commonly used to titrate calcium and magnesium together?

18 Which indicator is commonly used when titrating total calcium and magnesium with EDTA?

19 At approximately which pH is the EDTA titration of calcium plus magnesium commonly performed?

20 If calcium and total calcium plus magnesium are measured separately, how is magnesium commonly calculated?

21 During Kjeldahl digestion of an organic manure, what is the main function of concentrated sulphuric acid?

22 A g manure sample requires mL of N acid after Kjeldahl distillation, while the blank requires mL. Using as the atomic mass of nitrogen, what is the total nitrogen content?

23 Why is excess sodium hydroxide added to the Kjeldahl digest before distillation?

24 A Kjeldahl analysis gives an unusually low nitrogen result. Inspection shows that the digest was distilled before enough sodium hydroxide was added. What is the most likely cause of the error?

25 After digestion of an organic manure, phosphorus is determined colorimetrically using a molybdate-based reagent. Why must the absorbance of standards and samples be measured at the same wavelength?

26 A manure contains total phosphorus expressed as P. What is the corresponding value expressed as ? Use the conversion factor .

27 A digested manure sample produces an absorbance above the highest phosphorus standard. What is the most appropriate action?

28 A g sample is digested and made to mL. A mL aliquot is diluted to mL and found to contain mg P/L. What is the total phosphorus content of the sample?

29 In flame photometric determination of potassium, why should standards be prepared in a matrix similar to that of the sample digest?

30 A manure contains potassium expressed as K. What is its content expressed as ? Use .

31 A potassium standard gives a stable flame photometer reading, but repeated readings of a manure digest steadily decrease. Which action should be taken first?

32 A sample digest contains mg K/L. If the digest volume is mL and the original manure mass is g, what is the potassium content?

33 In the turbidimetric determination of sulphur, what causes the measured turbidity after barium chloride is added?

34 Why should the time between adding barium chloride and measuring turbidity be kept constant for standards and samples?

35 Gravimetric precipitation from a manure digest yields g of pure . Approximately how much sulphur does it contain? Use g/mol and g/mol.

36 A reagent blank in a sulphur turbidimetric analysis shows substantial turbidity. If the blank response is not corrected, how will the reported sulphur concentration most likely be affected?

37 In EDTA titration, total calcium plus magnesium is commonly determined near pH 10, while calcium alone is determined at a higher pH. How is magnesium obtained?

38 A digest requires mL EDTA for total Ca plus Mg and mL EDTA for Ca alone. Assuming identical aliquots and EDTA concentration, what volume corresponds to Mg?

39 Why is a pH 10 buffer used during the EDTA titration of total calcium and magnesium?

40 A manure contains magnesium expressed as Mg. What is the corresponding content as ? Use .

41 A g manure sample is analyzed by the Kjeldahl method. The ammonia collected requires mL of N acid, while the reagent blank requires mL. What is the total nitrogen concentration on the analyzed basis?

42 A manure contains nitrate as well as organic nitrogen. Which modification is most appropriate when Kjeldahl digestion is intended to measure total nitrogen, including nitrate nitrogen?

43 A manure is reported to contain total N on an as-received basis and moisture. What is its nitrogen concentration on an oven-dry basis?

44 During Kjeldahl distillation, a digest is made strongly alkaline, but the receiving flask is accidentally removed before steam enters the apparatus. Which error is most likely if the delay is prolonged?

45 A g manure sample is digested and diluted to mL. A mL aliquot contains mg P by colorimetry. What is the concentration expressed as , using ?

46 In molybdenum-blue determination of phosphorus, the absorbance of an intensely colored manure digest lies above the highest calibration standard. Which action best preserves analytical validity?

47 A phosphorus digest gives lower molybdenum-blue absorbance when prepared with a strongly acidic matrix than do aqueous phosphate standards of the same concentration. What is the most defensible correction?

48 A g sample digest is diluted to mL. A mL aliquot is diluted to mL and measured as mg P L. What is the phosphorus concentration in the manure?

49 A g manure sample is digested to mL. A mL aliquot is diluted to mL and gives mg K L by flame photometry. What is the total potassium concentration?

50 A potassium flame-photometric calibration is linear for dilute standards, but a concentrated manure digest gives a suppressed signal because of high dissolved salts. Which procedure best addresses this matrix effect?

51 A manure contains K on a dry-matter basis. What is the corresponding concentration as , using ?

52 An analyst dry-ashes manure at an unnecessarily high temperature and obtains consistently low potassium recoveries. Which mechanism most plausibly explains the result?

53 After complete oxidation of manure sulfur to sulfate, a corrected mass of g is obtained from a g sample. Using and , what is the sulfur concentration?

54 In a turbidimetric sulfate determination using , sample turbidity continues to increase long after the standards have been read. Which procedural defect is the most likely cause?

55 A manure contains substantial sulfide and organic sulfur. Direct extraction with water followed by sulfate turbidimetry is proposed as a total sulfur method. Why is the result likely to be biased low?

56 In gravimetric sulfur analysis, the collected precipitate is insufficiently washed and retains soluble barium chloride. How will the reported sulfur result be affected?

57 A g manure digest is diluted to mL. A mL aliquot consumes mL of M EDTA at pH 10 for Ca plus Mg, while an equivalent aliquot consumes mL for Ca alone. What are the Ca and Mg concentrations?

58 Why is calcium commonly titrated with EDTA at about pH 12 using a calcium-selective indicator when calcium and magnesium must be determined separately?

59 In an EDTA determination at pH 10, a manure digest contains appreciable Fe, Mn, and Cu that were not separated or masked. What is the most likely analytical consequence?

60 A laboratory reports Ca and Mg in a manure. What are the approximate concentrations as and , using conversion factors and ?