edited bv:
JOHN J . ALEXANDER University of Cincinnati Cincinnati. Ohio 45221
exam que~tionexchange
gone to America, others that he had met with an accident. The manager of the works suggested that he had fallen into the acid and had been dissolved, hair, flesh, boots, clothes, bones and all. The weeping wife now laid claim to his insurance money hut the assurance officials refused to pay out anything. "Produce us evidence of death," they said, "and we will give you the money. How do we know that your husband has not simply secretly left the country?" So the poor widow was in a sad plight and at her wits end what to do. She appealed to the manager ofthe works, and he resolved to solve the problem. Being a chemist he knew that the human hody contains quite a considerable amount of phosphorus, which must be found in the acid (if the man had really fallen into it) in the form of phosphoric acid. So he caused an analysis of the liquid to he made, and sure enough found a large amount of phosphorus present, such as represented the amount known to be in the body of a full grown man. This evidence was then presented, and the end of it all was that it was accepted as conclusive evidence of death, and the poor widow received the payments due to her. Applied chemistry is thus of great use, sometimes, in legal matters. althoueh - lawvers are not, as a rule, trained in such matten.
Stoichiometry to the Rescue (A Calculation Challenge) R. W. Ramette Carleton College Northfieid,MN 55057 This auestion would be suitable for atake-home exam or a prohlem set ingeneralchemistry or possihly in theanalytical course where a discussion of differences in analytical results would he appropriate.
Question We begin with a bittersweet tale from a book published in 1922, Modern Chemistry a n d its Wonders, Chapter 11,"The Romance of Some Simple Nitrogen Compounds", by Geoffrey Martin: The fate ofa world probably rests upon two simple compounds of nitrogen-namely, nitric acid and ammonia. This is a f a d sufficient to direct attention to these two substances, old friends of our schooldays as they are, and invest them with a fresh interest. Indeed they form the centre of attention in the scientific world a t the present time. Of the two nitric acid has oossihlv the ereater commercial imoortanee. The oure acid is teiriblv c&rosive. attackine" oreanic " material such as paper, wo8,d and skin extremely rapidly. Mort metalsdissolw in it, evolving poisonour nitrous fumes. Uitrir arid is ntsduiely indispensible fur the aniline dye and the rxplo.tiwu industry. Owing to the terribly corrosive properties of nitric acid many fearful accidents have happened, and of these the most dramatic was that which occurred some years ago in a large German dye factory. A workman overbalanced himself and fell into a large vat containing a boiling mixture of strong nitric and sulfuric acids, such as is used for dissolving dyes. There was no one in the building to hear his last despairing cry, and when, later, the man was missed, nowhere could a trace he found of him. His vanishing was an absolute mystery which no one could account for. Some people thought that the man had secretly fled the country and ~
~
~
~
Several methods for phosphate determination were well known by the turn of t h e century, when this tragedy occurred. If we faced the same problem today, it would be possible t o use spectrophotometry, which was uncommon before about 1940. T h e accompanying figure summarizes in a simplified way the steps in the proceduresl.2. Note t h a t they all begin with the separation of phosphorus from the acid samnle. T h e followine auestions ~ r o v i d ehwothetical data for a.nalyses represen;& each o f t h e methodiindicated in the chart. For each case, decide whether or not these data support the widow's claim; I n fact three of the data sets are supportive and one is not. It is essential t h a t you explain t h e basis for your conclusions in a clear fashion.
~
'
Lundell, G. E. F.: Bright. H. A,: Hoffman. J. I. Applied Inorganic Analysis, 2nd ed.: Wlley: New York. 1953: Chapter 45. Kolthoff, I. M.: Stenger, V. I. Volumetric Analysis: Interscience: New York, 1947; Vol. II, pp 143-144.
LARGE VAT OF NITRIC AND SULFURIC ACIDS, PERHAPS CONTAINING ONE DISSOLVED HUMAN
111
I"
I
Withdraw samplesforthe determinationofphosphak
1
Treat with "moiybdate reagent"
PRECIPITATE OF AMMONIUM PHOSPHOMOLYBDATE
(NH~~~~PIM~~~O~OII~~~HZO I31
Dryat 110°C
l-
Redis~olveand measure the absorbance of the yellow solution.
SPECTROPHOTOMETRIC PROCEDURE
F
Heat to 400 OC to
~ ~ ~ ~ ~ o " $ ~ GRAVIMETRIC PROCEDURE
Meineke. 1896 Organization of analytical investigation.
800
Journal of Chemical Education
4
Dissolve in excess atd. ktitrate NsOH, ~ 0the use 3 xsNaOH sM.HCI to
ACID-BASETITRATION
thmugh Jones reduetar. catrh in solution ofrs ferricsalt. Titrate the ion with std.ferrous prmanganate.
PROCEDURE
REDOX TPTRATION PROCEDURE
Pemberton, 1894
Emmerton. 1887
Assumptions i. The vat contained 8000 L of acid, which had become homogeneous. ii. All samples were 100 mL. iii. Blank determinations used 100-mLsamples from a fresh supply of the same acid mixture. A hlank determination is one made on a sample that consists only of a fresh mixture of nitric and sulfuric acids without any phosphorus having been added from dissolving human bodies or any other source. Hence, if any precipitate is obtained from a blank sample, this must reflect the presence of P-containing impurities in the original acid mixture (or the presence of other impurities that gave a precipitate with molybdate reagent). iv. The man weighed 70 kg. v. The human body contains about 6.3 ppt of phosphorus. vi. All phosphorus was converted to phosphoric acid by the nitric acid. a. Assuming that there really had been one "last despairing cry", calculate the mass of phosphorus in a 100-mL sample. b. In the gravimetric procedure the final mass of PzOs24 Moos was 0.3718 g. Its molecular weight is 3596.46. The mass of P205.24 Moos isolated from a hlank sample was 0.0331 g. c. The redox titration procedure is a determination of molybdenum in the precipitate, and assumes that the composition has the 121M O Pratio. The Mo in the precipitate bas a n oxidation state of VI, and when the solution is passed through the Jones reductor (a column of amalgamated zinc metal) the only redox change that occurs is
H,MoO,
+ Zn(Hg)
-
Ma3'
+ Zn2+
-
[unbalanced]
Finally, the ferrous ion produced by this procedure is titrated with a standard solution of potassium permanganate, using the reaction Mn0,-
-
+ Fez+
MnZt + Fe3+
+ HP0;- + 12 MOO?-
By titrating any slight excess NaOH with standard HCl to the point where the color of phenolphthalein just disappears, the ions are left in the states shown in this equation. After adding 50.00 mL of 0.1000 M sodium hydroxide, i t required 9.06 mL of 0.1000 M HC1 to reach the endpoint. Acceptable Solutlon a. Total quantity of P in vat = 70 X
lo3 g hody weight X
We need to determine the mass of P contained in 0.3387 g of P20~24Mo0,
x-
2 mol P 3 2 . 0 = 6,03 mol P2O5.24MoO3 mol P
Thus, the widow's claim is confirmed. c. First, the redox equations must be balanced: 2[H2Mo0, + 6H'
+ 3eC 3[Zn
2H2Mo04+ 3Zn + 12H'
--
Ma3++ 4H20] Zn2+ Ze]
+
ZMo3++ 3Zn2++ 8H,O
Thus, we can relate the number of moles of permanganate used to the number of moles of P in the sample: 1 mol Mn04- X 3 mol - Fez+ X 1mol Mo3+ 5 mol Fez+ 1mol Mo3+ 1m 0 1 ~ 2 ~ 0 0 ; X
[unbalanced]
I t required 21.16 mL of 0.01000 M potassium permanganate to reach the endpoint. The hlank determination required 3.55 mL. d. The spectrophotometric procedure is based on the yellow color of the phosphomolyhdate species. The precipitate was dissolved and diluted to 100 mL in avolumetric flask, and, with the solution in a 2.00-cm cell, the ahsorhance was measured at a suitable wavelength. The sample gave an absorbance of 0.542. A standard was prepared by adding 0.200 mmol of phosphoric acid to 100 mL of fresh acid mixture and was put through the same procedure. The ahsorhance was 0.648. T h e hlank ahsorhance was 0.046. e. The acid-base titration proceduredepends on the ability of a certain stoichiometric amount of sodium hydroxide being capable of transforming the precipitate as~follows: precipitate --33 NH,'
h. In a 100-mLsample, the mass of P20s.24Mo03 due to the sample is 0.3718 g from vat sample -0.0331 produced from pure acidmixture 0.3387 g P20~24Mo03
[unbalanced]
The solution of molyhdenum(II1) coming out of the zinc column flows directly into a solution of ferric sulfate, causing the rapid reaction Ma3++ Fe3+ H2MoO4+ Fez+
This mass of P would he contained in 8000 L. In 100 mL, we have
12 mol H,Mo04 -=1ma1 P
36 mol Mn0,5 mol P
In the titrationof a 100-mLsample, the amount of permanganate used is L X 0.0100 mol L-' = 1.761 X (21.16 - 3.55) X This corresponds to 1.761 X
mol
mol MnO; X-
mol 32 gP = 7.8 x 10-4 p x36 mol Mn0,m0l P
These data do not seem to confirm her claim d. For this data analysis, we need Beer's Law: absorbance = A = eC1 When the known amount of H3POIwas added to 100 mL of acid mixture, 0.200 X c=[p]=-~
mol H3P03 --~. 0.100 L
The absorbanceA attributed to added P = 0.648 - 0.046 = 0.602. Hence, we can determine r
For the sample from the vat,
6.3gP = 4 . 4 X 1 0 z g P 1000 g body weight
A = 0.542 - 0.046 = 0.498
Volume 65
Number 9
September 1988
80 1
amount of base reacting with precipitate =
Hence, in this sample,
(50.M) X 10~~L)(0.1000 mol L-')
- (9.06 X 10-~L)(O.1000L) = 4.094 X
And the quantity of P is 1.65 X
mol NaOH
Hence, the maas of P in the 100-mL sample is
32.0 g P mol P L-' x 0.100 L X - = 5.28 X mol P
gP
4.094 X
mol NaOH X
1molP 23 mol NaOH
Xw mol P
Spectroscopy supports the widow's claim. e. The equation for the reaction of the precipitate with base is (NH4),P(Mo,,0,)].l2H,O
802
-
+ 230H3NH,+ + HP0:+ 12Mo0:- + 23HZ0
Journal of Chemical Education
Thus, the titration method confirms the claim T h e net result is that, if t h e widow pressed her claim today, she would collect t h e insurance money.
