Edited by Gerard Baruch Lackey High School Indian Head. MD 20640
Soil Analysis for High
School Chemistry
Students
Mary A. Eisenmann J. J. Pearce High School 1600N. Coit Road Richardson, Texas 75080 Checked by: R o b e r t Stephenson, Cherry Creek School District Aurora, Colorado Neal Haywood, Arlington, Virginia Jerry L. Sarquis, Mark Raisch, Patricia D. Brown, Miami University-Middletown, Middletown, OH This lab can he done near the end of a student's first-year chemistrv course to eive annlication of several tmics .. ~ractical . .. discussed during the year. Thesr tupics include acid-hnse reactions. nH. ~>xidntion-redl~ction. orrcinitatim and solubility. he &dent should know how soiuhility provides a mechanism for the transportation of ions that plants need. Many of the student's lab skills which helshe developed earlier in the year are tested in this lab. These include working with a concentrated acid, collecting a gas by water displacement, identifying a gas by burning split tests, filtering a mixture, and testing to determine pH. This lab fits well with the lab experiments in "Modern Chemistry" by Metcalfe, Williams, and Castka, because i t uses similar lab tests and equipment. Students will examine the soluhilitv of nitrate ions. the insoluhilir~oi i r m hydroxide and in," carhonntr. and the reaction herween acid and carbonnres. This In11 was desianvd for an alkaline soil with a high carhonate content s u c h i s is found around Dallas, Texas, hut it can be adapted to other areas (see the Special Notes Section). Discussion Plants need nitroeen and nhos~horusto ~ r o d u c eamino acids. These amino acids are used to build theplant's protein. Commercial lawn and aarden fertilizer are characterized bv three numbers which igdicate the percent of nitrogen, phosphorus, and potassium present, in that order. For example, a 15-5-10 fertilizer has 15% N, 5% P and 10% K. Nitrogen stimulates above ground growth such as leaves, phosphorus stimulates root and fruit growth, and potassium promotes stamina in the plant. The nH of the soil is also imnortant because it controls the types of planti that can grow. hlmt plants require slightly basic soil. If the soil nH is t w hniic. necessar!. mctal ion> like iron may form insolible hydroxides or carbonates which are not assimilated by the plant. If the .bH is too acidic, the nutrients are so soluble that they are easily leached from the top soil or permeate so deeply into the ground that they become unavailable to the plant. Wine William R.. "Chemistry for the Consumer," Allyn and Bacon, B&m, 1978, pp. 157-160.
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A
I
.e "irk
.,
Various sul~stancescan In! added to the \oil to adjust its pH. Hasir soils can he ;at lea41 oartialls neutralired hv rhr addition of sulfur compounds, oak leaves, pine needle:, citrus fruit peels, or nut shells. If the pH is too acidic, CaC08 (limestone) or wood ashes can he added to raise the pH? A number of these important soil variables will be tested in this experiment, i.e., pH, carhonate, iron and nitrate content. (Nitrate is the form in which plants take up nitrogen from the soil.) Equipment and Materials Needed Amounts listed are per student or group of students.
watch glass 3 15 X 125 rnm test tubes
funnel (Buchner funnel, side funnel (Buchner funnel, side arm flask,and aspirator, if available) filter paper R wooden splints matches stirring rod 25 ml graduated cylinder
forgosgenerator and collector 2 gas collecting bottles 2-hole stopper to fit collecting bottle funnel tuhe 20 em glass bend 40 em piece of rubber tubing pneumatic trough glass plate (approx. 2 X 2 in.) 3 small test tubes and stoppers
eye droppers 250-ml glass jar that can bediscarded at end ofhb (studentscan bring their own) 250 ml soil (100g max) 6 M HCI (30-50 ml for ave. sail) distilled water (125ml) litmus solution bromthymol blue solution (can be made from solid by dissolving 0.1 gin 8 ml of 0.02 M NaOH and diluting to 250 ml with water)
blue litmus paper (4 pieces) FeSOd (s) (3 g) 0.1M K3Fe(CN)6(4 ml) 0.1M K I F ~ ( C N(4 ) ~ml) limewater solution (Optional) (Note: the solubility of Ca(0H)z in water decreases with temperature, i.e., 1 g dissolves in 590 ml H20 at 15". fi30 ml H20at 25', 1300 ml boiling water. Cloudiness may result from reaction with soluble Coy;therefore,you may need to prefilter the solution before use.) Procedure Part A: Testing for a Polyatornic Ion I ) Place a sample of soil the size of pea on a watch glass. Add one drop of fi M HCI. If the soil effervesces,go to step 2 to determine the
New experiments and directions in teaching chemistry through the use ofthe labaratoryare provided in this feature. Experiments will be fully detailed and will be field tested before they are published. Contributions should he sent to the feature editor. Volume 57, Number 12, December 1980 1 897
appn,ximately 25 ml of distilled water. Obtain a two-hole stopper that will I'lt this bottle. Insert a funnel tube so that the end of the funnel tube is nearly Louching the bottom of the bottle. CAUTION: Be sure t o observe safety rules for inserting glass tubing inta stoppcrs. In the ~ltllcrh d e ~f the ,twper, msrrr 9 Lent W r m pwrr d d l w t u h i n ~tJhtnm n pnrunutic r n w h nnd a pirrcd' r u l h w tuh~hg.\Itsth tlw runhcr tul,e tu th*,Iwnt elri.tulli~l* from thr h8.ttlc. Hjll tdl the tmugh with water. CompleteG fill a 250-ml bottle with water, slide nletp rwer the invert the hottle inta the trourh of -a "u l a q ~-.-.. . ~tnn. ~ nnd . water. Insert the ather end of the rubber tubing into the battle. After the bottle is under the water level, the glass plate is removed (see figure for set-up) ~~
~
~
.~.
~~~~~
~
~
the same way for nitrate. Compare your nitrate test with another lab group's soil sample to see if the amount of nitrate is the same.
Part D: Testing for lron I) lron ions can be identified by use of potassium hexacyanoferrate (111) solution (also called potassium ferricyanide), K:,Fe(CNk or potassium hexacyanoferrate (11) (ala, called potassium fermcyanide), K4Fe(CN)6.When one of these is in the presence of the iron with the different oxidation number, KFe[Fe(CN)fi].HxO is formed. This is a very dark blue compound known as Prussian Blue.:' The two iron ions in this compound have different oxidation numbers; one is +2 and m e is +3.
+ K4Fe(CN)s+ Few
K3Fe(CN)e Fez+
-
+
KFe[FeiCN)el
+ 2K1+
KFKFe[Fe(CN)fil+ 3K1+
Using the filtrate from part B, test a sample of your soil to determine if either Fez+ or FG'+ is present. 2) Decant some liquid from the soil-acid mixture left from part A and test for Fez+ and Few. Sample Data for Basic Dallas, Texas Soll
+
1. soil HCI 2. burning splint 3. glowing splint
A.
B.
1.
2.
C.
1.
2. 3. D.
T h e end of the funnelmust be under the water level. Add 10 ml of 6 M HCI slowly thmugh the funnel tuhe. When the reaction slows down, gently swirl the bottle to bring more mil in contact with the acid. Another 10 ml of HCI can be added to keep the reaction going. Collect a bottle of gas and discard it. Then collect two ur three test tubes of the gas. Be careful to keep the gas trapped until ready to test it. Test m e sample with agluwingsplit and one with a burning split. Is the gas hydnlgen, oxygen, or carbon dioxide? What polyatomic ion must have been present in the soil to produce this gas? What further Lest can he done to identify the gas? Save this soil-acid mixture for part D.
Part B: Testing Acidity or Alkalinity Place approximately 50 gof soil intoa 250-ml glass jar that can be thmwn away a t the end of the experiment. Add 50 ml of distilled water. Stir the soil For a few minutes. let soil settle to at least half the
parts C and D. Por reference, check the pH of your distilled H 2 0 in a similar Isshion. Compare the colors of your indicator solutions with an indicator color ehart or standard solutions provided by your teacher. What was the approximate pH of your soil sample? (Teacher may uptionally choose to have students cheek pH by using pH paper.)
Part C: Testing for Nitrates CAUTION: Concentrated acids may cause severe burns upon contact with skin or eyes. Flood affected area with w a t e r f o r a t least I5 minutes. 1) Perform the following test on asolution known tocontain anilrate such as a sample of commercial fertilizer. Dissulve 0.1 g of commercial fertilize fertilizer in 100 ml of distilled water. Pour 2 ml of the nitrate solution into a test tuhe. Add 3 M HBOI m e drop at a time until the solution is acidic. Add 2 ml of freshly prepared, saturated FeS04 solution. Tilt the test tube to a 45" angle. Slowly pour 1ml of concentrated HISOl down the inside of the test tuhe. Avoid unnecessary mixing. CAUTION: Be careful with this concentrated aeid.Test tuhe may also became warm due t o heat evolved upon mixing of acid a n d water. Note the faint brown ring that forms a t the interface. This ring will form only if nitrate is present.' This reaction is
4 HIt
+ NOa'- + 3Fe'+ NO
+ Fez+
-
-
+
3FG3+ NO
+ 2 H20
Fe(NO)Z+(brown)
2) Use 2 ml of the filtrate saved from part R. Test the sample in 898 1 Joumal of Chemical Education
1,
2. 3. 4,
litmus solution bromthymol blue fertilizer nitrate test soil filtrate nitrate test soil filtrate of another lab group + nitrate test soil water K3Fe3+(CN)s soil +water K,Fe2+(CN)e soil +acid + K,Fe3+@Nle soil +acid + K42'(CN)e
+
+
+ + +
effervesces goes out
goes out blue blue brown ring brown ring brown ring no reaction no reaction no reaction turns blue
Special Notes 1) Because of all thesoil brought to the lab, this lab can be messy. Disposable glass jars are used so that they can be thrown away with some soil in them to keep students from flushing a i l into the sinks. 2) This lab will take 50 minutes if the glass tuhing has already been inserted into the rubber stopper . . and if the filtration procedure goes w~,.uthl)~ ~ SUIP e e q : i , . I t unll p r ~ l u l d vr:ake tnu 5tl.m.n p r o d < i t th, ;tudcnr. :m gc in,: tud ?:it Y IL
Pre-Lab Concepts 1) Mention that in parts of this lah (e.g., Part D) specific directions are nut given. Therefore, the students must decide what equipment is to he used. 2) Ask tu he sure students understand the value uf nitrogen t o plants. 3) Review the concept of pH with students. Find the chart in their textbook that gives pH values for colors of indicators. 4 ) Emphasize the importance of pH in making iron available t o plants. 5) Ask students why deionized water is used in the lah instead of t a p water. This is to emphasize keeping other ions out of t h e soil. 6) Review the splint tests for hydrogen, oxygen, and COr. In hydrogen a trurningsplit pops. In oxygen aglowingsplint relights. In C01 110th splints will he extinguished. COr can he more positively identified hy huhhling it through limewater, C a ( 0 H ) i this procedure may need t o he reviewed with the students.
Pre-Lab Questions
iron ion is present? Why? 4) Why is cammercial fertilizer used in the test for nitrates? 5 ) Why must deionized o r distilled water he used in this laboratwy experiment? Post-Lab Q u e s t i o n s 1) Why is the first hottle of gas collected in part A-2 discarded? 2) What pas was produced in part A-2? :3) What polyatomic ion must have heen present to produce the gas in A-2? Write t h e equation for the reaction crf this hm with url 4 ) What type of plants would do well in this soil with this pH'? 5) What could be added to this soil t o make the pH closer to neutral? fi) Was inm present in your soil? If you could not identilj, iron in hoth the s o i l h a t e r filtrate a n d t h e soil-acid filtrate, explain why. 7) If only one of Fez+ ur Fe:'+ was present, explain why.
.."..
Acknowledgment
The author wishes to thank Dr. L. A. Melton, University of Texas at Dallas, for discussions and suggestions related to this work.
Volume 57, Number 12, December 1980 / 899
