ARTICLES A Laboratory Exercise on Semiquantitative Analysis of Ions in Nutrient Solutions David R. Hershey* and Gary W. Stutte ABSTRACT NUTRIENT SOLUTIONS Astudent laboratory exercise on the developmentof nutrient Nutrient deficiency solutions for N, P, K, Ca, Mg, S, deficiencyand toxicity symptomswas linked to analytical tech- Fe, Mn, Cu, Zn, B, and Mo were prepared according to niquesand ion chemistryby havingthe studentsdetermine which Hoagland and Arnon (1950) except that Fe was provided ions in the nutrientsolutions were missing or presentin excess. as Fe-EDTAat 90/~M Fe, and CaCI2 was used instead Qualitativeanalytical methodsfor CI, Ca, Mg,Na, K, Mn,S, of CaSO4for the minus-N solution. The complete solu- ammonium,and nitrate were madesemiquantitative by using tion was a Hoagland Solution Number 1. standards.Quantitative analytical techniquesfor B, Fe, P, and Various element toxicities were induced by adding one bicarbonatewere simplified. Mostreagents and samples were of the following: 460 #M B(OH)3, 90/aM MnCI2, in dropperbottles, allowingstudents to rapidlyrun the tests mMNaC1, or 20 mMNaHCO3. The toxic levels of B by addingthe propernumber of dropsof sampleand reagent(s) and Mn were ten times the normal Hoagland Solution to a test tube. Formost elements, a color changeor precipita- tion denoteda positive test. Mostreagents have stored well for Number1 level. 2.5 yr, minimizingreagent preparation after the 1st yr. Ammoniumtoxicity was induced by growing plants in a modified Hoagland Solution Number1. The solution for ammoniumtoxicity had CaCl2 substituted for LASS LABORATORYEXPERIMENTS in hydroponics, the Ca(NO)3, K2SO4 substituted for KNO3,and 15 mMam- C soilless culture of plants, are popular for demon- monium as (NH4)2SO4. strating plant nutrient deficiency symptoms(Epstein, Extra volumes of all but the minus-Zn, -Cu, and -Mo 1986; Moore,1974). Generally, nutrient deficiency symp- solutions were prepared for student analysis. Analytical tom laboratories are mostly observational in nature with techniques for Zn, Cu, and Mo were not attempted be- students focusing on symptom development. Tissue or cause the low concentrations of these elements are either nutrient solution analysis to confirm the nutrient defi- difficult to detect with simple techniques or difficult-to- ciency is usually not done due to a lack of analytical handle reagents are needed. Zinc analysis requires organic equipment. However, visual nutrient deficiency symp- solvent extraction with carbon tetrachloride (Epstein, toms have limited usefulness as a diagnostic procedure. 1986), Cu analysis requires toxic cyanides, and Mo Introductory inorganic chemistry laboratories often analysis requires atomic absorption spectrophotometry perform qualitative analysis of ions in solution whenstu- (American Public Health Association, 1985). The all- dents learn ion chemistry in a hands-on setting. A weak- nitrate-N, all-ammonium-N, nine deficient, and four ness of this classic qualitative analysis laboratory toxic nutrient solutions were assigned numbersand placed experience is that the ion solutions analyzed, particularly in 30-mL-glass-dropper bottles. the unknowns,are artificial. In the real world, plant scien- tists knowwhat they are analyzing be it irrigation water, fertilizer solution, plant tissue, or soil. PLANTS In order to extend the learning experience in a plant Leaf cuttings of Tolmiea menziesii (Pursh) Torr. & nutrient deficiency laboratory and link it more closely to Gray were rooted in 2 mMCaC12 in solution culture analytical chemistry, qualitative analysis laboratory tech- (Hershey,1989), placed in plastic jars in continuouslyaer- niques were utilized to analyze nutrient solutions. The ob- ated nutrient solutions, and set on a greenhouse bench jective was to determine if an ion or element was present for symptom development. The aeration manifold was at deficient, adequate, or toxic levels. like that of Hershey and Merritt (1986) as modified Sadof and Hershey (1989). There were four replications Department of Horticulture, Univ. of Maryland, College Park, MD per treatment, but not all jars were labeled for the stu- 20742-5611. Received 7 May 1990. *Corresponding author. dents. The unknownjars were labeled for the instructors, Published in J. Agron. Educ. 20:7-10 (1991). and wereused later to test the student’s ability to discern J. Agron. Educ., Vol. 20, no. 1, 1991 7 the identity of the deficient or toxic element via symptom Table 1. Directions for achieving desired molarity of reagents identification and solution analysis. The seed-propagated necessary for the nutrient solution analysis laboratory. The in- crops more often used in nutrient deficiency laboratories, structions for each test are given in the text. such as tomato (Lycopersicon esculentum Mill.) (Epstein, Reagent Preparation 1986) or sunflower (Helianthus annuus L.) (Moore, 1974) Chloride could also be used. The houseplant Tolmiea, knownas 17 g AgNO3L-1 0.1 .~/AgNO3 -1 piggyback plant, was chosen for a change of pace and 10 mMNaC1 584 mg NaCI L because its rosette form requires no staking. Sulfate 1 MBaC12 244-1 g BaC12.2H20 L 1 mMH2SO4 56-1 #Lconc. H2SO4L ANALYTICAL PROCEDURES Phosphate Reagent A 12 g ammoniummolybdate [(NH4)6Mo7024-4H20]in 250 mLH20 and Qualitative analysis procedures for Cl, Ca, Mg, Na, 0.3 g antimony potassium tartrate K, Mn, ammonium,nitrate, and sulfate (Masterton and (CaH4K2Sb2012’3H20)in 100 mLH20 are Slowinski, 1978) were used with modifications. Standard added to 1 L of 2.5 M H2SO41148 mL conc. H2SO4 L-1L mixed well, and qualitative analysis procedures usually utilize samples brought to 2 L with 20 mMconcentrations of each ion. This presented Reagent B 0.25 g ascorbic acid in 50 mLof Reagent A Imake fresh) sensitivity problemsfor certain ions since a HoaglandSo- 1 mMKH2PO4 136-1 mg KH2PO4L lution Number1 has ion concentrations of 15 mMorless Nitrate (Hoagland and Arnon, 1950). To overcome the problem, 18 M H2SO4 conc. H2SO4 solutions for the least sensitive tests, Mnand Mg, were 0.2 MFeSO4 2.8 g FeSO4-7H20in 50 mL(make fresh) 10 mM KNO3 1.01 g KNO3L-1 evaporated to dryness in an oven and reconstituted to Ca one-tenth of the original volume prior to class. With the 6 M NH40H 400-1 mL 30% NH4OHL 1 Mpotassium oxalate 184-1 g K2C204.H20 L exception of C1, the other tests were sensitive enoughto -1 detect deficient or adequate ion levels. The C1 test was 5 mMCaCl2 735 mg CaC12.2H20 L Mg only used to detect a Cl excess. 6 M HC1 500-1 mLconc. HC1L Quantitative analytical procedures for B (John et al., Mgreagent 0.1g p-nitrobenzene-azoresorcinol L-1 of 25 mMNaOH I1 ~ L -11 1975, bicarbonate (Chapmanand Pratt, 1961), phosphate 6 M NaOH 240 g NaOH L-~ (Watanabeand Olsen, 1965), and Fe (Chaney et al., 1972) 5 ram MgSO4 1.23-1 g MgSO~.7H20L were simplified by minimizing the solution volumes uti- Ammonium 6 M NaOH 240 g NaOH L-1 lized, measuring reagent volumes by drop-count instead 10 mMNH4C1 535-1 mg NH4C1L of pipettes, and visually estimating solution color instead K of using spectrophotometers. 10 mMKC1 746 mg KC1L- 1 For the majority of tests, 10 drops of sample are added Na to a 12- by 75-mm-glass-test tube and the appropriate 10 mMNaCl 584 mg NaCI L-1 Mn number of drops of one or more reagents added. A posi- 6 M HNO3 400-1 mL conc. HNO3L tive test is indicated by a changein solution color or for- Sodium bismuthate -1Solid NaBiO3 mation of a precipitate. Sections of 512 square, plastic 100 ~ MnCl2 20 mg MnCI2.4H20 L plug flats, used for seed germination in bedding plant Buffer masking reagent 125 g ammoniumacetate (CH3COONH4) production, are utilized as inexpensive racks for the 12- and 7.5 g Na-EDTAin 200 mLH20; by 75-mm-disposableotest tubes. Each plug cell is 14 mm slowly add 62.5 mLglacial acetic acid Azomethine-H reagent 0.45 g Azomethine-Hand 1 g ascorbic acid wide and 21 mmdeep. The flats are available from in 50 mLH20 (make fresh) Maryland Plants and Supply, Baltimore, MD.Most rea- 500 ~/B{OH)a 31 mg B(OHh -1 gents and nutrient solutions are in 30-mL-dropperbottles. Fe Standard solutions of specified concentrations are avail- 2.5 mMTPTZ (2,4,6 tri- pyridyl-s-triazine) 780 mg TPTZ L-1 able for each ion and allow the student to judge if the 4 M sodium acetate 544-1 g CHaCOONa-3H20L nutrient solution concentration is higher or lower than 2 M HC1 165 mL conc. HCI L-1 Ascorbic acid Solid the standard. Deionizedwater is available in dropper bot- 100 ~ Fe-EDTA 36.7-1 mg Fe-EDTA L tles so students can determine what a negative test result Bicarbonate looks like. Preparation of reagents for each test are sum- Phenolpthalein indicator 2.5 g phenolpthalein L-1 of 1:1 ethanol~water marized in Table 1. Details for each test are described Methyl orange indicator 1 g methyl orange L- 1 below. 25 mMH2SO4 1.39-~ mLconc. H2SO~L -1 Chloride. To 10 drops of sample add two drops of 0.1 10 mM NaHCO3 840 mg NaHCOaL M AgNO3.A cloudy white precipitate of AgCl indicates the presence of chloride. The dropper bottle of AgNO3 solution is wrapped in aluminum foil to prevent pho- Sulfate. To 10 drops of sample add three drops of 1 todegradation of AgNO3. A 10-mM-NaCl solution is MBaCl2 and mix well. A white, finely divided precipi- used as a standard. This test is not sensitive enough to tate of BaSO4 indicates the presence of sulfate. A 1-mM- detect the normal 18/d~/-chloride concentration in H2SO4 standard is used. Hoagland Solution Number 1 Hoagland Solution Number1, so it is used to detect an has 2 mMsulfate. excess of chloride. Phosphate. To one drop of sample add four drops of 8 J.