GEOLOGICAL SURVEY CffiCULAR 41 

Feb1'118l'y 1949 

FIELD METHOD FOR THE 

DETERMINATION OF 

ZINC IN PLANTS 

By Laura E. Reichen and H. W. I.akin 
UNITED STATES DEPARTMENT OF THE INTERIOR 

J. A. Krug, Secretary 

GEOLOGICAL SURVEY 

W. E. Wrather, Director 

WASHINGTON, D. C. 

Free on application to the Director, Geological Survey, Washington 25, D. C. 
FIELD METHOD FOR THE DETERMINATION 

OF ZINC IN PLANTS 

By Laura E. Reichen and H. W. L8.kin 

Abstract •••• · •••••••••••••••••••••••••• Introduction •••••••••••••••••••••••••• Acknowledgments •••••••••••••••••••••.• Apparatus ••••••••••••••••••• o ••• o ••••• 

CONTENTS 

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2 

2 

2 

2 

Reagents ••••••••••• ~ ••••••• •••••••••• Procedure •••••••••••••••••••••••••••• Expe rim en tal ••••••••••••••••••••••••• References •••••••••••••••••••.••••••• 

ILLUSTRATIONS 

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Plate 1. Hand kit for field determination of zinc in plants ••••••••••••••••••• ,,,. Facing 2 

Te.ble 

2. Box for transporting reserve supplies by truck in field determination of 

zinc in plants.................................... • • • • • • • • • • • • • • • • • • • • • Facing 3 

TABLES 

1. Zinc contents corresponding to various volumes of sample solution • • •••••••••• 2. Loss of zinc by volatilization in air-dried plant material ••••••••••••••••••• 

3. Comparison of results by wet oxidation and dry a.shing of fresh vegetation •••• 4. Comparison of laboratory and field results on fresh willow leaves •••••••••• • • 

ABSTRACT 

A field method for estimating zinc in fresh plant leaves is described 

whereby samples are collected with a leaf punch and ashed directly over a flame, the zin9 in the ash then being determined with dithizone. Results 

obtained by the field method compare favorable with those obtained by the more precise laboratory method. Forty or more samples can be tested for 

zinc in a day. 

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3 

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INTRODUCTION 

. In further continuation of a study of 

biogeochemical prospecting for zinc (Robirison, La~in, Reichen),l/ a field test for zinc in 

plant leaves has been devis ed that makes 

possible on-the-spot investigations in mineralized areas. The study of plant tissues 

offers numerous advantages for geochemical 

prospecting: Their composition may reflect 

the composition of the soils on which they 

grow (Robinson, Edgington, 1942); they may 

concentrate some elements abnormally (Williams, 

Lakin, Byers); and their extended root systems 

often sample large areas not readily studied 

by sampling the soils (Robinson, Edgington, 

1943). Almost always present, vegetation thus 

offers a means of studying the increasing or decreasing concentration of an element in the 

soil over extended areas. 

To be successful, a field test must be as simple as possible and require a minimum 

of reagents and apparatus. The usual methods 

of effecting the so1ution of vegetation, 

which require the weighing of air-dried or oven-dried samples and digesting with nitric 

and perchloric acids (Piper) or ashing at a controlled temperature, are time consuming 

and impractical for field use. Acids, 

moreover, are difficult and hazardous to 

transport. In the field test described below 

fresh vegetation is used, the sample being 

measured by leaf area (Harley, Lindner); the 

material then is ashed in a dish over a direct flame, and the zinc is estimated by an adaptation of a simple field method used 

for soils (Lakin, Stevens, Almond). 

ACKNOWLEDGMENTS 

The authors are indebted to Fred N. Ward 

for his assistance in the laboratory analyses~ 

to Rollin E· Stevens for advice and encouragement, and to Helen Cannon for aid in obtaining sui t'able samples. 

APPARATUS 

In field testing it is desirable to 

have reagent and equipment kits that contain 

everything needed yet are easily moved from 

place to place. The hand kit for estimating 

zinc in vegetation is shown in plate 1. It 

contains: 

20 pyrex test tubes, 18 by 150 millimeters, marked at 3, 10, _and 

11 milliliters. 

20 pyrex test tubes, 16 by 150 mlllimillimeters, marked at 5 milliliters. 

1 rack for test tubes. 

1 250-milliliter pyrex glassstoppered bottle for 

dithuone solution. 

1 250-millili ter_.Pyrex glass-stoppered 

bottle for buffer solution. 

1 250-milliliter pyrex glass-stoppered 

bottle for hydrochloric acid. 

2 250-milliliter pyrex glass-stoppered 

bottles for water. 

1 50-milliliter pyrexglass-stoppered 

bottle for sodium _thlosulfate. 

lJ A full list of publications oited will be 

found on P• 4• 

1 50-milliliter pyrex glassstoppered bottle for zinc 

standard B .• 

1 5-milliliter graduated pipette. fitted with stopcock at upper end. 

1 test tube to support graduated 

pipette. 

1 3-milliliter pipette. 

1 camel's -hair brusn. 

10 platinum or nickel crucibles, 

4 centimeters in diameter and 

2 centimeters high. 

1 pair platinum-tipped crucible tongs 

(if platinum dishes are used). 

1 porcelain plate on which to set 

platinum crucibles after ashing. 

1 leaf punch, Fisher Scientific Co. 

No. 2-846, cutting a disk 

1 square centimeter in area. 1 fused-quartz grating to place over burner. 

Corks. 

For ashing the samples a Coleman pocket 

stove is satisfactory~ Reserve supplies are transported by truck in a box illustrated in 

plate 2. 

REAGENTS 

Water.--Distilled in an all-pyrex still or passed through a resin demlneralizer such as the Bantam manufactured by Barnstead Still 

& Sterilizer Co. 

Acetate buffer.--Dissolve 248 grams 

sodium acetate (NaC2H302.3H20) in 900 milliliters of water. . Add ~1 milliliters glacial 

acetic acid and make up to 1 liter. Remove 

reacting heavy metals by shaking with 0.01-

percent dithizone solution. 

Sodium thiosulfate.--50 grams 

Na2S203.5H20 in 100 milliliters of water. 

Standard zinc solutions.--Solution A: 

0.01 percent in lN hydrochloric acid. Dissolve reagent-grade 30-mesh zinc in a slight excess of hydrochloric acid and dilute to 

volume. Solution B: 5 micrograms per milliliter. Add 35 milliliters acetate buffer and 

5 milliliters sodium thiosulfate to 2.5 millin.ter standard solution A and dilute to 

50 milliliters with lN hydrochloric acid. 

This solution should be made fresh daily. 

Carbon tetrachloride.--Purify reagentgrade carbon tectrachloride by distillation 

in an all~pyrex still. 

Hydrochloric acid.~Prepare lN hydrochloric acid from constant boiling hydrochloric acid distilled in an all-pyrex still. 

Dithizone solution.--0.0025 percent 

(weight per volume) in pure carbon tetrachloride. 

PROCEDURE 

Collect 20 disks (20 square centimeters) 

from the leaves of the plant with the leaf 

punch. Place these disks in a platinum 

(or nickel) dish and ash directly over the 

flame of the Coleman pocket stove, heating 

only long enough to burn the samples com- pletely. Transfer the ash to a test_ tube 

(18 by 150 millimeters)· with a camel 1 s-ha1r 

brush and rinse the dish with a little lN 

Plate 2.--Box for transporting reserve su~plies by truck in field determination 

of zinc in plants. 
hydrochloric acid. Dilute to 3 milliliters 

with lN hydrochloric acid, add 7 .milliliters 

acetate buffer and 1 milliliter sodium thio- sulfa~e, stopper with a clean cork, and mix 

by shAking. 

Prepare a standard by shaking for 1 

minute 1 milliliter of standard solution B 

(5 micrograms of zinc) with 5 milliliters of 

the dithizone solution in_ a test tube (16 by 

150 millimeters). The standard must be 

prepared frequently because the color fades. 

Put 5 milliliters of the dithizone 

solution in another test tube and add the 

sample solution in increments of 1 milliliter, 

shaking vigorously for 1 minute after each 

addition until' the color of the carbon 

tetrachloride layer matches as nearly as possible that of the standard. If less than 

1 milliliter of sample solution is needed 

to match the standard, dilute 1 milliliter of 

sample solution tenfold with water and 

determine the volume of this diluted solution 

needed to match the standard. 

Volumes of sample solution needed to 

match the standard correspond with the 

following zinc values for the sample: 

Table 1.--Zinc contents corresponding to 

various volumes of sample solution 

Milliliters required 

for closest matching of standard 

Original sample 

solution 

5 

4 

3 

2 

1 

Diluted sample 

solution 

6 

5 

4 

:3> 

2 

1 

Zinc con tent 

Micrograms 

·per 100 

square 

centimeters 

fresh 

material 

50 

60 

80 

125 

250 

400 

500 

600 

800 

1,250 

2,500 

Approximate 

micrograms 

per gram 

air-dry 

weight 

(parts 

per 

million) 

100 

120 

160 

250 

500 

800 

1,000 

1,200 

1,600 

2,500 

5,000 

"Micrograms per areaR is a comparatively 

unfamiliar manner of expressing trace-element 

content of plant material. For orientation 

purposes, therefore, these calculations are given both in micrograms per gram (parts per 

million) and in microgr~s per 100 square 

centimeters. Because of the variation irt the 

leaf-structure density of different kinds of 

plants, no accurate conversion from area to 

weight basis can be made; however, it has 

been found that multiplying the micrograms 

per 100 square centimeters by 2 will give 

the approximate micrograms per. gram. 

EXPERIMENTAL 

To determine the loss of zinc in ashing t~e plant materlal over a direct flame, 

3 

leaves were decomposed both by ashing and by 

wet oxidation with nitric and perchloric 

acid, and the zinc in each was then determined 

by the Holmes dithizone method (Holmes). 

Table 2 shows comparative data by wet oxidation and by ashing in platinum dishes, of 

samples of weighed, finely ground, air-dried 

plant material. 

Table 2.--Loss of zinc by volatilization in 

air-dried plan t material 

Zinc content 

(parts per million) 

Plant and After After Percent 

sample wet dry zinc 

number oxidation ashing lost 

Ragweed (1) 2,100 1,800 14 

Ragweed (2) 40 27 23 

Poplar (1) 1,800 1,500 16 

Poplar (2) ].40 120 14 

Horsetail ( 1) 5,300 3,400 36 

Horsetail ( 2) 70 18 72 

Table 3 shows comparative data by wet oxidation and by ashing in both platinum and 

nickel dishes, of samples of _fresh green 

leaves. 

-

Table 3r-Oomparison of results by wet oxidation and dry ashing of fresh vegetation.* 

Plant After wet After dry ashi~g 

oxidation In platinum In nickel 

Micrograms Zn in 5 gm. fresh material 

Zinc 

weed 44 52, 56, 38 -- Maple 58 66, 56, 60 -- Chrys- ~ 

anthemums 40 35, 27, 41 -- Micrograms Zn per 100 sq. em. 

Dogwood 50 50- 50-, 50-

Hickory 160 125 100 

Poplar 110 lOO 80; 80 

* The figures in horizontal rows represent 

determinations made on separate replicate 

samples. 

Platinum dishes are not ideal equipment 

for a field test because of the initial 

expense and the possibility of loss. Good 

results were obtained with nickel (table 3), 

inasmuch as the oxide coating formed during 

the ashing seems to prevent the nickel, which 

would interfere with the dithizone estimation 

of zinc, from being dissolved py the hydro- · chloric-acid rinse. Ashing in nickel takes 

longer, however, and because of greater volatilization may account for the slightly lower zinc 

content indicated for the samples ashed in 

nickel. Porcelain dishes are not satisfactory 

because of heat transfer and consequent slower 

burning, and pyrex test tubes are unsuitable in 
that they do not allow free access of oxygen. 

The loss of zinc through volatilization 

is not considered sufficiently large to 

affect the geochemical pattern of zinc content. 

In table 4 the results of the field 

method performed in the field are composed 

with the laboratory analyses of duplicate 

samples using nitric-perchloric acid digestion 

and determination of zinc by the Holmes 

method. The laboratory "micrograms per gram" were calculated from the air-dry weight of the 

20-square centimeter sample and the "micrograms per 100 square centimeters" directly 

from the area of the sample. The weight of 

the duplicate samples was assumed to be the 

same, and the field "micrograms per gram" were calculated from the air-dry weight of 

the corresponding sample used for laboratory 

analysis. 

Table 4.--comparison of laboratory and 

field results on fresh willow leaves 

Micrograms per gram Micrograms per 100 

Sample (parts per million) sq, c_m. 

Laboratory Field Laboratory Field 

1 370 190 120 60 

2 230 180 105 80 

3 210 170 105 80 

4 320 3l0 130 125 

5 270 210 160 125 

6 230 190 150 125 

7 780 630 310 250 

8 900 1,100 260 300 

9 470 53-0 310 300 

10 870 750 480 400 

11 750 l,OOO 300 400 

12 950 1,200 380 500 

13 930 1,000 560 625 

14 900 1,800 320 625 

15 1,100 1,300 780 830 

16 2,400 2,300 1,300 1,250 

Examination of table 4 indicates that th~ errors involved in the field test are not 

sufficiently. large to obscure the basic 

geochemical pattern. It is felt, therefore, 

that the test may prove useful in biogeochemical. prospecting. 

A discussion of the sampling of plant 

material for purposes of biogeochemical 

prospecting and an application of the field 

method for zinc will be published later. 

REFERENCES 

Harley, c. P., and Lindner, R. 0., -A rapid 

method for determination of nitrogen in 

plant tissue: Science, vol. 96, 

pp. 565-566, 1942. 

Holmes, R. s., Determination of total copper, 

zinc, cobalt and lead in soil and soil 

solutions: Soil Science, vol. 59, 

pp. 77-84, 1945. 

Lakin, H. W., Stevens, R. E., and Almond, Hy, 

Field method for determination of zinc in 

soils (manuscript in preparation). 

Piper, C. s., Soil and plant analyst~ 

pp. 258-275, New York, Interscience 

Publishers, 1944. 

Robinson, W. 0., and Edgington, G., Boron 

content of hickory and some other t~ees: 

Soil Science, vol. 53, pp. 309-312, 1942. 

Robinson, W. 0 ·., and Edgington, G., 

Occurrence of rare earths in plants and 

soils: Soil Science, vol. 56, pp. l-6, 

1943. 

Robinson, w. o., Lakin, H. ·w., and Reichen, 

L. E., Zinc content of ·plants on the 

Friedensville slime ponds in relation to 

biogeochemical prospecting: Economic 

Geology, vol. 42, pp. 572-582, 1947. 

Williams, K. T., Lakin, H. W., and Byers, H. G., 

Selenium in certain soils in the United 

States with a discussion of related topics, 

5th report: u. s. Dept. A.gr. Tech. Bull. 

758, P• 57, 1941.