Preparation of Water Sample 

for Carbon-14 Dating

GEOLOGICAL SURVEY 

CIRCULAR 480
Preparation of Water Sample for 

Carbon-14 Dating

By H. R. Feltz and B. B. Hanshaw

GEOLOGICAL SURVEY CIRCULAR 480

iVosnin0/on 

7963
United States Department of the Interior

STEW ART L. UDALL, SECRETARY

Geological Survey

THOMAS B. NOLAN, DIRECTOR

on application to tfw U.S. Geological Survwy, WaMngton, D. C. 20242
CONTENTS

Page Page

Abstract________________ __________ 1 Apparatus ___________________________ 2

Introduction_________________________ 1 Procedure __________________________ 3

Principles of isolation _______________ 1 Reference ___________________________ 3

Reagents ___________________________ 1

ILLUSTRATIONS

Page 

Figure 1. Apparatus for isolation of dissolved-carbonate species from water _______ ______ 2

2. Percent CO2 recovery as a function of cycle time __________ ________________ 2

3. Relation of alkalinity to minimum water sample volume required for carbon-14

measurement __________________________________________________________ 2

4. Minimum volume of concentrated H2 804 required to release CO2 from water

samples of known alkalinity_____________________________________________ 3

III
Preparation of Water Sample for Carbon-14 Dating

By H. R. Feltz and B. B. Hanshaw

ABSTRACT

For most natural water, a large sample is required to provide the 3 grams of carbon needed for a carbon-14 determination. A field procedure for isolating total dissolved-carbonate 

species is described. Carbon dioxide gas is evolved by adding sulfuric acid to the water sample; the gas is then collected in a sodium hydroxide trap by recycling in a closed 

system. The trap is then transported to the dating laboratory 

where the carbon-14 is counted.

INTRODUCTION

An assessment of the carbon-14 concentration of water in various parts of hydrologic 

systems is useful in determining both direc - 

tion and rate of water movement. Current 

interest in carbon-14 measurements has necessitated the development of a field procedure for isolating total dissolved-carbonate 

species from water systems. This paper describes an adequate yet simple method for 

readily accomplishing the isolation.

The authors acknowledge the early work by 

L.L. Thatcher and C. W. Naeser on the water sample preparation technique and thank 

Irving May for measuring CO2 content of 

NaOH traps used in determining the efficiency of the apparatus shown in figure 1.

PRINCIPLES OF ISOLATION

Sulfuric acid added to water samples liberates dissolved-carbonate species as carbon dioxide gas. The evolved gas is collected 

in a sodium hydroxide trap by recycling in a 

closed system.

For most natural water, a large sample is 

necessary to provide the minimum of 3 g of 

carbon required for an age determination by 

counting. Polyethylene bags manufactured 

for use by dairy companies in transporting 

liquids are suitable for collecting largevolume water samples. These two-ply lightweight containers, designed to hold as much 

as 6 gallons each, are supported by a cardboard carton provided with the bag. Water 

is removed without bubbling because the bags 

collapse as they are emptied; thus air contamination is minimized. Polyethylene carboys bound in plywood have also been used in 

sample collection and have served as the gas 

evolution flask during carbonate isolation.

When collecting ground water, adequate 

care must be taken to reduce air contamination of the sample and to pump the well until 

a representative sample of formation water 

is assured.

The apparatus shown in figure 1 was used 

to obtain sequential evolutions of CO2 from 

carefully weighed reagent-grade Na^COg 

standards. The overall efficiency of the procedure was determined using a gas-flow rate 

of approximately 30 cc per sec, In practice, 

screw compressor clamps on rubber connection lines were used to control the gasflow rate. Percent of CC>2 recovery as a 

function of cycle time is plotted on figure 2. 

To attain a minimum of 90 percent CC>2 recovery, the minimum cycle period should be 

150 minutes when using a 5-gallon evolution 

flask and 180 minutes when using the 13-gallon carboy.

REAGENTS

1. Sulfuric acid, concentrated, reagent 

grade.

2. Sodium hydroxide, ,5N, carbonate free 

(any method listed):

a. Dissolve 200 g NaOH (reagent grade, 

carbonate free) in demineralized CO2-free 

water and dilute to 1 liter.
PREPARATION OF WATER SAMPLE FOR CARBON-14 DATING

VACUUM

DRYER

EVOLUTION FLASK

Figure 1. Apparatus for isolation of dissolved-carbonate species 

from water.

IUU

80

oe

UJ

S 60

UJ

c

z 40

UJ

o

c

UJ

a.

20

0 ,

1111 ^-^^

x^x^"T

5 GAL 7^'^X'

/' , ' I36AL

^

, ,

" /'

- /

/ ' 1 1 1 1 1

0 30 60 90 120 ISO 10

TIME, IN MINUTES 

Figure 2. Percent COj recovery as a function of cycle time.

b. Dilute 400 ml of 12. 5N NaOH (prepared reagent,carbonate-free) to 1 liter with 

demineralized CO2-free water; dilute 500 ml 

of ION NaOH (prepared reagent, carbonate 

free to 1 liter, or use 5N NaOH (prepared reagent, carbonate free).

Dissolve 1 g of SrCl2*6H2O in a minimum of 

demineralized CO2-free water and add to the 

5N NaOH solution. Stopper container tightly 

and swirl the mixture. If a precipitate forms, 

allow it to settle, and use only the clear supernatant liquid. Dispense the sodium hydroxide solution by means of a double -action 

rubber-bulb pump fitted with a drying tube 

charged with Ascarite.

3. Silica gel, indicating.

4. Gas, Nitrogen.

APPARATUS

1. a. Bag, water collection, polyethylene, 

with carton. The Scholle Container Corp. 

product or its equivalent is recommended.

or

b. Bottle, water collection, carboy, polyethylene, 6^ gal or 13 gal capacity.

2. Flask,gas evolution, glass or polyethylene, 5 gal, 6j gal, or 13 gal capacity.

3. a. Pump, sealed-diaphragm type. The 

Fisher dynapump 1-092 series is satisfactory, 

or

b. Pump, peristaltic-action type.

4. Bottle, glass 1 liter.

5. Buret, 10 ml or 25 ml. A separatory 

funnel may also be used.

6. Tubes, gas dispersion, fritted, medium 

porosity.

1. Drying tubes, gas.

1,000

o

I 500

? 300

-j 

_j

I 200

100

70

50

30

20

I T Ii 1 I

20 30 50 70 100 200 300 500 

WATER SAMPLE VOLUME, IN LITERS

Figure 3. Relation of alkalinity to minimum water-sample volume required for carbon-14 measurement.
REFERENCE

8. Tubing, glass, £ in. ID.

9. Tubing, rubber,-^ in. ID.

10. Clamp, screw-compressor type.

11. Stoppers, rubber.

12. Plastic sheet. Saran wrap has been used 

successfully.

PROCEDURE

1. Determine the alkalinity (parts per million as HCO~3> of the water to be analyzed. 

The procedure described by Rainwater and 

Thatcher (1960, p. 137-140) is appropriate. 

From figure 3, determine the minimum volume of sample required for carbon-14 measurement.

2. Introduce all or part of the water sample 

into the evolution bottle which is being swept 

with nitrogen gas. Connect the bottle to the 

apparatus shown in figure 1. Minimize the 

free air space by choosing an evolution ves - 

sel that is not excessively large; the free air 

space should be not more than half a liter. 

Pack the gas drying tubes with freshly regenerated indicating silica gel.

3. Charge the NaOH trap with at least 300 

ml, but not more than 500 ml, of 5N NaOH. 

The trap should be swept continuously with 

nitrogen gas while being filled.

4. From figure 4, compute the volume of 

concentrated HfeSC^ needed to lower the pH 

of the water sample to 4.5. The addition of 

10 percent excess of acid over the stoichiometric requirement will hasten the evolution 

and assure nearly complete reaction. Add 

acid from the buret, close the buret, start the 

pump, and allow the gas to circulate for the

1,000

ow 7°°

1 500

2 300

V)

200

100

. 70 

I 50

30

20 I MINI

\ I h

0.01 O.O2 003 0.05 0.10 020 Q30 050

MILLILITERS OF CONCENTRATED H 2 S04

PER LITER OF WATER SAMPLE

Figure 4. Minimum volume of concentrated r%SO4 required to 

release COj from water samples of known alkalinity.

time required (fig. 2) to assure nearly complete evolution and entrapment.

5. Upon completion of the COg -evolution 

cycle, seal the NaOH trap with a plastic covered rubber stopper, wire the stopper in 

place and transport the trap to the carbon-14 

laboratory.

REFERENCE

Rainwater, F. H., and Thatcher, L. L., 1960, 

Methods for collection and analysis of water samples: U.S.Geol. Survey Water-Supply Paper 1454, 301 p.

GPa 864-908