Determination of Nitrates 

in Saline Minerals

By LAURA E. REICHEN and JOSEPH J. FAHEY

CONTRIBUTIONS TO GEOCHEMISTRY

GEOLOGICAL SURVEY BULLETIN 1214-F

A quantitative method of analysis 

based on the stoichiometric relation 

of the nitrates to the magnesium oxide 

that is formed when a sample is heated 

to 700°C in the presence of an excess 

of magnesium sulfate

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1968
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 - Price 10 cents
CONTENTS

Page

Abstract ____________________________________. Fl

Introduction ___________________ __________ __. 1

Procedure ________________________ _____ _ . 1

Discussion and results ________________________ __. 2

TABLE

Page

TABLE 1. Determination of nitrates __________-_____________ F3

III

CONTRIBUTIONS TO GEOCHEMISTRY

DETERMINATION OF NITRATES IN SALINE MINERALS

By LAURA E. REICHEN and JOSEPH J. FAHEY

Abstract

The alkali nitrate is heated in the presence of magnesium sulfate for 40 

minutes at 600°C, whereby magnesium oxide is produced in stoichiometric 

relation to the NnOs present. The amount of magnesium oxide is measured 

by titration with sulfuric acid, and the equivalent N2 0s is calculated.

INTRODUCTION

When an aqueous solution of sodium nitrate and magnesium 

sulfate is evaporated to dryness and the residue heated to 600°C, 

magnesium oxide is formed: 2NaN03 + MgS04 -* MgO + N205 + 

Na2S04 . As is shown by the equation, the magnesium oxide bears 

a stoichiometric relation to the nitrate in the sample. A careful 

search of the literature revealed no previous reference to this 

relation.

If a lake water is concentrated by evaporation it deposits its 

saline constituents. These minerals consist mostly of chlorides, 

nitrates, sulfates, carbonates, bicarbonates, and borates of sodium, 

potassium, calcium, and magnesium. Of these saline minerals, the 

only ones which need be considered in this procedure are those 

that are soluble in water, as all the others are easily separated 

from the nitrates by a water leach.

PROCEDURE

Prepare the following reagents:

MgS04 :10 percent solution

Standard H2S04 : 0.1 N

Standard NaOH: 0.1 N

Ag2S04 : Hot, saturated solution (about 1.5 percent) 

Take a sample of such size that no more than 250 mg of N2 0S 

is present. (A full 50-ml burette of 0.1 N H2S04 for the titration

Fl
F2 CONTRIBUTIONS TO GEOCHEMISTRY

of MgO is equivalent to 270 mg N205 .) Treat the sample in a 

small beaker with 25 ml of water at room temperature, filter, and 

wash into a porcelain dish. This removes all insoluble material 

from the readily soluble nitrate of the sample.

Add a drop of methyl red and neutralize carefully, making the 

final adjustment with dilute sulfuric acid or sodium hydroxide so 

that the solution is exactly neutral. If the sample contains 

carbonate, sweep carbon dioxide-free air through the system 

during neutralization until it is free of carbon dioxide.

If chlorides are present, gradually add hot silver sulfate solution, while stirring, directly to the nitrates in the dish until the 

silver chloride precipitate coagulates and no more precipitate 

forms. Digest briefly on the steam bath until the precipitate 

settles; it is not necessary to filter off the silver chloride.

Add 10 ml of the magnesium sulfate solution to the neutral 

alkali nitrate and evaporate to complete dryness so that there will 

be no spattering or decrepitation when the dish is heated over 

a flame. Heat the dish over a flame being careful not to exceed 

600 °C until no more gases seem to be coming off so that the 

muffle furnace will not be attacked unduly by the fumes. Heat at 

600 °C for 40 minutes in muffle. Add a calculated excess of standard sulfuric acid to the cooled dish and heat on the steam bath 

to dissolve the magnesium oxide. Titrate the excess sulfuric acid 

with standard sodium hydroxide using methyl red indicator. 

Calculate the percent N205 from the magnesium oxide which 

reacted with the sulfuric acid (1 ml 0.1 N H2S04 o 0.002016 g 

MgO o 0.005401 gN2 05 ).

DISCUSSION AND RESULTS

The optimum temperature for the formation of magnesium 

oxide, stoichiometrically equivalent to the nitrate of the sample, is 

600°C. At 700°C, magnesium sulfate alone produces a few milligrams of magnesium oxide, and below 600°C the nitrate-magnesium oxide reaction is incomplete. At 500°C, 98 percent of the 

nitrate present reacts with the magnesium sulfate to produce 

magnesium oxide; at 400°C, about 93 percent reacts, and at 300°C 

only 64 percent reacts.

The stoichiometric relation of the magnesium oxide formed to 

the nitrate present in the sample was demonstrated by using 

sodium .nitrate, potassium nitrate, and known volumes of standard nitric acid (table 1). The nitric acid was neutralized with 

sodium hydroxide before the addition of the magnesium sulfate. 

An excess of about 1 g of magnesium sulfate was added before 

evaporation.
DETERMINATION OF NITRATES IN SALINE MINERALS F3 

TABLE 1. Determination of nitrates, in grams

Source Mg° found N2°s teken N2°5 £ound

HNOi

KNO«

NaNOa

0.0040

.0040 

.0403 

.0403 

.0806

_ _ .0403

.0403 

.0806

_ _ .0040

.0403 

.0806

0.0041

.0041 

.0404 

.0409 

.0806

.0401

.0400 

.0813

.0037

.0399 

.0805

0.0108

.0108 

.1079 

.1079 

.2159

.1079

.1079 

.2159

.0108

.1079 

.2159

0.0110

.0110 

.1082 

.1096 

.2159

.1074

.1072 

.2178

.0099

.1067 

.2157

Humberstonite l _____ .0341 .0340 .0914 .0910

1 Ericksen, G. E., Fahey, J. J., and Mrose, M. E., 1967. Humberstonite, Na7K3Mg2 (SO4 )e 

(NO3) 2 .6H2O: A new saline mineral from the Atacama Desert, Chile [abs.]: Geol. Soc. 

America Program 1967 Ann. Mtjfs., Nov. 20-22, ia67, New Orleans. La., p. 69.

Determining the magnesium oxide by filtering and weighing is 

difficult because it sticks so tenaciously that the dish cannot be 

scrubbed clean. Also, the magnesium oxide contains a small 

amount of magnesium sulfate in spite of all attempts to wash it 

out. The magnesium oxide is best measured by dissolving it in a 

known volume of standard sulfuric acid and titrating the excess 

acid with standard sodium hydroxide.

The solution must be neutralized with sulfuric acid before 

evaporation; any carbonates or bicarbonates present will be 

converted to sulfates and, as such, will not interfere. The alkali 

chlorides react with magnesium sulfate and produce magnesium 

oxide stoichiometrically. Either the total magnesium oxide from 

the nitrate and the chloride can be measured and a correction 

made for the chloride present, or the chloride can be inactivated 

by precipitation with silver sulfate. It is not necessary to filter 

off the precipitated silver chloride.

Although nitrites do not occur as minerals, and therefore are 

not listed as interfering substances, they react with magnesium 

sulfate stoichiometrically, as do nitrates, and thus could be quantitatively measured. Sodium iodate does not react with magnesium 

sulfate to form magnesium oxide, but it is decomposed below 

600 °C and would interfere in the determination of nitrates.

Sodium borate was used to simulate the soluble alkali berates 

(borax, tincalconite, and kernite). It was heated to 600°C with 

magnesium sulfate and then allowed to cool to room temperature;
F4 CONTRIBUTIONS TO GEOCHEMISTRY

all the salts present were soluble in water, indicating that no 

magnesium oxide was produced. However, when standard sulfuric 

acid is added and back titrated with standard sodium hydroxide, 

it is found that sulfuric acid, equivalent to as much as 2 mg of 

magnesium oxide, reacted with the sample. This is equivalent to 

as much as 5 mg of nitrate. The error is not dependent on the 

amount of sodium borate present. Many determinations have 

shown that sodium borate does not cause erroneous results if 

5 mg or more of nitrate is present in the sample.

* U.S. GOVERNMENT PRINTING OFFICE : 1968 O - 313-756