Having obtained the full texts for two old British patents related to
a simplified method for producing azides , I am sharing this
obscure and interesting information with others who may wish
to experiment with and elaborate upon this process .

GB128014 , Complete Specification ,
Improvements in and relating to the Production of Azides
application date: June 17 , 1918 No. 9952 / 18
complete accepted : June 17 , 1919

I , William Richard Hodgkinson , C.B.E. , of Ordnance College ,
Woolwich , London , S.E. 18 , Professor of Chemistry and Metallurgy ,
do hereby declare the nature of this invention and in what manner
the same is to be performed , to be particularly described and
ascertained in and by the following statement :

This invention relates to the production of azides .

The object of the invention is to provide a process of producing
azides from salts of hydrazine without the employment of
ethyl benzoate or the like , and the formation of benzoyl hydrazine .
According to the invention , salts of hydrazine are diazotised
by means of a suitable nitrite under such conditions that the
reaction mass is at no time so distinctly acid as to redden
litmus paper , but appears on the border line of acidity and alkalinity .
The conditions of acidity under which the reaction is carried out
according to the invention , are preferably comparable with the
conditions of acidity as indicated by litmus paper , of ,
for instance a solution of boric acid . Thus in accordance with
the invention , the nitrite solution is brought together with a
salt of hydrazine in approximately the proportions required on the
basis of theory to diazotise the hydrazine , under such conditions
that the acidity never exceeds the degree of acidity indicated above ,
and an acidified salt of the metal of which the azide is required ,
is introduced into the mixture , or alternatively the mixture
is added to the solution of the salt of the metal .

For example , silver azide is formed by adding a neutral solution
of hydrazine sulphate and sodium nitrite in molecular proportions
to a solution of silver nitrate of a degree of acidity with nitric acid
enabling the conditions in regard to acidity set forth above to
be realised , while avoiding the precipitation of silver sulphate .
Thus , for example 130 grams of hydrazine sulphate is converted
to the neutral hydrazine ammonium sulphate , which is more soluble
in water than is the simple hydrazine sulphate , by the addition of
ammonia solution , the mixture being made up to about 500 ccs .
To this solution contained in a vessel in which it can be violently
agitated , the calculated amount of sodium nitrite dissolved in
500 ccs. of water is added slowly so that the temperature does not
rise much above 30 degrees Centigrade .

The reaction resulting in the formation of sodium azide may be
represented by the following equation :

N2H4-H-NH3-H-SO4 + NaNO2 ------> NH4HSO4 + NaN3 + 2 H2O

From this equation it will be seen that the solution would become acid
as the result of the reaction were it not for the fact that sodium nitrite
is never pure but is alkaline , the alkali in the nitrite ordinarily
being sufficient to maintain the solution on the border line of
acidity and alkalinity , and addition of a small quantity of alkali
being made should such not be the case . The solution thus made
contains sodium azide , some unchanged reagents , and the
by-products of the reaction .

The sodium sulphate and sodium azide contained in the solution
may be separated by crystallisation but preferably the solution
is added gradually to an acidified solution of silver nitrate produced
by adding 1. 0 cc. of 70% of nitric acid to 1000 ccs. of a
normal solution of silver nitrate which is violently agitated
during such adding , the temperature of the reaction mass being
prevented from rising much higher than 30 - 40 degrees Centigrade .
With adequate cooling and agitation the yield of silver azide may
be obtained equivalent to 85 % of the calculated possible amount .
Instead of adding the diazotised solution to the acidifed silver nitrate
solution , the latter may be added to the former .

When starting from hydrazine nitrate which is also an acid salt ,
a solution of ammonia should be added in order to form the
neutral hydrazine ammonium salt or , if desired , the neutral
hydrazine sodium salt may be produced , the remaining steps
of the process being then carried out as above described .

In producing lead azide , a solution of a hydrazine salt
of an acid which will not result in the precipitation of
an insoluble lead salt of the acid in question , for instance
hydrazine nitrate , is employed , and the reaction mass
in this case should be only just acid , as lead azide is
particularly easily acted on by acids , while in producing
silver azide the acidity of the reaction mass may be greater
in order to avoid the precipitation of silver sulphate
and agitation indicated in the foregoing example .

The azides produced in accordance with the invention
are usually amorphous , and in this state they are
safer to handle than when crystalline .

Having now particularly described and ascertained the nature
of my said invention and in what manner the same is to be
performed , I declare that what I claim is :

1. The process of producing azides from salts of hydrazine
in which salts of hydrazine are diazotised by means of
a suitable nitrite under such conditions that the reaction mass
is at no time so distinctly acid as to redden litmus paper ,
but appears on the border line of acidity and alkalinity .

2. The process as claimed in Claim 1 in which the conditions
of acidity under which the reaction is carried out according
to the invention are preferably comparable with conditions
of acidity as indicated by litmus paper , of , for instance ,
a solution of boric acid .

3. The process of producing azides from salts of hydrazine ,
substantially as hereinbefore described .
Dated this 10th day of December , 1918


Regarding the second patent GB129152 ,
It simply states that another method of isolating
sodium azide is to agitate in cool water a suspension
of silver azide with slightly less than the theoretical amount
of sodium chloride for a few minutes , and filter the precipitated
silver chloride . The residual solution of sodium azide is
evaporated on a steam bath to yield a concentrated solution
of sodium azide . The reaction proceeds according to the equation

AgN3 + NaCl -----> AgCl + NaN3

The sodium azide produced may be used in double decomposition
reactions to form whatever other azides are desired .

Wow, that's a fascinating process if it works as it says. Preparing sodium azide in this way sure beats bubbling ammonia into liquid sodium metal to make the amide, and then reacting that to form the azide. It looks like lead azide will still have to be made from sodium azide in the usual way.

Hydrazine salts should be well within the means of amateur experimentors. In fact the procedure to make hydrazine on my website is derived from the procedure to make hydrazine sulfate; instead of concentrating the hydrazine hydrate thus formed from ammonia and bleach, one adds sulfuric acid. The full procedure has been posted on The Forum before.

I don't much care for purifing the sodium azide with silver nitrate... too expensive. I suppose by "The sodium sulphate and sodium azide contained in the solution
may be separated by crystallisation" is fractional crystallization?

Yes , the "Hodgkinson Process" for azides would sure
greatly simplify things when going the synthetic route
from OTC precursors .

Hydrazine is more easily made from urea and 10 percent
sodium hypochlorite pool chlorinator . I have done it a
dozen times . The hydrazine is isolated as the monohydrazine
sulfate . There is a fully detailed description of the method
and its refinements posted at the Hive and also over at
Mad Science . Feel free to import those files here if you wish .

The more modern but more difficult route to sodium azide
requires freebasing the hydrazine from the sulfate and extracting
the hydrazine hydrate with methanol portions . Then the methanolic
extract of hydrazine hydrate is basified with a slight equimolar excess
of sodium hydroxide , and treated dropwise with a slight equimolar excess
of isopropyl nitrite , the reaction being performed over about two hours
with the reaction flask in a plain ice bath . A closed glass system that
can maintain a slight pressurization also complicates things for the
more modern method , although it works very well and yields pure
crystalline sodium azide directly .

The Hodgkinson patent describes a method which requires no elaborate
glassware or volatile precursors . A good stirrer and some ventilation
is about all that is required in the way of special equipment .

The only complication is that close control of pH is absolutely essential
for the reaction to work . A good narrow range color indicator and
perhaps a pH buffer scheme of some sort would be refinements
which could be the basis for experiments .

The Hodgkinson patent method is probably general and there is room
for many experiments to elaborate on the process .

For many purposes it will not be needed to isolate the sodium azide
but only to use its solution . Or the sodium azide could be isolated
by fractional crystallization . The silver azide double decompostion
method is simply a shortcut route to a solution of pure sodium azide ,
and the silver is recyclable in the process .

Regarding the production of lead azide by Hodgkinsons method ,
the hydrazine sulfate must be first converted to neutral hydrazine nitrate and diazotised , and the solution treated with lead nitrate . Hydrazine sulfate can be treated with calcium nitrate and the calcium sulfate
filtered , to leave a solution of hydrazine dinitrate .
This is neutralized and then treated with sodium nitrite ,
to form a solution of sodium azide useful for producing
lead azide .

Since obtaining copies of these patents I have been studying carefully
the texts in preparation for experiments with Hodgkinsons patent process .

Marvin , or others who may be knowledgeable about the chemistry
involved , I invite your input and ideas .

I have two significant , ( yet reconcilable ) issues
concerning the Hodgkinson patent .

In the above patent there is information which I must
legitimately question , regarding both the mechanism
of the reaction and the pH .

According to Hodgkinsons description of the reaction ,

[QUOTE]
The reaction resulting in the formation of sodium azide may be
represented by the following equation :

N2H4-H-NH3-H-SO4 + NaNO2 -----> NH4HSO4 + NaN3 + 2 H2O

From this equation it will be seen that the solution would
become acid as the result of the reaction were it not for
the fact that sodium nitrite is never pure but is alkaline ,
the alkali in the nitrite ordinarily being sufficient to maintain
the solution on the border line of acidity and alkalinity ,
and addition of a small quantity of alkali being made
should such not be the case . The solution thus made contains
sodium azide , some unchanged reagents , and the by-products
of the reaction .
[END QUOTE]

The observation I can make is that even *if* the Sodium Nitrite
used by Hodgkinson was alkaline due to the presence of an
"impurity" of an *added* equimolar amount of Sodium Hydroxide ,
( which is certainly a great deal more than a "small quantity"
of added alkali like Hodgkinson describes ) ......
the reaction product would still be so acid as to be more than
sufficient to redden litmus . Let us consider the reaction
equation for the situation I describe and this will be evident .

N2H4-H-NH3-H-SO4 + NaNO2 + NaOH -----> NaNH4SO4 + NaN3 + 3 H2O

The NaNH4SO4 is a "stipulated compound" to illustrate acidity ,
but in reality would probably be an indeterminate mixture of
normal and acid sulfates of Sodium and Ammonia .

There is a ( possible ? ) way of reconciling this descrepancy .
Where Hodgkinson describes preparing a solution of
*neutral* hydrazine ammonium sulfate , no particular quantity
of ammonia is specified as being added to achieve that *neutral*
solution . There may be actually present in such a "neutral"
solution an "excess" of alkali not accurately reflected by
the simplified equation for the reaction as written by Hodgkinson .
This excess alkali , together with the unspecified amount of alkali
described as being present in Hodgkinsons "impure nitrite" ,
may satisfy the accounting for some of the missing amount of
additional alkali not detailed by Hodgkinson .

The second issue regarding pH as described by Hodgkinson relates
to the comparison of the pH required for the reaction as being
comparable to the pH for a solution of Boric Acid .
The pH for a .1 M solution of Boric Acid is pH 5.2 .
The color change range for Litmus is pH 6 to pH 8 ,
so at pH 5.2 , the reaction mixture would already be
quite red to litmus .....in direct contradiction with
Hodgkinsons description .

In spite of these discrepancies which seem most obvious
and somewhat perplexing , I believe that Hodgkinsons
process probably does indeed work , although not all
of the finer details are disclosed completely by the patent .

It would be nice to have a more detailed and modern description
of the process after the subtleties are worked out from
experiments . Only from experiments shall we know
"the rest of the story" about Hodgkinsons patent process
for producing azides from hydrazine salts .

Rosco

Update :

After many careful experiments have been done without success
at reproducing the results described by GB128014 , I am reasonably
sure the method described by the patent is BOGUS and unworkable .

I used pure chemicals , accurately measured and controlled
conditions of pH in different experiments , in increments of
two tenths of a pH across the range from pH 5 to pH 7.2 .

I tried following the patents process description exactly ,
and I tried variations of every sort I could contemplate ,
with no joy in producing azides except in useless trace quantity
mixed with inert undesired by products .

On rare occasion I have encountered this scenario before ,
where a patent makes disclosure of a technology which does
not withstand scrutiny . There is no way of being sure about
the validity of what is reported by a patent except to experiment
and thereby put the matter to the test , which I have done .

My "finding" regarding GB128014 is that the information it
discloses is either incorrect or incomplete , and the process
it describes is *NOT* a straightforward nor valid method for
the synthesis of azides .