NATIONAL BUREAU OF STANDARDS REPORT

b, . 10 621

THE ENTHALPIES OF NEUTRALIZATION AND OF HYDROLYSIS OF

NBS STANDARD REFERENCE MATERIAL NO. 724a [TRIS(HYDROXYMETHYL)AMINOMETHANE]

Technical Report

to Office of Standard Reference Materials

National Bureau of Standards

Washington, D. C.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

3161752 October 1971

NBS REPORT

10 621

THE ENTHALPIES OF NEUTRALIZATION AND OF HYDROLYSIS OF

NBS STANDARD REFERENCE MATERIAL NO. 724a [TRIS(HYDROXYMETHYL)AMINOMETHANE]

Marthada V. Kilday and Edward J. Prosen

Technical Report

to Office of Standard Reference Materials

National Bureau of Standards

Washington, D. C.

IMPORTANT NOTICE NATIONAL BUREAU OF ST>

for use within the Government,

and review. For this reason, the whole or in part, is not authori Bureau of Standards, Washingto

the Report has been specifically

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

s accounting documents intended

subjected to additional evaluation

listing of this Report, either in Office of the Director, National

i the Government agency for which

pies for its own use. 


U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The Enthalpies of Neutralization and of Hydrolysis of

NBS Standard Reference Material No. 724a [ tris (hydroxyme thy 1) ami nome thane]

by Marthada V. Kilday and Edward J. Prosen

Abstract

An adiabatic solution calorimeter was used to measure the enthalpies

of neutralization of crystalline tris (hydroxyme thy l)aminome thane

,

TRIS, in aqueous HC1 solution and of hydrolysis in aqueous NaOH

solution. These measurements resulted in the following certified

values for the enthalpies of solution of NBS Standard Reference

Material No. 724a at constant pressure and at a concentration of

3

5 grams per 1000 cm of solution and other conditions specified in

the certificate:

(1) in 0.100 N hydrochloric acid solution

ZTi(298 . 15 K) - -245.76 ± 0.26 J.g~\ and

(2) in 0.0500 N sodium hydroxide solution

AH (298 . 15 K) = 141.70 ± 0.19 J-g"

1

The results of other experiments are given comparing the reactions of

SRM.724 and SRM 724a, and showing some conditions under which high

values were obtained for reaction (1).

Key words: tris (hydroxyme thy l)aminome thane , 2 -amino- 2- (hydroxyme thy 1)

1-3-propanediol, (HOCH^) ^CHN^, heat of soln. of TRIS in

HC1, and heat of soln. of TRIS in NaOH, TRIS.

1
1. Introduction

In 1964, the U.S. Calorimetry Conference requested that the National

Bureau of Standards issue a standard sample of tris (hydroxymethyl)

aminome thane, or TRIS, for solution calorimetry. It was assumed that

certification of the enthalpy of solution of the material would involve

only a series of simple calorimetric experiments to confirm the early

results reported by Irving and Wadso [1] in Sweden. At that time, a new

adiabatic solution calorimeter had been constructed in the Thermochemistry

Section at NBS which was still untested, but was believed to be capable

of high precision and accuracy, and well-qualified to do the certification

work. However, the first results obtained late in 1967 with the new

Standard Reference Material No. 724 were greater than 0.1% more exothermic

than those reported by two other groups who made extensive studies of

the TRIS reaction and achieved results of high precision. The experimental

precision of the results obtained with the new calorimeter and SRM 724

was about 0.04%,

An intensive effort began to locate the cause of the descrepancy in

the results. No difference was found in the wlues obtained with samples

from different sources, nor in samples which were stored for years in

darkness as opposed to those stored in the presence of fluorescent lighting,

nor in samples stored in the room atmosphere as compared to those stored

in a hygrostat of 50% relative humidity. The calibrations of our bridge,

potentiometer, standard resistors, standard cell, and electronic counter

were checked; a dummy heater with its leads directly in the calorimeter

solution (thus at calorimeter temperature before leaving the vessel) was

2
compared with a permanent calibrating heater in a Pt well -- the difference

between the results obtained with the two heaters was less than 0.004%;

temperature differences at various points between the adiabatic shield and

the vessel were measured and the currents in the shield heaters were

readjusted to produce minimal departures from the vessel temperatures;

and the platinum resistance thermometer was replaced by a quartz oscillator

thermometer -- none of these produced any detectable change in the results.

Having eliminated the sample composition and treatment, and the measuring

equipment as likely sources of error, we decided to measure for comparison

another reaction on which published results were available. The heat

of reaction of Gunn's sample of H^SO^ was measured in 0.02N and in 0.08N*NaOH;

seven experiments in each group agreed with Gunn's values [2] to within

a few hundredths of a percent for both reactions, and the standard deviation

of the mean of our experiments was 0.01%. Thus, our efforts were directed

to investigating conditions of the TRIS reaction in HC1 solutions

which might explain the existing disagreement.

Much is still not understood about this reaction, but we report here

the results of some of our measurements which have led to our confidence

in the experiments on which the certified values are based.

3
2. The Calorimeter

The adiabatic solution calorimeter employed in the measurements

reported in this paper has been previously described in NBS Technical

Reports [3] which are available on request to the authors.

Figure 1 is a drawing of the calorimeter vessel showing the well for the

platinum resistance thermometer (A), the platinum stirrer (B), and

the well for the calibrating heater (C) . The vessel is lined with

platinum which is surrounded by approximately 3 mm in depth of pure

silver. The solution level is 0.5 cm below the cover of the vessel.

Five platinum tubes leading from the cover of the vessel carry leads, etc.,

from the calorimeter. Three of these tubes are shown in figure 1, and

in a vertical plane at a 90° angle at the stirrer, are two additional

tubes: one carries the push-rod for opening the sample holder, and the

other (on the opposite side of the stirrer) is the vent tube. An

enlarged view of the vent tube arrangement originally used is shown

in figure 2. The weighted platinum rod seats on a polytetraf luroe thylene

(Teflon) ring with a hole 1.6 mm in diameter. When the rod is in

place, this forms a seal for the space above the calorimeter solution;

when the rod is withdrawn there is a positive vent to the atmosphere.

In some measurements the Pt rod was withdrawn and the upper end of the

vent tube was connected by rubber tubing to an open-end manometer containing

mineral oil)

either mareol or mercury-- thus changes in pressure in the vapor space A

above the calorimeter solutions could be observed during the experiments.

In other measurements, the Pt rod was removed and a Teflon capillary

4
scale, centimeters

SOLUTION CALORIMETER VESSEL
Weighted Pt rod

Original vent arrangement

FIGURE 2
tube was passed through the rubber tubing and the vent tube. The PTFE

seat was removed. The lower end of the capillary tube terminated a

little above the stirrer and the upper end could be connected to a gas

introduction line. This permitted flushing a gas through the calorimeter solution, and the exit gases passed through the annular space in the

vent tube and the rubber tubing to the atmosphere. When the flushing was

completed, the Teflon tube was disconnected from the gas inlet line and

placed inside the top of the open-end manometer where the rubber tubing was

again connected. Thus, the calorimeter solution was essentially saturated

with the gas under study, and the space above the solution was filled

with the gas at atmospheric pressure.

The venting of the calorimeter was also effected by the type of

o-ring used at the top of the Pt tube containing the stirrer shaft (shown

at top in figure 1). Normally a PTFE (Teflon) o-ring is used here and is

held in place by a brass fitting (not shown). The o-ring is not compressed

and functions as a bearing for the stirrer shaft, not as a seal; thus,

the calorimeter vapors may diffuse slowly through the tube to the

atmosphere, and the pressure above the solution is always near

atmospheric pressure. For a special project it was desirable to

make the calorimeter "gas-tight," and the PTFE o-ring was replaced by a lubticated Buna N rubber o-ring which was compressed by the fitting

to form a seal around the stirrer shaft. Under these conditions

when the vessel was filled with helium gas to a pressure of 15 cm

of Hg, the leakage rate with the stirrer running was about 3 mm

of Hg per minute.

5
In the measurements reported in this paper four venting arrangements

were used

:

(1) In the "unvented" system, the Buna N rubber o-ring was used to

seal the stirrer shaft, and the vent tube was sealed as shown

in figure 2.

(2) In the "vented" system, the Pt rod (fig. 2) was removed, and

the top of the vent tube was connected to the open-end

manometer by rubber tubing; a Teflon o-ring was used as a

bearing at the top of the stirrer tube.

(3) In the systems vented to the atmosphere, the arrangement

was similar to (2) except that the rubber tubing was disconnected

from the manometer.

(4) In the systems where the effects of various atmospheric gases

on the TRIS reaction were observed, the Buna N rubber o-rings

sealed the stirrer shaft, and the Teflon capillary tubing

was placed in the vent tube and used as described above.

The Pt sample holder has been described [3]. In this work,

3

the largest sample holder, 2.8 cm , was used, and the samples (approximately

1.5 grams) nearly filled the cylinder but were not tightly packed.

6
3. Calibrations, Units, and Corrections

The experiments reported were performed between February, 1970, and

May, 1971, except some preliminary work on the endothermic reaction in

January to March, 1969.

Certified NBS calibrations were made as follows:

Capsule-type platinum resistance thermometer No. 769892, October, 1961;

saturated standard cell No. 2625-2293, July, 1969; and standard

resistors No. 117443 (0.1 ohm), No. 119222 (10 ohms), and No. 119184

(10,000 ohms), June, 1969. The records of numerous calibrations over

several years for the standard cell and standard resistors provide

additional confidence in the values used. An unofficial measurement

of the ice point with the Pt resistance thermometer early in 1970

agreed with the correct value within 0.01K, indicating no significant

change in the absolute values since the original calibration.

Calibrations were made in this laboratory of the G-3 resistance

thermometer bridge dials in January, 1969, and November, 1970, and of the

6-dial potentiometer in January, 1969. The electronic counter used

in timing the electrical heating periods was checked against the

standard seconds signals available at the National Bureau of Standards.

In some experiments a quartz oscillator thermometer replaced the

platinum resistance thermometer for measuring the calorimeter temperature changes

The calibration of the quartz thermometer was based on direct comparisons

with the platinum resistance thermometer.

7
The 1961 atomic weights [4] were used in the calculations.

The unit of energy is the joule (J), and 4.1840 joules =

1 thermochemical calorie.

For reduction to weights in vacuum, the following densities

-3 -3

were used: 1.35 g*cm for TRIS and 0.00118 g*cm for air.

8
4 . Experimental Procedures

The general calorimetric procedures followed were described

previously [3]. The procedures and conditions used for the TRIS sample

and its reactions were those recommended by the Standards Committee,

U. S. Calorimetry Conference, October, 1966, which may be obtained upon

request to the authors of this technical report.

In the endothermic reactions of TRIS with sodium hydroxide

solutions, precisely measured electrical energy was added during the

chemical reaction to prevent a drop in the calorimeter temperature

which could not be followed by the adiabatic shield. (During rating

periods the stirring energy is sufficient to maintain a temperature rise

in the calorimeter and assure operation of the controls for the

adiabatic shield.) The electrical heating was started at least 1 minute

before opening the sample holder and the calorimeter temperature was

rising rapidly when the reaction began. Thus, the departures of the

shield temperature from the calorimeter temperature were negligible.

In earlier experiments, when the chemical reaction and electrical heating

were initiated simultaneously large differences between the shield

and calorimeter temperatures occurred and corrections were made

which were as much as 0.1% of the total heat absorbed. The addition of

electrical energy during endothermic reactions is generally desirable to

prevent possible vaporization errors during a drop in calorimeter

temperature

.

O
The stock solutions of 0.1N HC1 were prepared from reagentgrade concentrated HC1 diluted with distilled water which was in

equilibrium with air, except as noted in special solutions where the

distilled water was boiled to remove CO^ and then dispensed from

containers guarded by Ascarite tubes. The solutions were analysed by

titration with standard 0.1N sodium hydroxide solutions using a recording pH meter.

The stock solutions of 0.05N NaOH were prepared by dilution of ION

sodium hydroxide with CO^-free distilled water, and analysed by

titration with potassium acid phthalate (SRM 39g) using a recording

p H meter.

The initial and final system for each reaction was calibrated

electrically.

10
5. Experimental Results

A quartz oscillator thermometer was used for the calorimeter

temperature measurements in the experiments given in table 1 which

show the effects of vented and unvented systems [see page 6, (1)

and (2)] using the two TRIS samples, SRM 724 and SRM 724a. The

average enthalpies for the two samples in vented systems agree within

the uncertainties, but in unvented systems the enthalpies are about

3% too high and are less reproducible than in the vented systems.

The electrical energy equivalents have not been corrected for small

variations in the amount of the calorimetric solutions (the spread of

the weights was less than 0.04%), however, they all agree within about

0.1% regardless of whether the system was vented or unvented. This indicates

that the high reaction enthalpies obtained in unvented systems were characteristic of the TRIS reaction and not a property of the calorimeter since the

venting did not effect the electrical energy experiments. In the vented

experiments the manometer indicated that the pressures above the solutions

never exceded 5 cm of marcol during the entire experiment.

The expt. nos. omitted from table 1 are as follows: nos. 385 and

396 were calibrations of the quartz oscillator thermometer vs. the Pt

resistance thermometer, and nos . 403 and 404 were experiments in which

an attempt was made to degas the calorimetric solution by pumping with

a vacuum prior to beginning temperature measurements. However, the

Teflon o-ring seal on the sample holder leaked under the reduced pressure

and the calorimetric solution was drawn into the sample holder when

atmospheric pressure was restored above the solution.

11
Table

1.

Results

of

measurements

with

TRIS

(SRM

724

and

724a)

in

OolN

HC1

solutions

in

vented

and

unvented

systems.

nJ CM

O'£

co

LO

00o\

CM*

50

*4

QJ CO C 4-J W fiQ)

i—I r-l

cti CO O P> •H *H U 0

4-j cr o w

<D

<—

i

w

a. o

x s

w

TO

<u vo vO <0

4-1 i 1 1 1 | 1 1 1 |

ii 1 • 1 i 1 1 1 1 • c i 1 1 1 1 1 1 1 | r-l 1 CM 1 i 1 1 1 1 tH

<u | 1 1 1 1 1 1 1 1 CM 1 vO 1 i 1 1 | 1 <±

?> i 1 1 1 | 1 1 1 1 CO 1 co 1 | 1 1 | 1 CO c O' O' O' 3

TO vO LO o vO O'

CD • 1 • | i • 1 • 1 6 1 1 • i 1 • | • • 4J 1 LO 1 1 O' 1 O' 1 1 1 1 O' i | vO 1 CO CO

CS r-l r-l 1 CM 1 1 CM 1 rH 1 1 1 1 rH i | rH | tH tH

<U | tH 1 t-4 1 1 rH 1 rH 1 1 1 1 fH i 1 rH | fH tH

P> r-l r-- r-' O' O' O- O' O' O

e

• r-l TO CO a> o O' o 00

4-) 1 1 1 1 1 i • 1 | 1 • 1 1 i 1 1 1 1 • a 1 1 i 1 1 i 00 1 1 1 CO 1 1 1 | I 1 1 o

<D 1 1 i 1 1 i 00 1 1 1 CO 1 1 1 | 1 I 1 <1-

p> 1 1 i 1 1 i CM 1 1 1 CO 1 1 I 1 1 | 1 CO c O' O' O' 3

TO vO rH O o ON <t o rH

<0 1 • • • i 1 | • 1 1 ! 1 • • 1 • 1 •

4-J 1 00 l r-l <1- i | | o 1 1 | 1 o CM | vO 1 vO

so 1 r-l l r-l r-l i 1 1 co 1 1 1 1 rH t-4 | rH 1 tH

a> 1 r-l i rH rH i | 1 r-l 1 1 | rH rH 1 rH 1 tH

P> p>- O' O' O' O' O' O'

COa

cO

t—

1

uo 00 LO 00 CM On o O' VO <f 00 O' <f rH O CM

r—

1

•

CM rH CO r-l 00 00 O CM CO VO 00 00 o 00 O rH CO m

s rH o o O o O O o O o o o o rH O O tH

•H 1-0 CO CO co CO CM CO CO CO CO CO CO CO CO co CO CO CO co

p*4 I-"- 1"- O' O' O' O' O' O' o- O' O' O' O' O- O' O' o- fH rH r-l rH rH r-l fH fH rH fH rH rH fH rH fH fH rH rH

i—

i

CM LO LO o\ 00 co CO LO vO vO CM 00 <t CM (O CO CM CM

CO tH O p'- T—

1

00 vO O'* o CM O' 00 o o m O O' CM CO in

•H

4J O'! 00 O' O' O' oo 00 O' 00 00 rH 00 00 O' 00 CO 00

•H • CM CM CM CM CM CM CM CM CM CM CM CO CM CM CM CM CM CM c O' o- O' O' I"'- O' O' O- O- O' O' O' O' O- O- o- O' O' H r—

1

r-l r-l r-l !—1 1-4 r-l r-l t—

1

rH rH r-l t-4 rH rH rH tH rH

av O fH

1

CM CO O- 00 cr. o rH CM m VO O' 00 Or o

00 CT\ OV CO (TV c^ O'. CO co o o O o O o O O rH

co CO CO CO CO CO CO CO co 'Cl

-

12
A study was made of the effects of dissolved gases (the major components of air: A, and CO^) on the TRIS reaction in 0.1N HC1

solution. In order to introduce the gas under study into the calorimeter,

a capillary teflon tube was passed through the vent tube and into the

calorimetric solution as described on page 6, (4). The compressed

gas from a commercial cylinder was passed through absorbers containing

magnesium perchlorate, Ascarite (this was by-passed when CO^ was used),

and then through a bubbler containing 0.1N HC1 solution before entering

the calorimeter. The gas was bubbled through the calorimeteric

3

solution at approximately 50 cm per minute for about 90 minutes to

saturate the solution and displace the air above the solution with

the gas under study. The upper end of the teflon tube was disconnected

from the gas introduction line and the vent tube was connected to the

open end manometer—thus the atmosphere above the solution was initially

at atmospheric pressure.

When the gases were bubbled into the solutions small droplets of

solution accumulated on the vessel cover as a result of the bubbles

bursting at the surface. These droplets probably caused errors due to

abnormal evaporation during the calormetric measurements, therefore,

the results are not regarded as calorimetrically precise, but are useful

in showing gross effects. The enthalpies of reaction for these experiments

are given in table 2. We believe results for A, and 0^ are essentially

the same and the spread of the values is the result of the droplet

errors. However, in the case of CO^ the enthalpy values are about TL

higher than those obtained with the other three gases. This appears

to be a significant difference. The result of expt. No. 454 (-7160

cal. mol was not included in the table because the conditions were

13
-1

Table 2. -£H(298. 15K) , cal-mol , for TRIS in 0.1N HC1

with various gases above and saturating the

solution

Expt

.

No.

r Atmosphere

saturating

above and

the solution

N A 0o C0o 2 2 2

447 7146

448 7129

449 7194

450 7121

451 7113

452 7160

453 7122

455 7555

456 7609

14
different from the others. This was the first experiment using CO^ gas.

After the normal flushing period, the open-end manometer containing

marcol was connected to the vent tube as usual. However, the pressure

over the calorimetric solution dropped rapidly due to the absorption

of CO^ i- n the system, and it was necessary to open the connection to

the manometer to prevent the oil from being drawn into the calorimeter.

This was repeated several times and finally the vent tube was closed

off leaving the solution and the atmosphere above the solution

no longer saturated with CO^. In expt. Nos. 455 and 456, mercury

was used in the manometer instead of marcol and it was possible to

proceed normally.

Figure 3 is a plot of the pressures above the solutions vs.

time observed in four of the experiments given in table 2; the results

were similar in the other corresponding experiments. A factor of 15

was used to convert cm of Hg to cm of marcol in the case of the CO^

atmosphere, and the larger circles indicate the greater uncertainty in

the manometer readings. The vertical lines indicate the time of the

electrical heating periods for the initial and final calibrations and the

time at which the TRIS reaction was initiated. Negative readings

indicate reduced pressure above the solutions. The pressure changes

during the first 100 minutes were due to the gaa solubilities and

equilibrations within the system, but during the calibrations and chemical

reactions they were caused by the 0.2K temperature change. In the

atmospheres of N , 0^. and A, the pressure changes were essentially the

same for all calibrations and reactions, as would be expected since the

15
'
}0?JT>!<S ^.C? ’UC a ' ? ’OHttvau V M3JL3W0/WIV

FIGURE
temperature rises were the same. However, for the CO^ atmosphere,

although no significant pressure change was detectable during the

calibrations, a 0.9 cm of Hg increase in pressure above the solutions was

observed during the chemical reactions even though the temperature

change remained the same. We cannot fully explain this effect, but

it does provide further evidence that C0o effects the TRIS reaction

in a closed system.

The platinum resistance thermometer was used to measure the

calorimeter temperature in all experiments hereafter in this report.

The measurements for the four experiments given in table 3 were on

systems vented to the atmosphere [see page 6, (3)]. In this

calorimeter, the "standard" systems contained 1.5 grams of TRIS and

302.4 grams of 0.1N HC1 solution. In expt. Nos. 530 and 531 these

standard weights were increased by 5%, and in expt. Nos. 532 and 533

the standard weights were decreased by 5%. In the first two experiments

the vapor space above the solution was almost eliminated and in the

last two experiments the vapor space was approximately 1 cm in depth.

We assume that in expt. No. 531 the solution accidentally sealed the

vent giving a high enthalpy value as in an unvented system, however,

the other three values are in excellent agreement and show that

changes in the vapor space do not effect the enthalpy of the reaction.

16
Table 3. Results of measurements with non-standard weights

of HC1 solution and TRIS

Expt

.

No.

wt. of HC1

soln (in air)

h Electrica

Equiva

1 Energy

lents

Initial Final

grams J.Q"

1

JrQ”

1

-AH

(298. 15K)

532

533

(5% more than the standard wt . of HC1 and TRIS

17 , 756.4

17,764.

(5% less than the standard wt . of HC1 and TRIS

530 317.36 17,730.6

531 317.38 17,731.9

cal * mol

7115.5

7156.

-1

287.38 16,498.1 16,516.9 7115.1

287.47 16,505.7 16,527.4 7115.2

-1

40.98 J*Q « g for initial system

.-1 -1

41.26 J«Q « g for final system

17
The experiments in table 3 also provided information for calculating

factors (at the bottom of the table) for correcting the electrical

energy equivalents of the initial and final systems to the standard

weights of HC1 in order that the precision of the measurement of

electrical energy equivalents could be determined. The weights in air

of the HC1 solutions are given and the corrections were applied

accordingly to obtain the values for the electrical energy equivalents

given in table 4; here the standard deviation of the mean was less than

2 J • Q

^ or ±0.01 percent. The mean energy equivalents for each

experiment before correction to the standard weight of HC1 solution was

multiplied by AR^ reaction to obtain AH at the mean temperature of

reaction, T reaction. The correction to 298.15 K is the product of

AGp, 1.435 J*g ^'K and (T reaction -25. °C). The slopes in the

initial and final rating periods (which were a least squares fit of

5 points) were extrapolated to the time of initiating the reaction to

obtain the corrected temperature rise, AR^. A bridge dial correction

was also applied. Since this is an adiabatic calorimeter the slopes in

the initial and final rating periods are almost the same, and the time

chosen for the time of reaction is not very significant. The

"Stirring Energy Correction" given in table 4 is the net result of

the extrapolations of the initial and final rating periods, and this

correction is uncertain by 1 or 2 percent for fast reactions such

as TRIS in HC1 solution.

18
Table

4.

Results

of

measurements

leading

to

the

certified

values

for

the

exothermic

enthalpy

of

reaction

of

SRM

724a,

TRIS,

in

0.1 N

HC1.

-4 /-s 1 *5< * * CO r» * * rH

r—< Q O CM CO 1^- 'd- O U0 rH 1—4 CM vO U0 vO O E T—1

CM O in ON i—4 CM O CO o- U0 U0 vO UO o o m CM CM CO r-l r-4 CM +1 +l r-4 r-4 rH rH r-4 rH +1 +1

i—i <—4 r—4rH rH rH rH i—l v_^- r-4 i—l i—l r-4 rH r-l

V • 03 O' i-"- O' r- i"- O' O' C' O- O' O' O'

00 O

CM

-I v£> r-4 o- vO o 00 00 co o CM UO uo rH vO CM r-4 K 1 CTN o CM O' ON ON o o O' 00 O' 00 O' o o 00 • • • • • • 1 • in vO VO in in UO o o uo uo uo uo U0 LO o o *—> <t <t +1 +1 <r +1 +1 CM CM CM CM CM CM CM CM CM CM CM CM N-' o

-u —4 LO | ON ON o o rH r-4 rH o o 00 • H 00 CM CM o o o O o CO CO CM • u oo *“> o 1 i 1 i + + 1 0 ON 1 o CM ao

•tH VO in o- vO co CO 00 00 CM u On on ON ON ON ON ON o o o o o o- ON ON ON ON ON CM CM oo

CtJ O

0) <t <r <t uo uo u ' CM CM CM CM CM CM CM CM CM CM IHCoH

-i-i CM CO o rH vO O' CO vO o T—1

LO 00 ON VO <f CM O- CO UO

rO in <t 00 ON vO ON oo VO CO oo

0) G r—< rH r-4 rH rH rH i—i rH rH t-4 G CM CM CM CM CM CM CM CM CM CM O o o o O O O o o o O • • • • • S5 o c - 0 CM o- in ON 00 ON 'Cf 00 •H CM on ON ON I"- O' r-4 i—l uo c 4-1 m <t <f <} <t <T in in uo

•H a o o o o o O O o o o u 60 cu G ' o o o o o O O o o o u U u o o o o o o O o o o •tH a; u

4-J C o o

id.jl; ' r4 /^S

r-4 o rH CO o ON o ON UN ON CM VO r-4

r-4 r-4 • • • • • • o 60 CC 1 r-4 vO CO o 1-4 r-4 o ON O vO CO uo rH rH •

5-4 CO c G in <f +1 +1 co <t CO <t <t +1 o 0) 4J •r- • T—1

i—f r-4 rH rH rH 'w' rH rH 1-4 r-4 r-4 C c f*4 to •S . *s •N *N - ^ •\ •N •N •N *S •s I W a) " O' O' O' O' o» O' o- O' O' O' rH rH1

t-4 rH i—i rH rH rH p-4 rH rH r-4 rH

pH ctf cC p> /-N o •H G r-l rH

•H 0 r-( n O'* CO uo rH CO 00 o vO ON <J- in <t o o o u cr 03 -4

4-1

w 4 r- 1 r-4 vO co uo 00 i—l o ON vO co O' O' r-4 O O 4—4 G CM r-4 CM rH i-4 rH +1 +1 r-l r-4 rH rH rH rH +1 +1

cu •r-f • r-l rH r-4 r-4 i—4 rH V—'

r-4 r-4 rH rH rH rH 'W' rH c •“3 *\ •N •N •s •0 • *\ •0 •N w M o- O' O' O' O- O' O' O' O' O' • • i—l r-4 rH rH r-4 r-4 rH rH rH rH rH r-4 /-s •H C -u

cti T— a) in O o o O UO o in o UO C 4J CM r-4 00 00 CM rH oo o- r-4 C 4-4 w <u <r <t <t CO co • • <t <t co <t

•H o 60 E

p- o CM CM CM CM CM 0) CM CM O' r" CM • C c o o O o o u o O 00 00 o 4-1 X 03 CO co CO CO co <u CO CO CM CM CO E -Ca T—J o o o E

14-1 >4 VO uo oo o 4-1 00 <t o o CO O eg on r-4 CM i"- 00 CO UO O' ON UO

CO E vo CM CM CM oo ON O O' uo r-4

COw 00 ON ON CM co o o CM CM in co CM

CO o <t <t in UO uo 4-J uo UO <J- <J- in

rH rH r-4 rH r-4 0D 1-4 i—4 rH rH i—i TJ <u

QJ 4-J • 4-J C

4—1 • C r-4 CM CO <t O'. c E ,a» oo ON CM CO Mt G E

Oi o CM CM CM CM CM 03 no tO? > CM CM CO CO co C33 rU 5^2 X s > in in in UO UO aj w V UO UO UN UO uo a) CO w S S

19

Initially

solutions

were

CO

-free
The first five experiments given in table 4 were vented to the

marcol manometer [see page 6, (2)], and the last five were vented to

the atmosphere [page 6, (3)]. The electrical energy equivalents in the

two groups are the same within the uncertainties, but a lower mean

enthalpy of reaction was obtained in the second group and the precision

was significantly improved. It appears that even the small confinement

of the atmosphere above the solution imposed by the open-end marcol

manometer produced somewhat high enthalpy values for the reaction.

The last five experiments in table 4 are the basis for the certified

enthalpy of reaction of SRM 724a (TRIS) in 0.1N HC1 under the

specified conditions.

In an effort to detect an effect on the enthalpy of reaction by

the presence of small amounts of CO^ in the HC1 solutions, the

experiments marked by an asterisk in table 4 employed HC1 solutions

prepared from boiled distilled water which was guarded by Ascarite

absorbers. However, the solutions were weighed and transferred to

the calorimeter in the presence of air, and approximately 3 hours

elapsed before the TRIS reaction occurred, consequently the solution

was probably almost in equilibrium with the CO^ in air by the time

the reaction occurred. No difference was apparent in the results

obtained with these '’CO^-free" HC1 solutions and those originally

in equilibrium with air.

20
Four additional measurements of the TRIS reaction in 0.1N HC1 solution

were made to obtain the temperature coefficient of the reaction. These

vented to the atmosphere; the results are

Expt. No. T reaction -m (t)

-i

°C J-g

535 29.938 239.15

536 20.478 252.25

537 20.556 252.32

538 29.948 238.36

A least squares fit of these data plus the last 5 experiments in table 4

resulted in the following equation for the relationship of temperature

and enthalpy of solution (in °C and J*g :

£H(T) = -245.76 + 1.435 (T-25),

. . * A t i J

' *

•

. v i

5

.

. except that the value of the constant enthalpy term at 25 °C was

reduced by 0.04 J«g ^ to correspond to the certified value.

: The standard errors of the two constants were ± 0.07 and ± 0.023,

respectively, and the standard error of the estimate was ± 0.22 J*g \

Thus 4Cp for the reaction is 1.435 ± .023 J»g ^*K ^ or 41.54 ± 0.67

: /" ; • i

’ ' cal*mol »K

21
In the endothermic reactions of TRIS with 0.0500 N NaOH solutions,

the reaction occurred while electrical energy was added to the

calorimeter (see section 4) . Table 5 gives the results of 9 experiments

which are the basis for the certified value for the enthalpy of

reaction of SRM 724a with 0.0500 N NaOH solution at a concentration

3

of 5 grams of sample in 1000 cm of solution. All of the measurements

were on systems vented to the atmosphere except the three which are

narked with an asterisk and were vented to the marcol manometer

[see page 6, (2) and (3)]. For this reaction there appears to be no

difference in the enthalpy values obtained with the two venting

conditions

.

The electrical energy equivalents given in table 5 have not been

corrected as in table 4 for small differences in the weights of the

calorimetric solutions from the standard system and are the energy

equivalents of the actual systems. In Expt. No. 541, there was a malfunction of the electronic counter during the final calibration

and an estimated value of 17,380 J*Q ^ was used for the calculations.

Experiment No. 546 was lost because the counter malfunction necessitated

leaving the calorimeter assembled overnight and the sample holder

obviously leaked as a result of a drop in temperature.

22
Table

5.

Results

of

measurements

leading

to

the

certification

of

the

endothermic

enthalpy

of

reaction

of

SRM

724a

with

0.0500N

NaOH 1 CM 00 |H ip

m rH CM fp in in 00 vO

rH « o vO rH cn O s

rH 8

p- vO CM i—t o 6 o rH ON O o ov O O +1 o

rH rH o rH iH rH rH o T—1

rH rH

•s •s •\ •s •s CO <f <1- <f <f <f <r <f pf <f H CM CM rp <f |p m CM co <f o vO

S'! 3

CO 1 00 o cn vo ip vD VO m 00 ip O vO

60 • tH CM rH rH rH iH rH rH rH »H o' O rH

”> «vf <f <f <f <t <1- <f <f +1 s-/ <f

rH tH iH rH rH i—4 rH rH i—t

iH i—l

4-1 55o £ 1—1

CM CM CM CM CM CM 00 00 00 • o o 1 CM CM CM CM CM CM rH tH rH u in CO 00 u o s • O o o’ o* O o’ o o o o • •o l 1 1 1 1 1 i 1 i o o c

cO

<De

<u x

4-1

T3U • in H CM CM CM p-f rp ip p- CO m CO U tH 1 o o o o rH rH rH o rH "O u o • 00 e c O 4J O0 • o o o o o o o o o CO CO o CTv CM o 1 1 + + i 1 + i X

4-1 CO o

•H

4-1 pf rHm CO CM 00 O oo u CM CM 00 vO vO vO vO i"- <1-

CO O O O O-N On rH rH fH On H

0) 0 • U m

m <f

m

mm <t

m

IH i CM CM CM CM CM CM CM CM CM

<u CO 00 CM in tH o pf

m

m

44 c s, r> 00 o in CM o <t i"- O o a> <f CM CT\ tH |p 00 rH m

•H 00 00 in 40

mm in vO o O 4-1 • G vO vO in in m in m m vD

KJ O 4-1 o o o O O O o o o

<3 CO CJ CO CO o

P5

W

00 O > ' , rH in vO <f rH CO o>

m vO e tP 4-1 CO o vO vO o <1- p~ tH ip •H 00 o rp 00 I-"- VO 00 oo rp ip in u t4 CO G o o o o o o O O O u (0 u O o o o o o O O O

•H c u O o o o o o o O o

4-4

w o

CO o O

to CM rH vO CO 00 vO CM VD

00 tH -1 • • • u CO 1 CM OV CM CTv vO O CO IP

CD CO C G 00 P'- i. 00 P- P- 00 00 IP C 4-1 •H CO CO i CO CO CO co CO co W C pH •s •s •N •s •\ CU p- p> rp P- IP IP IP

rH tH |H rH rH rH iH iH iH rH

cO cO •H *H rH CO o CM o o U 3

4-J U* •H H o rH o pT MT o vO pf rH o w 4J 1 vO vO vO vO vO vO m vO vO

a) •r4 CO CO CO CO CO co co CO CO

rH fi •s •s H M H) p' p- P- P- IP ip IP p^

|H rH rH iH rH rH rH iH rH

w

4-1 • OV o tH CM co in ip oo

0- o CO *3- pf pf- X 25 m in in in in in in in

CM

P>

cox

to O •H

4-1 / ? • •H IP p- IP IP P- IP CTv av CTv C

5C u rH 00 oo oo 00 00 00 00 00 00 O • o

CO CO

CO o

pfo

<1- o 3

pfo 3

<ropfo

pfO

•H

4-J CO

4-1 o CO Cl o o •H X z > <u

TJ

<U •Ucs* U

CO rH

•HB

•r4CO vO

4-1Oc

CO 3

23

Standard

deviation

of

the

mean

±0.03

±0.7
The procedure for calculating AH(298.15 K) in table 5 was

similar to that used for the exothermic reaction and described above,

except that the "Electrical Energy added During Reaction," given

in table 6 was subtracted from the net heat evolved during the

reaction period which was the product of AR^ and the mean electrical

energy equivalent.

For the correction to 298.15 K, ACp = 1.033 J»g ^ was used;

this value was determined by the use of 4 additional experiments:

Expt. No, T reaction . -AH(T) in 0.0500N NaOH

°C J-g”

1

549 29.980 146.64

550 22.293 139.17

551 22.409 139.58

552 29.919 147.52

A least squares fit of the data for the mean temperature of reaction

and the enthalpy values in 0.0500N NaOH resulted in the following

equation for the relationship of temperature and the enthalpy of

reaction (in °C and J*g ^)

:

AH(T) = 141.70 + 1.033 (T - 25),

except that the value of the constant enthalpy at 25 °C was reduced by

0.10 J.g ^ to correspond to the certified value. The standard errors

of the constants were ± 0.09 and ± 0.041, respectively, and the

-1

standard error of the estimate was ± 0.32 J*g

24
Table

6.

Electrical

energy

added

to

the

calorimeter

during

the

endothermic

reactions

of

SRM

724a

(TRIS)

in

0.0500N

NaOH

solutions.

"O

<u

r—l "O

CO vO vD <t CM o UO o co vt o CO p i—

1

rv o oo CM co MO 00 vt

•H o *-> % u 00 •H co CM rv P CM co CM o Pv U P 4-1 o o 00 00 00 00 oo On P

O u •f-l O <t vt r—l r—l 1—1 T—^ r—l r—l CM

CD 0) u CO r—

1

r—l H i—

1

r—l r-l r—l i—i r—l r—

1

c p <U H w Q Pi

Pi oo CO UO in rv vt CM MO CM m P p lO P MO M0 M0 vt

II <t vt vt vt vt vt vt vt CO G 00 oo 00 00 oo 00 00 oo oo M

sO p P p p M0 P MO sO W cr> CT> <T> os CT> C^ ON Os

o vt rv oo CM rv MO 00 Os vt CM vt uo CO oo 00 P r—l o cn 00 p CM oo oo r—l rv rv o CM I—

1

CM CM rv CM CO vt CM

4-1 w o o O O ON o o o O

os o^ co CO CM co CO co uo

'

cn CO CO CO CO CO CO co 1—1

in rv oo rv T—

1

rv 00 UO co

• r—l o m ON co co OS T—

1

00 a rv rv <t CO OS CO P Os CM e vD P P P CO CO CM 00 r—l H %

- C0 CM CM CM CM CM CM CM CM uo os c^ CTi CO O'* Os Os Os

.—1 1—1 r—l r—l 1—

1

r—l i—

1

T—

1

CM o

CM m iv os CO o o 00 r—l vt •vt o vt p vt MO oo uo

CO <Ti ON rv p CM rv o o 00 M 4-1 uo in m UO CO CM CM oo rv

r-l p p P P M0 P P P m

o> oo 00 oo 00 00 oo 00 00 00

» r—l r—

1

^1 r—l T—

1

r—l T—

1

r—l CM

4-1 00 O • p <u • CO

4-1 •H U 4-1 aj hN< H|^ i 4 H|t HIM1 H|^ H|^ HlM1 C0|^

n) O 4-1 o o 4-1 g <u CO 4-1 c0 p r—l 1—

1

T—

1

r—l r—l CM CM r—l •H r—

1

<u 0) cu c H w p P V4 •He

4-1 O'* O r—l CM CO m rv 00 a o* CO vt <t vt vt vt vt vt vt X p m in uo uo U0 m uo UO UO w

25
The enthalpy of reaction is dependent on the concentration of the

NaOH solution, and the corrections to 0.0500 N NaOH in table 5 were obtained

from the following equation (in normality and kJ*mol : AH(298 . 15 K) - 17.680 - 10.59(N) + 5.19(N)

2

The standard errors of the constants were ±0.013, ±0.18, and

±0.19 respectively; and the standard error of the estimate was ±0.022

kJ'mol The equation was obtained from a least squares fit of 11

preliminary experiments using SRM 724, the first NBS standard sample

of TRIS, in which the concentration of NaOH was varied:

Expt. Normality AH(298.15 K)

No. of NaOH

kJ*mol

264 0.005 17.643

265 .005 17.632

274 .049 17.170

275 .049 17.148

276 .051 17.155

273 .051 17.151

272 .051 17 . 148

270 .144 16.231

271 .144 16.305

268 .944 12.320

269 .944 12.300

The heats agree within the uncertainties of the NaOH analyses

with those reported by Hill, et al [5].

26
At the bottom of table 5 are the results of 6 preliminary

experiments using SRM 724. A difference in procedure was that the

TRIS reaction was started only a few seconds after beginning the electrical

heating. This resulted in a drop of the calorimeter temperature

because of the heater lag; consequently, there was a relatively large

difference between the calorimeter and shield temperatures, and a correction was made which was about 0.1% of the heat absorbed by the

TRIS reaction. Although the experimental precision appears better

in the older experiments, the corrections for departure from adiabatic

conditions are quite uncertain, and we believe the experimental

precision in the more recent experiments is more realistic. However,

the agreement of the mean values for the two samples is excellent.

In table 6 are given the time intervals between the beginning of electrical

heating and the initiation of the TRIS reaction for the corresponding

experiments in table 5.

In table 6 are the values used in determining the quantity of electrical

energy added to the calorimeter during the endothermic TRIS reactions

(see table 5). The voltage drop across the heater, E, was calculated from the

voltage drop across a 10-ohm standard resistor in series with a 10-K ohm

standard resistor, and the combination is in parallel with the calorimeter

heater. The current through the heater, I, was calculated from the

potential drop across a O.l^ohm standard resistor in series with the

calorimeter heater. The time of heating, t, was measured on an electric

counter (actuated through a mercury switch) which counted a 10 K HZ standard

frequency available at the National Bureau of Standards. The product, E*I*t,

is the amount of electrical energy added to the calorimeter during the

TRIS reaction.

27
The ratio, E/I, is equivalent to the average heater resistance

and is an indicator of the precision of the current and potential

measurements. It is also interesting to note that in the last

experiment in table 6 where the voltage was increased by 10 volts,

the heater resistance decreased by 0.01% as a result of the higher

temperature even though the heater is in very close contact with

a platinum well immersed directly in the calorimetric solution. For

this reason, both E and I should be measured if results of highest

accuracy are to be achieved.

The following possible errors and corrections to the AH

soln

were considered for the worst conditions and found to be negligible

:

(1) Heater lead corrections, -0.004 percent;

(2) Factor switch on potentiometer, -0.004 percent;

(3) Condensation correction in vapor space above solution,

about -0.001 J or < 0.0005 percent;

(4) Vaporization correction for air in sample holder,

< 0.05J or < 0.02 percent.

The first three corrections are opposite in sign to the fourth

resulting in a ne^ correction of about 0.01% in the worst case.

28
6. Discussion and Conclusions

The effects of various venting conditions reported in this paper

for the exothermic TRIS reaction in HCL solutions have not been observed

in any other reactions studied in this laboratory and appear to be characteristic of this reaction. The presence of traces of CO^ in the

calorimetric solutions apparently has no significant effect on the

enthalpy' of the exothermic reaction under normal conditions; however,

we have observed some significant effects in a closed system. Until

these effects can be thoroughly investigated, understood, and

explained, a certified value of higher accuracy than that chosen in

this paper for the exothermic TRIS reaction in HCI solution cannot

reasonably' be expected. Therefore, we have assigned a larger uncertainty

(0.10 percent for other possible systematic errors, see below) to the

certified value than would be normal if based only on our experimental

results . j ...

Two other groups have made extensive studies of the TRIS reaction

and report results of high precision. For the AH(298.15 K) of

3

SRM 724 in 0.1 "N HCI at a concentration of 5 grams per 1000 cm of

solution, Hill, et al, report -245.54 ± 0.03 J*g ^[5] and

Gunn reports -245.47 ± 0.01 J«g ^[2]. The uncertainties given

here were the standard deviations of the means of the experimental

values, and both measurements were made in isoperibol calorimeters,

the first at constant pressure and the second at constant volume.

We have been unable to detect a difference in the values obtained

with SRM 724 and with SRM 724a. Our certified value for AH(298.15 K)

of SRM 724a in 0.1 N HCI solution at a concentration of 5 grams per

q _ i

1000 cm of solution is -245.76 ± 0.26 J*g under the specified

29
conditions. The uncertainty (0.11%) is the square root of the sum of

the squares of the following uncertainties: the experimental

precision at the 95% confidence level, 0.03%; inhomogenie ty of the

sample, 0.01% [6]; and other possible systematic errors, 0.10%. These

measurements were made in an adiabatic calorimeter at constant pressure.

The endothermic reaction of TRIS in NaOH solutions has been

-1

studied by Hill, et al, who report a value of 141.90 ± 0.04 J*g

for AH(298.15K) for the reaction of SRM 724 in 0.0500N NaOH at

3 a concentration of 5 grams per 1000 cm of solution. The uncertainty

is the standard deviation of the mean of the experimental values.

Our certified value for the reaction of SRM 724a in 0.0500N NaOH

-1

under the specified conditions is 4H(298.15K) = 141.70 ± 0.19 J*g

The uncertainty (0.13%) is the square root of the sum of the squares

of the following uncertainties: the experimental precision at the

95% confidence level, 0.10%; analysis of sodium hydroxide solutions,

0.07%; and other possible systematic errors, 0.05%. Higher

accuracy in the enthalpy value is not expected for this reaction

because of its sensitivity to the concentration of the NaOH solution

and the possible effects of atmospheric CO^.

30
7. References

[1] Irving, R. J., and Wadso, I., Acta Chem. Scand. ,18, 195-201,

(1964) .

[2] Gunn, S. R., J. Chem. Thermodynamics 2

_} 535-547, (1970).

[3] Kilday, M. V., Prosen, E. J., and Wagman, D. D., "Heats of

Solution of BeO(c) in HF(aq) and in HCl(aq)," Chapter 2 NBS

Report No. 10 004, January, 1969;

Kilday, M. V. , and Prosen, E. J., "The Enthalpy of Solution of

NBS Standard Reference Material No. 1654 (cy-quartz) in Aqueous

Hydrofluoric Acid," NBS Report No. 10 561, April, 1971.

[4] Cameron, A. E., and Wichers, E., J. Am. Chem. Soc. 84, 4175,

(1962).

• • [5] Hill, J. 0., Ojelund, G., and Wadso, I., J. Chem. Thermodynamics 1969

,

111-116.

[6] Marinenko, G., "Coulometry, " pg. 56, NBS Technical Note 543,

November, 1970.

31


II —