Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE At pH <5, boric acid B(OH)3 is the


dominant form and at pH > 12 then borate anion B(OH)4-


is the dominant form.


At pH between these two values and excluding outside factors, both forms will be present. The higher the pH, the greater the tendency to form the borate anion.


For solutions at concentrations higher than 0.025 molar, boric acid in water tends to form polyborate complexes of various types, all dependent on concen- tration, pH and the environment.


Afton Chemical manufactures a range of boric acid derivatives for use as metalworking corrosion inhibitors. These derivatives are produced at a high temperature to ensure full reaction between boric acid and the various alkanolamines used. Water of reaction is evolved during the process generating a complex mixture of borate and polyborate reaction species. With this as the starting point, Afton Chemical undertook a program to develop an analytical method, based on 11


capable of determining the free boric acid content in the POLARTECH®


B NMR BA


series additives at a level of 0.1%. 11


H and 13


B NMR analysis is difficult in comparison to the more familiar 1


C NMR.


H, determination of boric acid at low levels is always going to be a problematic. There were a number of challenges to be overcome for this program.


With a spin number of I=3/2 and a relative sensitivity approximately 4% that of 1


NMR method development A Bruker 400 MHz NMR spectrometer, capable of multi-nuclear analysis and equipped with a BBO probe was used for this analytical programme. Conditions were developed to maximise the sensitivity towards 11


B and required the


use of ultra-low boron quartz tubes, Norrell®


S-5-500-QTZ-7. This


methodology was established as part of the calibration process and it was possible to calibrate the instrument down to very low levels of free boric acid.


Calibration was achieved by preparing boric acid in deuterated water [D2


O] at


various concentrations from 5.0 to 0.05% by weight. Boric acid is set at 0 ppm chemical shift. Chemical shifts other than 0 ppm indicate the boron atom is


22 LUBE MAGAZINE No.110 AUGUST 2012 Fig 3 Boric acid in D2 O at 0.05% by weight


chemically associated with other chemicals such as alkanolamine or present in more complex forms such as polyborate. With boric acid arbitrarily set at 0 ppm, this leads to other boron species having negative chemical shifts away from the boric acid. Each boron species has its own unique chemical shift.


In both cases the free boric acid is clearly visible as the sharp peak at 0 ppm and the limit of detectability is thought to be below 0.02% by weight. The broad peak is borate anion and hydrated polyborate anions.


Additive analysis


A series of investigations were conducted to first determine how to analyse for boric acid alkanolamine condensate derivatives, then determine the free boric acid content.


Fig 1 11 B NMR of boric acid solutions, aqua = 5% by


weight, light green = 2%, brown = 1%, yellow = 0.5%, purple = 0.2%, green = 0.1%, red = 0.05% and blue = 0%.


O at calibration points at 2.0% and 0.05% are seen in figures 2 and 3.


A more detailed observation of boric acid in D2


The first phase of the program was to determine how to analyse the additives. A laboratory prepared reference product was prepared under conditions designed to replicate the typical plant conditions. The reaction was monitored by the normal QC analytical methods and water of reaction was evolved and collected. The sample prepared was determined to be indistinguishable from the typical plant product by normal QC analytical methods.


As produced, the borate condensates are viscous concentrates which makes them difficult to analyse in the neat form. Analysis of the neat additives gave an unsatisfactory broad spectrum from which it was difficult to extract meaningful data.


Starting with the lab prepared borate condensate sample, the reaction product of boric acid and


O significantly improved resolution and enabled isolation of single species peaks. We found that dilution of borate condensate 50% to 10% active in D2


monoethanolamine, sequential dilution in D2


O Fig 2 Boric acid in D2 O at 2% by weight.


was sufficient to provide the resolution necessary, with the greatest resolution at 10%. However as we were looking for boric acid, which we expected to be at a low initial level, we decided to accept a compromise resolution at 50% active.


An interesting observation from this dilution work came from assessing the hydrolytic stability of the borate condensates. Many formulators have experienced precipitation when using boric acid products; the common belief is that it is boric acid that is precipi- tating. This may be the case where boric acid salts are used, but even at the low


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