inorganic lab report questions

Anonymous
timer Asked: Nov 29th, 2016

Question description

Experimental procedure:
Day 1. Prepare brushite-based cements.
Safety note: Phosphoric acid is very corrosive! Similar to other acid-base reactions, reaction of
phosphoric acid with HA is exothermic. Be careful, as the hot glass looks exactly the same as cold glass.
In this lab you will synthesize brushite-based cements according to the following reaction:
Ca5(PO4)3OH + 2H3PO4 + 9H2O → 5CaHPO4·2H2O (8)
On an evaporation dish or a watch glass, add 1.6 g of HA synthesized during the last lab. Dissolve sodium
hexametaphosphate in 2 mL of phosphoric acid such that the concentration is 0.04 M. Add the 
phosphoric acid solution to the HA powder in small portions while carefully mixing it with a glass rod.
Continue mixing for approx. 30-60 s until you have a homogeneous paste. Pour the paste in a form and
let it sit for 30-60 min until it hardens.
Submit to your sample for SEM and XRD.
Synthesis of α-TCP
Tricalcium phosphate is one of the compounds that cannot be precipitated from solution and needs to
be synthesized by a solid-state reaction. There are two forms of tricalcium phosphate: α-TCP (stable at
temperatures above 1100 °C) and β-TCP (low-temperature phase). β -phase is significantly less soluble
than α-TCP (see Figure 4 again), which results in a very slow reaction of hydrolysis. Thus, α-TCP is more
often used as precursor for bone cements. Unfortunately, α-TCP is not stable at room temperature and
it needs to be chemically stabilized. In this lab SiO2 (Si is a biogenic element, which also improves the
bone regeneration6
) will be used as a stabilizer, according to the following reaction:
(1+x)CaCO3 + (1-x)Ca2P2O7 + xSiO2 → Ca3-x(PO4)2-2x(SiO4)x + (1+x)CO2 (9)
Calculate the amount of the reagents to synthesize 2g of the product, where x = 0.05. Thoroughly grind
the mixture with mortar and pestle and place it into an alumina crucible. The mixture will be annealed at
1300 °C for 3h in order to synthesize the desired α-TCP.
Day 2. Prepare α-TCP -based cements:
In the second part of the lab you will synthesize HA-based cements by hydrolysis of tricalcium
phosphate. As it was discussed earlier, calcium phosphates with Ca/P < 1.67 produce acid upon
hydrolysis (reaction 10). However, the HA structure may adopt different substitutions and the reaction
may go without release of phosphoric acid, producing calcium-deficient hydroxyapatite according to the
reaction 11:
5α-Ca3(PO4)2 + 3H2O → 3 Ca5(PO4)3OH + H3PO4 (10)
3α-Ca3(PO4)2 + H2O → 3 Ca9(HPO4)(PO4)5OH (11)
For the cement, prepare 2.5 wt% solution of Na2HPO4 in water (it will accelerate the hydrolysis process).
Mix 1g of the α-TCP and Na2HPO4 solution such that the liquid to powder ratio is 0.4 mL/g (the ratio
might be adjusted to have a good paste). Put the paste into a form and let it react for ~1h. In order to
further accelerate the reaction, the form could be heated on a water bath at a temperature of 50 °C or
higher.
Submit your sample for SEM analysis.

1) How the concentration of [A+] depends on pH of solution if we dissolve the compound AX,
where HX is a weak acid? Derive the equation, where [A+
] is expressed through Ksp, Ka and pH (or
[H+]).
2) One of the problems with the reaction we used for brushite-based cements is that it is way too
fast. Sodium hexametaphosphate plays a role of inhibitor in this process. Suggest a possible
mechanism of the inhibition.
3) Analyze SEM data. What structural changes do you see?


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